All posts by alisterchapman

S-Log, Latitude, Dynamic Range and EI S-log. Or how to modify your exposure range with EI S-Log

The big issue most people have when working with log and exposing mid grey at 38 is that when you look at it on a standard monitor without any lookup tables it looks underexposed. The assumption therefore is that it is underexposed or in some way too dark to ever look right, because that’s what people used to working with conventional gammas have become programmed to believe over many years from their experience with conventional gammas.

So, for confidence you add a lookup table which converts the log to a Rec-709 type gamma and now the image looks brighter, but as it now has to fit within Rec-709 space we have lost either some of our high end or low end so we are no longer seeing the full range of the captured image so highlights may be blown out or blacks may be crushed.
It’s important for people to understand the concept of gamma and colour space and how the only way to truly see what a camera (any camera) is capturing is to use a monitor that has the same gamma and colour space. Generally speaking lookup tables don’t help as they will be taking a signal with a large range and manipulating it to fit in a small range and when you do that, something has to be discarded. If you were to take an F3 set to S-log and expose mid grey at 38 and show that on one of the nice new Sony E170 series monitors that have S-log gamma and place that next to another F3 with Rec-709 shooting mig grey at 45% and a similar but conventional 709 monitor the lower and mid range exposures would be near identical and the S-log images would not look under exposed or flat. The S-log images however would show an extra 2 stops of dynamic range.

Furthermore it has to be remembered that log is log, it is not linear. Because of its non linear nature, less and less brightness information is getting recorded as you go up the brightness range. As our own visual system is tuned to be most accute in the mid ranges this is normally fine provide you expose correctly putting mid tones in the more linear, lower parts of the S-log curve. Start putting faces to high up the S-log curve and it gets progressively harder to get a natural look after grading. This is where I think a lot of people new to log stumble. They don’t have the confidence to expose faces at what looks like a couple of stops under where they would with a standard gamma, so they start bringing up the exposure closer to where they would with standard gamma and then have a really hard time getting faces to look natural in the grade. Remember that the nominal S-Log value for white is 68 IRE. Part of the reason for this is that above about 70 IRE the amount of compression being applied by log is getting pretty extreme. While there is some wriggle room to push your exposure above or below the nominal mid grey at 38 it’s not as big as you might expect, especially dealing with natural tones and overexposure.

If you do want to shift your middle grey point this is where the EI S-log function and a light meter comes into it’s own, it’s what it’s designed for.

First something to understand about conventional camera gain, dynamic range and latitude. The latitude and sensitivity of the F3 is governed by the latitude and sensitivity of the sensor, which is a little under 13 stops. Different amounts of gain or different ISO’s don’t alter the sensors latitude, nor do they alter the actual sensitivity, only the amount of signal amplification. Increasing the camera gain will reduce the cameras output dynamic range as something that is 100 IRE at 800 ISO would go into clipping if the actual camera gain was increased by 6db (taking the ISO to 1600) but the darkest object the camera can actually detect remains the same. Dark objects may appear brighter, but there is still a finite limit to how dark an object the camera can actually see and this is governed by the sensor and the sensors noise floor.

EI (Exposure Index) shooting works differently, whether it’s with the F3, F65, Red or Alexa. Let’s consider how it works with the PMW-F3. In EI S-Log mode the camera always actually outputs at 800 ISO from the A/B outputs. It is assumed that if your working with S-Log you will be recording using an external 10 bit recorder connected to the A/B outputs. 422 is OK, but you really, really need 10 bit for EI S-Log. At 800 ISO you have 6.5 stops of over exposure and 6.5 under when you shoot mid grey at 38 or expose conventionally with a light meter.
Now what happens when you set the camera to EI 1600? Understand that the camera will still output at 800 ISO over the A/B outputs to your external recorder, but also note that 6db gain (1 stop) is added to the monitor output and what you see on the LCD screen, so the monitor out and LCD image get brighter. As the cameras metering systems (zebras, spot meter, histogram) measure the signal on the monitor side these are also now offset by +6db or + 1 stop.
As the camera is set to EI 1600 we set our light meter to 1600 ISO. If we make no change to our lighting the light meter would tell us to stop down by one stop, compared to our original 800 ISO exposure.
Alternately, looking at the camera, when you switch on EI 1600 the picture gets brighter, your mid grey card would also become brighter by one stop, so If we use the cameras spot meter to expose our grey card at 38 again we would need to stop down the iris by one stop to return the grey card to 38 IRE (for the same light levels as we used for 800). So either way, whether exposing with a light meter or exposing using the cameras built in metering, when you go from EI 800 to EI 1600 for the correct exposure (under the same lighting) you would stop down the iris by one stop.
Hope those new to this are still with me at this point!
Because the cameras A/B output is still operating at 800 ISO and you have stopped down by one stop as that what the light meter or camera metering told you to do because they are operating at EI 1600, the A/B output gets darker by one stop. Because you have shifted the actual recorded output down by one stop you have altered you exposure range from the original +/- 6.5 stops to + 7.5 stops, -5.5 stops. So you can see that when working at EI 1600 the dynamic range now becomes + 7.5 stops and -5.5 stops. Go to EI 3200 and the dynamic range becomes +8.5 stops and -4.5 stops.
So EI S-log gives you a great way of shifting your dynamic range centre while giving you consistent looking exposure and a reasonable approximation of how your noise levels are changing as you shift your exposure up and down within the cameras dynamic range.
EI S-Log doesn’t go below 800 because shifting the dynamic range up the exposure range is less beneficial. Lets pretend you have an EI 400 setting. If you did use it, you would be opening up the iris by one stop, so your range becomes +5.5 and -7.5 stops compared to your mid grey or light metered exposure. So you are working with reduced headroom and you are pushing your mid range up into the more highly compressed part of the curve which is less desirable. I believe this is why the option is not given on the F3.

Setting Exposure with Standard Gammas – Use your judgement!

grey-boxes Setting Exposure with Standard Gammas - Use your judgement!First take a long look at the image to the left. look at the 3 small grey boxes. What do you see?
Is the middle of the three grey boxes brighter than the others? Does the bottom small grey box look about the same brightness as the top one?

This post comes as the result of a discussion going on elsewhere about how to correctly expose when using standard gammas. Basically discussing how to expose when your not going to do anything to your footage in post,  for what I would call “direct to air”.
There are many ways of setting exposure. You could use a light meter, you could use zebras,  you could use a waveform monitor or histogram.

Lets imagine that grey box is a face. If you were using zebras you would normally set them to between 65% and 70% and then expose the shot so the face exhibited the zebra pattern over any parts of the face not overly highlighted or in shadow. This is the textbook way to expose using zebras. Another way to expose might be to use a mid grey card (also known as an 18% grey card). With standard and cinegammas you would normally expose this at 50% using the cameras histogram, waveform monitor or spot meter. Again this is a textbook, technically correct exposure. But this is the real world and the real world is very different to the theoretical world because light plays tricks with our eyes and the overall brightness of a scene can change the mood of the shot.

Lets say you have a room with dark coloured walls.  At one end is a window and you have an actor standing at each end of the room, one against the dark wall, one against the window. We have two shots in our scene, one looking at the actor against the dark wall, one looking at the actor against the window. What happens if we expose both faces using zebras to exactly the same textbook 65% level? Well the face against the window will look darker than the face against the black wall. Look back at the grey boxes on the left. The top and middle grey boxes are exactly the same brightness but because the middle box is against black, to our eye’s it appears brighter than the top one. Now if we were to use a histogram or waveform monitor to expose these two shots, all the extra white in the window shot might tempt you to reduce the exposure, this would make the problem even worse. In fact to expose these two shots so that the faces match as you cut between them you need to reduce the exposure on the darker shot. Looking at the grey boxes again the lowest box is actually at 45% while the other two are at 65%, yet the lower box appears to be about as bright as the top box.
So what am I trying to say? Well exposure isn’t all about setting object “X” at exposure “Y”. You must use your judgement and a known monitor or viewfinder to asses your pictures. Learn to interpret what your monitor is telling you, learn to recognise scenes that may need to be exposed away from the text book values and methods. Above all else don’t be afraid to expose for what looks right, as opposed to object “X” at value “Y”.
I suppose to follow up on this I should tell you how to calibrate your viewfinder or monitor… I’ll do that soon in a later article. Did you find this useful? let me know, I’m planning on writing more about dealing with light and lighting.

Metabones EF to E-Mount smart adapter.

IMG_1237-300x224 Metabones EF to E-Mount smart adapter.
Metabone EF to E Mount adapter on Sony NEX-FS700

The new $400 Metabones EF to E-Mount smart adapter allows you to use Canon EF mount lenses on almost any camera with an E-Mount. So that means cameras like the FS100, VG10, VG20 and the NEX series stills cameras. I’ve been trying to get hold of one of these for some time, but they have always been out of stock due to popular demand. However I was lucky enough to track one down from a UK dealer a couple of weeks ago. It is a small compact device, there are no wires, cables or remote control boxes so it couldn’t be simpler to use. Simply attach it to the Sony E-Mount and then attach your Canon EF lenses to the adapter. The camera will then control the iris just as it would with a Sony lens. So in the case of a FS100 or FS700 the iris wheel will control the iris with an accurate display of the iris setting on the cameras LCD screen. You can also use the auto iris functions. For quick focus checks there is a small button on the barrel of the adapter that momentarily fully opens the iris so you have minimum depth of field, which makes it easier to see if you are in focus. The adapter doesn’t work with auto focus so no focus functions, but it does allow any image stabilisation built into the lens to work. It works with the vast majority of lenses although there are a few that don’t work or have some limitations, best to check the Metabones web site for details. I really like this adapter for it’s simplicity and transparent operation, you really don’t know it’s there. Just wish they could do one for the Sony F3.

Coming Soon: A review of the new Transvideo PMW-F3 base plate that completely replaces the underside of the F3 with a really nice bottom end and of course the Alphatron EVF-035W review.

Quick Reference Guide to Picture Profiles.

Here’s a quick reference guide to what the various settings in a Picture Profile do. Not all of the settings are available on every camera.

Detail Level: Changes apparent image sharpness by artificially boosting contrast around edges and coarse image details by adding a black or white edge. Does NOT actually increase resolution, only apparent sharpness. See here for more in depth information.

Detail Frequency: Alters thickness of the applied Detail sharpening edges. Positive value is thiner than negative value. High positive values can lead to flickery edges.

Detail Black/White Limit: Sets upper and lower limits for how bright or how dark the detail correction edges can be.

H/V Ratio: Alters the balance of detail applied in the horizontal and vertical axis. ALmost never needs adjusting.

V DTL Creation: Chooses the source signal for generation of the detail correction information. Normally set to Y (luma) but can be taken from G only which in some cases can lead to reduced noise visibility, but images with little green will be soft.

Knee Aperture Level (Detail): Not to be confused with Gamma Knee Aperture. Detail Knee Aperture controls addition of detail correction in knee highlights. You don’t want extra sharpening in the compressed knee highlights.

Crispening: Sets the threshold level for the application of Detail enhancement. Low value makes image look sharper, but may increase noise visibility, high value decreases appearance of noise at the expense of a softer image. See here for more in depth information on crispening.

Aperture: Adds high frequency boost to fine details making them look sharper. Most noticeable on textures and subtle details. High values will make noise more apparent, no effect on contrast. See here for more info on aperture.

Matrix: Allows selection of differing colour matrices that will give different colour ranges and saturation levels. Note that different gamma settings will also alter colourimetry. See my Video on the EX matrix here and read this for differences between white balance and the matrix.

Saturation or Matrix Level: Controls colour level of the image. As there is a limit to how much colour can be recorded, too high a saturation level can result in strong colours reaching that limit and getting clipped. This is then very difficult to fix in post. Be careful using high saturation levels, often better to do this in post production.

Hue: Changes the overall colour phase of the camera. Too much adjustment may result in  very strange colour response, use with care.

Color Correction: Allows selection of a single narrow colour range that can have it’s hue shifted. Allows you to pick a colour in your scene and change that colour to another.

R-G, R-B, G-R, G-B, B-R, B-G: Individual level and phase adjustments for the 6 primary colour vectors. In each case the level of the first character is adjusted while the phase of the second is adjusted. So increasing R-G increases the Red level and shifts the Green phase. This interaction makes adjusting these very tricky.

Multi Matrix: Similar to Color Correction (above) but allows for multiple adjustments of different color ranges. Easier to adjust than the R-G etc. BUT the narrow and specific color ranges can cause adjusted colors to suddenly “pop” as for example a person with a colored shirt moves from shade to light. Use with care.

Preset White: Sets the colour temperature for the preset white balance setting. Normally 3200 for tungsten (indoors), 5600 for daylight (outdoors). 4400 for Fluorescent lighting.

Offset White: Allows an offset to be applied to the A, B or ATW white balance adjustment. Can be used to warm or cool the image. Positive value warms (redder), negative cools (bluer).

Gamma: Alters the gain of the camera at different brightness levels to match the gamma of the viewing device. The standard gamma for HD TV is REC-709. Std 3 on an EX is REC-709. STD 5 on a PMW-200 or F5.F55. Non standard gammas result in less accurate image reproduction, but sometimes this give a visually more pleasing image. As gamma affects gain image may be brighter/darker and noisier or less noisy depending on the chosen curve. See here for more information on gamma and gamma curves.

Knee: Compresses image highlights to improve dynamic range beyond the 6 stops of a conventional gamma.

Knee Point: Sets the luma (brightness) level at which Knee compression starts, typically 80-90%. Lower value allows for more compression and thus greater DR, but then you must watch exposure of faces etc to stay out of knee. Too low a knee without appropriate slope adjustment may prevent recordings reaching 100/109%

Knee Slope: Sets the amount of knee compression. Note that too much compression may prevent exposure ever reaching 100/109%

Knee Saturation (Knee Aperture): Sets the saturation or colour level in the knee area. As the knee luma level is compressed the saturation level must also be reduced to match otherwise you get over saturated colours in the scene highlights.

CineGamma or HyperGamma: Special non standard gamma curves developed to give improved dynamic range and improved high light handling. May look a little flat compared to standard gammas. Highlight compression starts around 70% and progressively gets stronger so avoid over exposing skin tones. Best to shoot a little under and correct in post.

Black Gamma/Black Stretch: Adjusts the gain of the lower (darker) parts of the gamma curve. Makes the picture look more/less contrasty. Negative value makes image darker, but note that too much negative black gamma can lead to crushed or clipped blacks. For more info on scene file black gamma click here.

Black Level: Sets the pedestal or zero level for black. High negative values will cause dark areas of the scene to be clipped. Positive values will result in reduced contrast and grey looking image. Rarely needs adjustment beyond +/- 4.

Low Key Sat: Changes the saturation in low key or darker parts of the image. Useful for reducing colour in dark scenes. Can help keep noise levels under control as chroma (colour) noise can be objectionable in dark scenes.

Skin Detail Correction: Allows user to select a specific narrow colour range and then reduce the amount of detail correction or sharpening applied to anything that colour. Idea is to allow selective softening of faces.

Alphatron EVF035 Electronic Viewfinder Review Coming!

IMG_1243-300x224 Alphatron EVF035 Electronic Viewfinder Review Coming!
Alphatron EVF-035W on PMW-F3

DHL delivered a nice box on Friday,just in time for the long Jubilee weekend. In that box was one of the long awaited Alphatron EVF-035W viewfinders. I had a play with a pre production unit at NAB and I was really very impressed. The EVF uses the same screen as the iPhone 4s and as a result it is higher resolution than the current Zacuto and Cineroid EVF’s. It has a full range of markers, focus aids and exposure aids. It’s is also very nicely built. I’m preparing a full review which should go online later in the week. In the mean time here are a few pictures of it on my F3 configured for ENG with a Canon 2/3″ B4 zoom lens via the MTF B4 adapter.

