What is “Exposure”

March 24, 2015 alisterchapman Leave a comment

What do we really mean when we talk about exposure?

If you come from a film background you will know that exposure is the measure of how much light is allowed to fall on the film. This is controlled by two things, the shutter speed and the aperture of the lens. How you set these is determined by how sensitive the film stock is to light.

But what about in the video world? Well exposure means exactly the same thing, it’s how much light we allow our video sensor to capture. Controlled by shutter speed and aperture. The amount of light we need to allow to fall on the sensor is dependant on the sensitivity of the sensor, much like film. But with video there is another variable and that is the gamma curve…. or is it????

This is an area where a lot of video camera operators have trouble, especially when you start dealing with more exotic gamma curves such as log. The reason for the problem is down to the fact that most video camera operators are taught or have learnt to expose their footage at specific video levels. For example if you’re shooting for TV it’s quite normal to shoot so that white is around 90%, skin tones are around 70% and middle grey is in the middle, somewhere around the 45% mark. And that’s been the way it’s been done for decades. It’s certainly how I was taught to expose a video camera.

If you have a video camera with different gamma curves try a simple test. Set the camera to its standard TV gamma (rec-709 or similar). Expose the shot so that it looks right, then change the gamma curve without changing the aperture or shutter speed. What happens? Well the pictures will get brighter or darker, there will be brightness differences between the different gamma curves. This isn’t an exposure change, after all you haven’t changed the amount of light falling on the sensor, this is a change in the gamma curve and the values at which it records different brightnesses.

An example of this would be setting a camera to Rec-709 and exposing white at 90% then switching to S-log3 (keeping the same ISO for both) and white would drop down to 61%. The exposure hasn’t changed, just the recording levels.

It’s really important to understand that different gammas are supposed to have different recording levels. