Category Archives: Shooting Tips

Shooting in the dark – which gamma curve should I use?

This question comes up a lot, which gamma curve will give me the best results on a low key (dark) shoot. A common example would be “Should I use Gamma 2 which is brighter or should I use Gamma 5 which is darker but with a bit of gain”? or “Which is better to use in low light, normal gamma or log gamma”?

Log gamma (SLog, LogC etc) is great for capturing scenes with a large dynamic range, but it comes at the price of compressed highlights and overall less data per stop of exposure. Standard gammas have a lower dynamic range, but as a result the amount of data you are recording per stop of exposure is greater. It’s also vital to remember that if you do shoot in the dark with log you still need to expose correctly keeping middle grey and white at the correct levels in order that you keep mid tones and skin tones out of the heavily compressed part of the curve for the best results, so your images may look very dark and very flat. This can make focussing a big challenge (Top Tip – Use a large waveform monitor to check focus, as you go in and out of focus you should see the fine details in the waveform increase in amplitude as you become more focussed).

If you don’t need 13 or 14 stops of range, then use another gamma, don’t use log. Again with any of the extended range gammas like Cinegamma or Hypergamma watch where you place your skin tones, your exposure levels with these types of gamma curve are normally a little lower than with standard gammas (typically -0.5 to -1 stop).

A basic principle to understand with any video camera is that the actual sensitivity of the camera is purely a function of the sensor. Gain or raising the ISO is simply amplifying the signal off the sensor and results in more noise as the noise gets amplified too. Just like a music amplifier where you can make the music louder by turning up the volume (adding gain) you can make your pictures louder (brighter) by adding gain or raising the ISO. Listen to the music amplifier at high volume levels and what do you hear in the background? More hiss and noise, it’s the same with your video camera, more gain = more noise.

Gamma is also a form of gain providing different amounts of gain for different sensor output (brightness) levels to achieve specific recording and camera output levels, but it’s still gain (it can sometime be negative gain). The nearest you have to a gamma control on an audio amplifier would be an equaliser or bass and treble tone controls. Turn up the treble control on an audio amp and again you will here more hiss and noise, it’s nothing more than a selective gain control that shapes the sound you hear. Video camera gamma is very similar, it shapes the tonal range of the images you see.

So – and this is the bottom line: With any given gamma, combined with any given ISO or gain level, to achieve any particular brightness of output, for the same input level the total system gain (gain + gamma) is the same for that particular point in the scenes brightness range. So the noise level for any given brightness of output will be almost exactly the same whether you mix ISO “A” with Gamma “B”  or ISO “B” with Gamma “A” because the combined gain of A + B is the same as B + A.  At the end of the day the real sensitivity is governed by the sensor and anything else that makes the image brighter is gain, either regular “gain” or gamma gain.

So, choose a gamma curve that can deal with the dynamic range that you need and no more, don’t waste recording data on unused dynamic range. That way when you grade or do any post production noise reduction you have more data per stop to work from and that will help with the final image. If you do use log or a high range gamma (Hypergamma, Cinegamma etc) watch your exposure levels, the same rules apply in the dark as in daylight, you don’t want to over expose any skin tones as they won’t grade nicely.

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Correct exposure levels with Sony Hypergammas and Cinegammas.

When an engineer designs a gamma curve for a camera he/she will be looking to achieve certain things. With Sony’s Hypergammas and Cinegammas one of the key aims is to capture a greater dynamic range than is possible with normal gamma curves as well as providing a pleasing highlight roll off that looks less electronic and more natural or film like.

Slide3 Correct exposure levels with Sony Hypergammas and Cinegammas.
Recording a greater dynamic range into the same sized bucket.

To achieve these things though, sometimes compromises have to be made. The problem being that our recording “bucket” where we store our picture information is the same size whether we are using a standard gamma or advanced gamma curve. If you want to squeeze more range into that same sized bucket then you need to use some form of compression. Compression almost always requires that you throw away some of your picture information and Hypergamma’s and Cinegamma’a are no different. To get the extra dynamic range, the highlights are compressed.

exposure2-300x195 Correct exposure levels with Sony Hypergammas and Cinegammas.
Compression point with Hypergamma/Cinegamma.

To get a greater dynamic range than normally provided by standard gammas the compression has to be more aggressive and start earlier. The earlier (less bright) point at which the highlight compression starts means you really need to watch your exposure. It’s ironic, but although you have a greater dynamic range i.e. the range between the darkest shadows and the brightest highlights that the camera can record is greater, your exposure latitude is actually smaller, getting your exposure just right with hypergamma’s and cinegamma’s is very important, especially with faces and skin tones. If you overexpose a face when using these advanced gammas (and S-log and S-log2 are the same) then you start to place those all important skin tone in the compressed part of the gamma curve. It might not be obvious in your footage, it might look OK. But it won’t look as good as it should and it might be hard to grade. It’s often not until you compare a correctly exposed sot with a slightly over shot that you see how the skin tones are becoming flattened out by the gamma compression.

But what exactly is the correct exposure level? Well I have always exposed Hypergammas and Cinegammas about a half to 1 stop under where I would expose with a conventional gamma curve. So if faces are sitting around 70% with a standard gamma, then with HG/CG I expose that same face at 60%. This has worked well for me although sometimes the footage might need a slight brightness or contrast tweak in post the get the very best results. On the Sony F5 and F55 cameras Sony present some extra information about the gamma curves. Hypergamma 3 is described as HG3 3259G40 and Hypergamma 4 is HG4 4609G33.
What do these numbers mean? lets look at HG3 3259G40
The first 3 numbers 325 is the dynamic range in percent compared to a standard gamma curve, so in this case we have 325% more dynamic range, roughly 2.5 stops more dynamic range. The 4th number which is either a 0 or a 9 is the maximum recording level, 0 being 100% and 9 being 109%. By the way, 109% is fine for digital broadcasting and equates to bit 255 in an 8 bit codec. 100% may be necessary for some analog broadcasters. Finally the last bit, G40 is where middle grey is supposed to sit. With a standard gamma, if you point the camera at a grey card and expose correctly middle grey will be around 45%. So as you can see these Hypergammas are designed to be exposed a little darker. Why? Simple, to keep skin tones away from the compressed part of the curve.

Here are the numbers for the 4 primary Sony Hypergammas:

HG1 3250G36, HG2 4600G30, HG3 3259G40, HG4 4609G33.

Cinegamma 1 is the same as Hypergamma 4 and Cinegamma 2 is the same as Hypergamma 2.

All of the Hypergammas and Cinegammas are designed to exposed a little lower that with a standard gamma.

Exposure levels using EI ISO and zebras with the PMW-F5 and raw.

The PMW-F5 and F55 are fantastic cameras. If you have the AXS-R5 raw recorder the dynamic range is amazing. In addition because there is no gamma applied to the raw material you can be very free with where you set middle grey. Really the key to getting good raw is simply not to over expose the highlights. Provided nothing is clipped, it should grade well. One issue though is that there is no way to show 14 stops of dynamic range in a pleasing way with current display or viewfinder technologies and at the moment the only exposure tool you have built in to the F5/F55 cameras are zebras.

My experience over many shoots with the camera is that if you set zebras to 100% and don’t use a LUT (so your monitoring using S-Log2) and expose so that your just starting to see zebra 2 (100%) on your highlights, you will in most cases have 2 stops or more of overexposure headroom in the raw material. Thats fine and quite useable, but shoot like this and the viewfinder images will look very flat and in most cases over exposed. The problem is that S-Log 2’s designed white point is only 59% and middle grey is 32%. If your exposing so your highlights are at 100%, then white is likely to be much higher than than the designed level, which also means middle grey and your entire mid range will be excessively high. This then pushes those mids into the more compressed part of the curve, squashing them all together and making the scene look extremely flat. This also has an impact on the ability to focus correctly as best focus is less obvious with a low contrast image. As a result of the over exposed look it’s often tempting to stop down a little, but this is then wasting a lot of available raw data.

So, what can you do? Well you can add a LUT. The F5 and F55 have 3 LUTS available. The LUTS are based either on REC709 (P1) or Hypergamma (P2 and P3). These will add more contrast to the VF image, but they show considerably less dynamic range than S-Log2. My experience with using these LUT’s is that on every shoot I have done so far, most of my raw material has typically had at least 3 stops of un-used headroom. Now I could simply overexpose a little to make better use of that headroom, but I hate looking into the viewfinder and seeing an overexposed image.

Why is it so important to use that extra range? It’s important because if you record at a higher level the signal to noise ratio is better and after grading you will have less noise in the finished production.

Firmware release 1.13 added a new feature to the F5 and F55, EI Gain.  EI or Exposure Index gain allows you to change the ISO of the LUT output. It has NO effect on the raw recordings, it ONLY affects the Look Up Tables. So if you have the LUT’s turned on, you can now reduce the gain on the Viewfinder, HDSDI outputs as well as the SxS recordings (see this post for more on the EI gain). By using EI gain and an ISO lower than the cameras native ISO I can reduce the brightness of the view in the viewfinder. In addition the zebras measure the signal AFTER the application of the LUT or EI gain. So if you expose using a LUT and zebra 2 just showing on your highlights and then turn on the EI gain and set it to 800 on an F5 (native 2000ISO) or 640 on an F55 (native 1250ISO) and adjust your exposure so that zebra 2 is once agin just showing you will be opening your aperture by 1.5 (F5) or 1 (F55) stop. As a result the raw recordings will be 1.5/1 stop brighter.

In order to establish for my own benefit which was the best EI gain setting to use I spent a morning trying different settings. What I wanted to find was a reliable way to expose at a good high level to minimise noise but still have a little headroom in reserve. I wanted to use a LUT so that I have a nice high contrast image to help with focus. I chose to concentrate on the P3 LUT as this uses hypergamma with a grey point at 40% so the mid range should not look underexposed and contrast would be quite normal looking.

When using EI ISO 800 and exposing the clouds in the scene so that zebras were just showing on the very brightest parts of the clouds the image below is what the scene looked like when viewed both in the viewfinder and when opened up in Resolve. Also below is the same frame from the raw footage both before and after grading. You can click on any of the images to see a larger view.

xd-norm-800-1024x311 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
P3 LUT, XDCAM recording, 800 EI ISO (PMW-F5).
raw-norm-pre-1024x325 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
Raw footage, EI 800 ISO pre-grade.
raw-norm-grade-1024x322 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
Raw, 800 ISO after grade. NO clipped highlights.

As you can see using LUT P3 and 800 EI ISO (PMW-F5) and zebra 2 just showing on the brightest parts of the clouds my raw footage is recorded at a level roughly 1.5 stops brighter than it would have been if I had not used EI gain. But even at this level there is no clipping anywhere in the scene, so I still have some extra head room. So what happens if I expose one more stop brighter?

xd-plus1-1024x316 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
The XDCAM recording, LUT P3, 800 EI, +1 stop, zebras showing over almost all clouds.
raw-+1-pre-1024x314 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
Raw clip at +1 stop prior to grade.
raw-+1-grade-1024x313 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
Raw at +1 stop after grade, no sign of any clipping.

So, as you can see above even with zebras over all of the brighter clouds and the exposure at +1 stop over where the zebras were just appearing on the brightest parts of the clouds  there was no clipping. So I still have some headroom left, so I went 1 stop brighter again. The image in the viewfinder is now seriously over exposed.

XD-+2-1024x316 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
The XDCAM recording at +2 stops, the sky and clouds look very overexposed.
raw-+2-pre-1024x318 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
Raw clip, pre grading (LUT P3, EI 800). Looking scarily over exposed.
raw-+2-grade-1024x316 Exposure levels using EI ISO and zebras with the PMW-F5 and raw.
After the grade the raw is looking much better, but there is a bit of clipping on the very brightest clouds.

The lower of the 3 images above is very telling. Now there is some clipping, you can see it on the waveform. It’s only on the very brightest clouds, but I have no reached the limit of my exposure headroom.

Based on these tests I feel very comfortable exposing my F5 in raw by using LUT P3 with EI gain at 800 and having zebra 2 starting to appear on my highlights. That would result in about 1.5 stops of headroom. If you are shooting a flat scene you could even go to 640 ISO which would give you one safe stop over the first appearance of zebra 2. On the F55 this would equate to using EI 640 with LUT P3 and having a little over 1.5 stops of headroom over the onset of zebras or EI 400 giving about 1 stop of headroom.

My recommendation having carried out these tests would be to make use of the lower EI gain settings to brighten your recorded image. This will result in cleaner, lower noise footage and also allow you to “see” a little deeper into the shadows in the grade. How low you go will depend on how much headroom you want, but even if you use 640 on the F5 or 400 on the F55 you should still have enough headroom above the onset of zebra 2 to stay out of clipping.

 

 

What is PsF, or why does my camera output interlace in progressive?

This one keeps coming around again and again and it’s not well understood by many.

When the standards for HDSDI and connecting HD devices were originally set down, almost everyone was using interlace. The only real exception was people producing movies and films in 24p. As a result the early standards for HDSDI did not include a specification for 25 or 30 frame per second progressive video. However over time 25p and 30p became popular shooting formats, so a way was needed to send these progressive signals over HDSDI.

The solution was really rather simple, split the progressive frames into odd and even lines and send the odd numbered lines in what would be the upper field of an interlace stream and then send the even numbered lines in what would be the lower field. So in effect the progressive frame gets split into two fields, a bit like an interlaced video stream, but there is no time difference (temporal difference) between when the odd and even are were captured.

This system has the added benefit that even if the monitor at the end of the HDSDI chain is interlace only, it will still display the progressive material more or less correctly.

But here’s the catch. Because the progressive frame split into odd and even lines and stuffed into an interlace signal looks so much like an interlace signal, many devices attached to the PsF source cannot distinguish PsF from real interlace. So more often than not the recorder/monitor/edit system will report that what it is receiving is interlace, even if it is progressive PsF. In most cases this doesn’t cause any problems as what’s contained within the stream does not have any temporal difference between the odd and even lines. The only time it can cause problems is when you apply slow motion effects, scaling effects or standards conversion processes to the footage as fields/lines from adjacent frames may get interleaved in the wrong order. Cases of this kind of thing are however quite rare and unusual.

Some external recorders offer you the option to force them to mark any files recorded as PsF instead of interlace. If you are sure what you are sending to the recorder is progressive, then this is a good idea. However you do need to be careful because what will screw you up is marking real interlace footage as PsF by mistake. If you do this the interlaced frames will be treated as progressive.  If there is any motion in the frame then the two true interlace fields will contain objects in different positions, they will have temporal differences. Combine those two temporally different fields together into a progressive frame and you will see an artifact that looks like a comb has been run through the frame horizontally, it’s not pretty and it can be hard to fix.

So, if you are shooting progressive and yet your external recorder say’s it’s seeing interlace from your HDSDI, don’t panic. This is quite normal.

If you are importing footage that is indicated as being interlace, but you know it’s progressive PsF into most edit packages you can normally select the clips and “interpret footage” or similar to change the clip header files to progressive instead of interlace.

Tips for shooting in very cold weather.

With winter well upon us I thought it would be good to share some of my arctic shooting experience. I’ve shot in temperatures down to -40c in the arctic in winter.

Overall modern tapeless cameras do OK well in extreme cold. The most reliable cameras are larger solid state cameras. Larger cameras cool slower than small ones and larger cameras will hold on to heat generated internally better than small ones.

Condensation:

Condensation is the big deal breaker. When you take the very cold camera inside into a house/hotel/car/tent you will get condensation. If the camera is very cold this can then freeze on the body of camera including the glass of the lens. If there is condensation on the outside of the camera, there will almost certainly also be condensation inside and this can kill your camera.

To prevent or at least reduce the condensation you can place the camera in a large ziplock bag BEFORE taking it inside, take the camera inside in the bag. Then allow the camera to warm up to the ambient temperature before removing it from the bag. Peli cases are another option, but the large volume of the pelicase means there will be more moisture inside the case to condense and the insulating properties of the case mean that it could take many, many hours to warm up.

I don’t recommend storing a cold or damp camera in a Pelicase (or any other similar waterproof case) as there is nowhere for the moisture to go, so the camera will remain damp until the pelicase is opened and everything dried out properly.

Rather than moving a camera repeatedly from outside to inside and repeatedly generating risky condensation you should consider leaving the camera outside. You can leave the camera outside provided it does not get below -25c. Below -25c you risk the LCD panel freezing and cracking. LCD  panels freeze at between -30 to -40c. If you are using a camera in very cold conditions and you notice the edges of the LCD screen going blue or dark you should start thinking about warming up that LCD panel as it may be close to freezing.

LCD displays will become slow and sluggish to respond in the cold. Your pictures may look blurry and smeary because of this. It doesn’t affect the recording, only what you see on the LCD.

Very often in cold regions houses will have an unheated reception room or porch. This is a good place to store your camera rather than taking it inside into the warm. Repeatedly taking a camera from cold to warm without taking precautions against condensation will shorten the life of your camera.
If you can, leave the camera on between shots. The camera generates some heat internally and this will prevent many issues.

BATTERY LIFE:

Li-Ion batteries are effected by the cold but they are not nearly as bad as Nicads or NiMh batteries which are all but useless below freezing. li-Ion battery life gets reduced by between 25 and 50% depending on how cold it is. Down to about -10c there is only a very marginal loss of capacity. Down to -25c you will loose about 20%-30% below -25c the capacity will fall away further.

Keep your spare batteries in a pocket inside your coat or jacket until you need them. After use let the battery warm up before you charge it if you can. Charging a very cold battery will reduce the lifespan of the battery and it won’t fully charge. One top tip for shooting outside for extended periods is to get a cool box. Get some chemical hand warmers and place them in the cool box with your batteries to keep them warm. If you don’t have hand warmers you can also use a hot water bottle.

Watch your breath

If your lens has and snow or ice on it, don’t be tempted to breath or blow on the lens to blow the ice off.  Also try not to breath on the lens when cleaning it as your warm breath will condense on the cold glass and freeze.  Also try to avoid breathing out close to the viewfinder. A small soft paint brush is good for keeping your lens clean as in very cold conditions you’ll simply be able to brush and snow or ice off. Otherwise a large lens cloth.

Covers.

Conventional rain covers become brittle below about -15c and can even shatter like glass  below -20c. Special insulated cold weather covers often called “polar bears” can be used and these often have pockets inside for chemical heat packs. These are well worth getting if you are going to be doing a lot of arctic shooting and will help keep the camera warm. As an alternative wrap the camera in a scarf or cut the sleeves of an old sweater to make a tube you can slide over the camera. If you have a sewing machine you could make a simple cover out of some fleece type material.

Your lens will get cold and in some conditions you will get frost on the front element. To help combat this wrap some insulating fabric around the body of the lens. Wrist sweat bands are quite good for this or an old sock with the toes cut off.

Brittle Plastic.

Plastics get brittle at low temperatures so be very gentle with anything plastic, especially things made from very hard, cheap plastic. The plastic Sony use appears to be pretty tough even at low temps. Wires and cables may become ridged. Be gentle, bend then too much and the insulation may split.

Other considerations are tripods. If outside in very low temps for more than 30mins or so the grease in the tripod will become very thick and may even freeze, so your fluid damping will become either very stiff or freeze up all together. Contact your tripod manufacturer to see what temperatures their greases can be used over. Vinten and some of the other tripod companies can winterise the tripod and replace the normal grease with arctic grease.

Looking after yourself.

I find that the best way to operate the camera is by wearing a pair of large top quality mittens (gloves are next to useless below -15c), consider getting a pair of Army surplus arctic mittens, they are very cheap on ebay and will normally have an additional “trigger finger”. This extra finger makes it easier to press the record button and things like that.  If you can get Swedish or Finnish military winter mittens, these are amongst the best. I wear a pair of thin “thinsulate” gloves that will fit inside the mittens, i can then slip my hands in and out of the mittens to operate the camera.

I keep a chemical hand warmer inside the mittens to warm my fingers back up after using the camera. The hardest thing to keep warm is your feet. If you’ll be standing in snow or standing on ice then conventional hiking boots etc will not keep your feet warm. A Scandinavian trick if standing outside for long periods is to get some small twigs and tree branches to stand on and help insulate your feet from the cold ground. If your feet get cold then you are at risk of frostbite or frost nip. Invest in or hire some decent snow boots like Sorel’s or Baffin’s. I have an arctic clothing guide here; Arctic Clothing Guide |

Back to Basics; White Balance.

While chatting to a few people at IBC it was brought to my attention that some people find my blog a little too advanced and intimidating. So in order to try to address that I’m going to go back to basics and write about some of the basic principles of digital video. The plan is to look at some basic concepts and then expand on these so that even beginners will gain a deeper understanding of these topics.

To kick off I’m going to look at White Balance.

What Does it Do?

Accurate white balance ensures that white objects in your video appear white, in addition it ensures that colours look natural and similar to the colours you see with your own eyes while shooting the scene.

How do I set it?

You can use the cameras auto mode, often called ATW (auto tracing white balance), a preset white balance, calibrate the white balance using a white or grey object or manually dial in the white balance.

Background:

Colour-Temperature-200x300 Back to Basics; White Balance.
Colour Temperature

Different light sources have different colour temperatures. For example an incandescent light bulb, the type with a glowing filament has a low colour temperature while an arc lamp which is much hotter will have a higher colour temperature. Low colour temperature light sources like a filament light bulb are at the red or orange end of the visible light spectrum while hotter light sources like an arc lamp or the sun are at the blue end of the light spectrum. Our own visual system will adapt to these varying colour temperatures but a video camera needs to be set to the correct colour temperature to reproduce colours correctly. The colour temperature of a light source is measured in “degrees Kelvin” or “K”. Lower numbers will be at the red end and higher numbers at the blue end. So a filament light bulb will glow orange at typically 2,300k while the mid day sun will be around 5,500k and the light from a clear blue sky is around 9,000k. If you shoot with a camera set for a low colour temperature i.e. 3,200k outside on a sunny day your pictures will be very blue. If you shoot with a camera set for a higher temperature like daylight, typically 5,600k under filament lamps the pictures will be very orange.

color-balance-sample-1 Back to Basics; White Balance.

Different Types of White Balance Setting.

Full Auto, ATW, Auto Tracking White, Auto White: This is generally a fully automatic mode that tries to guess the correct white balance. It normally works by assuming that the brightest parts of the scene are white objects and will try to correct these objects so that they appear white. As a result ATW is often easily fooled especially by bright slightly off white walls. As ATW is always active if the scene you are shooting changes then so may the white balance and this can be seen as a colour shift during the shot. ATW can be a little unpredictable and trying to later correct a shot where the white balance changes part way through can be very difficult. One advantage with ATW is that if you move from one place with one colour temperature to a place with a different colour temperature, for example moving from an interior room lit by filament lights to outside daylight then the ATW should compensate for the change in colour temperature automatically. On many pro  cameras ATW can be assigned to the white balance “preset, A, B” switch “B” position.

Preset White: Preset White allows you to pre set the colour temperature manually. Normally this preset will be for 3,200K (Tungsten filament lamps) or 5,600K (average daytime exterior). Many more recent Sony cameras will have a switch to select preset white and then a further button to alternate between 3,200 and 5,600K.  Preset white is useful when you have a multi camera shoot to get all cameras matching or for setting the white balance when shooting under coloured lights such as stage or concert lighting where ATW would really struggle or manual white balancing may give an incorrect colour balance. It’s is also often the white balance method of choice on projects that will be colour graded in post production as it gives a constant setting from shot to shot, scene to scene which colourists often find easier and faster to work with.

color-balance-FS700 Back to Basics; White Balance.
White Balance Switch and White Balance Set Button on FS700

Manual White Balance: This is sometimes misunderstood, but when done correctly will give the most accurate results. This is where you have a 3 way switch marked “Prst” (preset) “A” and “B”. When in the “A” or “B” position (assuming ATW has not been assigned to the “B” position) a white balance memory is used to determine the colour balance of the camera. To get the correct colour balance the camera needs to be sample and memorise the colour temperature of the light in the scene you want to shoot. This is done using a white or grey card or object. It doesn’t really matter which you use provided the card or object is not coloured in any way and can be correctly exposed. Be careful what you use, a lot of paper is dyed slightly blue to make it appear brighter. If you have a choice the best thing to use is a grey card designed for photography or video exposure and white balance. Place the card at the center of your shot, you want the primary light source for your scene to be falling on the card. Point the camera at the card, it should fill at least 50% of the frame and expose it so that it is around the middle of your exposure range, not bright white and not dark grey, right in the middle. Then with the white balance switch in the “A” or “B” position press the “white balance” button (often found on the front of the camera under the lens or close to the white balance switch). The camera will then adjust the gain of the red, blue and green channels so that the grey/white card shows no colour and will often display the measured colour temperature in the viewfinder or a simple “OK” message. If you get a “NG” or error message you may need to adjust your exposure up or down a little. Look at the pictures on a colour screen and check that they look OK. If not, check that the grey/white card is in the centre of your frame again and try repeating the process. You can store two manual white balance settings one in the “A” memory and one in the “B” memory and switch between them as needed.

Advanced White Balance Techniques: Sometimes it can be useful to use a white balance colour temperature that may be different from the actual colour temperature of the scene. For example if you are shooting a sunset the colour temperature of the actual light might be around 2,000 to 3,000K. If you were to white balance the camera to match this it would neutralise the orange glow of the sunset making it much less colourful. So to shoot a sunset you might want to have the camera set to a higher colour temperature to accentuate the orange sunset light. Setting the camera to preset 5,600k might work, but perhaps this takes things too far and makes it too orange. In this case you might want to dial in an in between value for your white balance like 4,400K. Some cameras will allow you to do this by changing the preset white value (in picture profiles or scene files on most Sony cameras). Alternately some cameras will allow you to add an offset to your manually set white balance, but do remember if you do this to remove it again once your done with it.

Another way to white balance the camera with a warmer or cooler white balance than the actual lighting conditions is to use a coloured card instead of a white/grey card. If you use a card with a slight blue tint then the camera will think the scene is bluer than it really is. This results in a final image that is warmer (more orange) than if you had used a white/grey card. If you use a card with a slight orange tint then the camera will think the ambient light is warmer resulting in a final image that looks cooler (more blue).  Instead of using coloured cards (often called warming or cooling cards) you can achieve exactly the same effect by doing your white balance through a blue or orange lighting gel or filter.

What to avoid: Try to avoid mixing different light sources with different colour temperatures. For example a person sat close to a window, lit by a filament light will be illuminated by the orange 3,200K light from the lamp as well as the blue 5,600K light from outside. This can result in some strange colours and the scene may look odd (of course you may deliberately choose to have this colour contrast to enhance your scene). Some light sources contain peaks and gaps in the light spectrum that they emit and this can result in odd colour reproduction. A common example of this is fluorescent lights used in homes and offices that often have a pronounced green tint to them that white balancing alone will not remove.

 

Why Nailing Your Mid Range Will Make Post Production Happy. Even with Cingammas and Hypergammas.

One of the concepts that’s sometimes hard to understand is why mid range exposure is so critical with most video cameras, even cameras with extended dynamic range. Cameras that use Cinegammas, Hypergammas  may give you great dynamic range and extra latitude but it’s still vital that you get your mid range exposed correctly. In many cases, the greater you cameras ability to capture a wide dynamic range the more critical mid range exposure becomes. I’ve often heard comments from users of XDCAM cameras complaining that they find it harder to work with cinegammas and hypergammas than the standard REC-709 gamma.
So why is this, it seams counter intuitive, surely a greater dynamic range makes exposure more forgiving?

exposure1 Why Nailing Your Mid Range Will Make Post Production Happy. Even with Cingammas and Hypergammas.
Typical Standard Gamma

First lets take a look at a standard gamma curve. These graphs are not accurate, just thrown together to illustrate the point. The standard gamma for HD, REC-709 can be considered to be near linear. Certainly in terms of “what you see is what you get” the idea behind REC-709 is that if the camera is set to 709 and the TV or monitor is 709 compliant then we will get a linear 1:1 reproduction of the real world. However REC-709 is based on the gamma curves used at the very beginnings of television broadcasting where TV’s and cameras had very limited dynamic range. True REC-709 only allows for about 6 stops of dynamic range and as a result the version of REC-709 used in most video cameras is tweaked somewhat to allow a greater dynamic range in the region of 8 to 10 stops while still producing a pleasing image on most TV’s. Another way of increasing dynamic range is to introduce some form of signal compression. The simplest form of this in common use is the cameras knee circuit. This simply takes anything above a certain brightness level (typically between 80 and 95%) and compresses it. We normal get away with this compression because it’s only affecting highlights like clouds in a bright sky or a bright window or lamp in the shot. Our own visual system is tuned primarily to mid tones, faces, plants and things like that so we don’t tend to find highlight compression overly obtrusive.
When considering your post production workflow and grading in particular, it’s important to remember that in most cases whenever anything is compressed then some of the original data is being discarded. In addition if the amount of compression is non-linear (increases or decreases with amplitude) then when we add a linear function to that, like adjusting the signal gain the non-linearity is also increased.
Based on these assumptions, you should be able to understand that anything exposed in the linear part of a gamma curve will grade very well because there is no extra compression and gain adjustments will behave as expected. Now if you look at the graph of a typical standard gamma curve (as above) you can see that everything below the knee point is pretty linear, so anything exposed in this range will grade easily and well (assuming it isn’t actually overexposed). For this reason standard gamma can be very forgiving to small over exposure problems as a slightly bright face should still be in the linear part of the curve. However overexpose to the point where the face starts to enter the knee area and all is lost, you’ll never make it look natural.

exposure2 Why Nailing Your Mid Range Will Make Post Production Happy. Even with Cingammas and Hypergammas.
Typical Cinegamma or Hypergamma

Now look at the curve for a typical Cinegamma or Hypergamma. You can see that this curve starts to become more curved and less linear much earlier than a standard gamma. This is how the extra latitude is gained. Compression is used to allow the camera to record a greater brightness range. This extra compression though comes at a price and that is linearity. The further up the exposure range you go the less linear the response (it’s actually becoming logarithmic). The result is that even though you have more dynamic range, if you do overexpose faces and skin tones by even just a small amount they will start to creep into the non linear part of the curve and this makes them harder to grade naturally. You may be less likely to get those ugly blown out highlights on a shiny face typical of video knee compression with cine/hypergammas, but you must still be very careful not to overexpose.