IMG_1247-300x224 Alphatron EVF035 Electronic Viewfinder Review Coming!
Alphatron EVF-035W EVF with flip up monocular.

I’m using a GenusTech F3 top cheese plate with a home made viewfinder bracket to mount the EVF. With this mount I can slide the EVF forwards and back as well as side to side, yet unlike a noga arm it won’t sag or droop and the viewfinders horizon matches the cameras, so no wonky shots.

IMG_1246-300x224 Alphatron EVF035 Electronic Viewfinder Review Coming!
F3 ready for shoulder mounted ENG.

Uncompressed 240 fps possible with FS700 and Convergent Design Gemini. Tested!

IMG_1232-300x224 Uncompressed 240 fps possible with FS700 and Convergent Design Gemini. Tested!
Using Convergent Design Gemini to capture the 50p buffer stream from a Sony FS700

Great news. I was able to confirm yesterday that it is possible to record the super slow mo buffer stream from the Sony FS700 to an external recorder. When you shoot Super Slow Mo with the FS700 the camera buffers the slow motion footage into an internal frame buffer before reading this frame buffer out and encoding it to AVCHD to write to the internal compact flash card. The footage is read from the frame buffer at 50/60p depending on which country area the camera is set to.

IMG_1233-300x224 Uncompressed 240 fps possible with FS700 and Convergent Design Gemini. Tested!
Setting up the output.

The cameras output can be set in the menu to either 25/30p or 50/60p, if set to 50/60p the HDSDi output is 3G. In either case the buffering happens at 50/60p. So FOR THIS TO WORK CORRECTLY THE RECORDER MUST BE 3G CAPABLE AND BE ABLE TO RECORD 50/60P, if the camera output is set to 25/30p you only get one field from each frame in the buffer stream on the output Sdi at 1.5G. So you end up with a slow motion interlace video stream with recorders that can only record up to 25/30fps. The end result of this is that if you shoot at 200 fps with the camera output at 25p you would end up with an external recording the equivalent of 100 fps with a 180 degree shutter. Every external HD recorder that I know of can record this as it is nothing more than 1080p25 (or 1080p30). A very useful and a nice option. To get the full benefit of external high speed recording you need to set the FS700 to output 50/60p over 3G SDi and then you must use an external recorder that has 3G SDi and can record at 1080 50/60p, something like the Convergent Design Gemini. Using a 50/60p external recorder you can then record every frame from the frame buffer. If you then take this 50/60p recording and slow it down by 50% in the edit suite you end up with full HD resolution, uncompressed 200/240 frames per second video. Very impressive indeed!

BlackMagic Design Hyperdeck Shuttle 2 3G Confusion….. created by BlackMagic!

I kept coming across posts and messages from people trying to get their Blackmagic Shuttle 2’s to work with the 3G ports on the F3. I decided to investigate this for myself as I am developing an exciting accessory for the Shuttle. I had been led to believe the Shuttle 2 had 3G HDSDi because of this, from the BMD press release on their web site:

BMD-Shuttle-Press BlackMagic Design Hyperdeck Shuttle 2 3G Confusion..... created by BlackMagic!

But when you check out the tech specs, also on the BMD web site you will find this:

BMD-shuttle-specs BlackMagic Design Hyperdeck Shuttle 2 3G Confusion..... created by BlackMagic!

When I tested a shuttle 2 with the 3G Sdi on my F3 it didn’t recognise it at all. So my guess is that the press release is incorrect and misleading and the tech specs are correct. So be warned, don’t believe everything you read in a BMD press release. I feel sorry for anyone that purchased a Shuttle 2 expecting it to have 3G HDSDi and the usual signal and recording capabilities that it normally implies.