So there you have it. Greater dynamic range does not necessarily equate to more exposure tolerance. In fact it’s often the opposite. You might get better highlight handling, but you may find you need to be even more careful with how you expose. As we go forwards (or sideways at least) and linear raw becomes more common place then you will be able to shift you mid tone exposure up and down with a lot more flexibility as with a linear raw camera the last stop of exposure has the same linearity as the first, so in theory your mid tones can sit anywhere in the exposure range. Sony’s F65 is a great example of this. It has 14 stops of linear dynamic range. A face lit with a 3 stop range could be placed in stops 11-14 and would grade down to wherever you want just perfectly.

Calibrating your viewfinder or LCD.

smpte-arib-bars-sample Calibrating your viewfinder or LCD.One of the most important things to do before you shoot anything is to make sure that any monitors, viewfinders or LCD panels are accurately calibrated. The majority of modern HD cameras have built in colour bars and these are ideal for checking your monitor. On most Sony cameras you have SMPTE ARIB colour bars like the ones in the image here. Note that I have raised the black level in the image so that you can see some of the key features more clearly. If your using a LCD or OLED monitor connected via HDSDI or HDMI then the main adjustments you will have are for Contrast, Brightness and Saturation.

First set up the monitor or viewfinder so that the 100% white square is shown as peak white on the monitor. This is done by increasing the contrast control until the white box stops getting brighter on the screen. Once it reaches maximum brightness, back the contrast level down until you can just perceive the tiniest of brightness changes on the screen.

Once this is set you now use the pluge bars to set up the black level. The pluge bars are the narrow near black bars that I’ve marked as -2% +2% and +4% in the picture they are each separated by black. The -2% bar is blacker than black so we should not be able to see this. Using the brightness control adjust the screen so that you can’t see the -2% bar but can just see the +2% bar. The 4% bar should also be visible separated from the 2% bar by black.

Color is harder to set accurately. Looking at the bars, the main upper bars are 75% bars so these are fully saturated, but only at 75% luma. The 4 coloured boxes, 2 on each side, two thirds of the way down the pattern are 100% fully saturated boxes. Using the outer 100% boxes increase the saturation or colour level until the color vibrance of the outer boxes stops increasing, then back the level down again until you just perceive the color decreasing. I find this easiest to see with the blue box.

Now you should have good, well saturated looking bars on you monitor or LCD and provided it is of reasonable quality it should be calibrated adequately well for judging exposure.

I find that on an EX or F3 the LCD panel ends up with the contrast at zero, colour at zero and brightness at about +28 on most cameras.

When should I use a Cinegamma or Hypergamma?

Cinegammas are designed to be graded. The shape of the curve with steadily increasing compression from around 65-70% upwards tends to lead to a flat looking image, but maximises the cameras latitude (although similar can be achieved with a standard gamma and careful knee setting). The beauty of the cinegammas is that the gentle onset of the highlight compression means that grading will be able to extract a more natural image from the highlights. Note than Cinegamma 2 is broadcast safe and has slightly reduced recording range than CG 1,3 and 4.

Standard gammas will give a more natural looking picture right up to the point where the knee kicks in. From there up the signal is heavily compressed, so trying to extract subtle textures from highlights in post is difficult. The issue with standard gammas and the knee is that the image is either heavily compressed or not, there’s no middle ground.

In a perfect world you would control your lighting (turning down the sun if necessary ;-o) so that you could use standard gamma 3 (ITU 709 standard HD gamma) with no knee. Everything would be linear and nothing blown out. This would equate to a roughly 7 stop range. This nice linear signal would grade very well and give you a fantastic result. Careful use of graduated filters or studio lighting might still allow you to do this, but the real world is rarely restricted to a 7 stop brightness range. So we must use the knee or Cinegamma to prevent our highlights from looking ugly.

If you are committed to a workflow that will include grading, then Cinegammas are best. If you use them be very careful with your exposure, you don’t want to overexpose, especially where faces are involved. getting the exposure just right with cinegammas is harder than with standard gammas. If anything err on the side of caution and come down 1/2 a stop.

If your workflow might not include grading then stick to the standard gammas. They are a little more tolerant of slight over exposure because skin and foliage won’t get compressed until it gets up to the 80% mark (depending on your knee setting). Plus the image looks nicer straight out of the camera as the cameras gamma should be a close match to the monitors gamma.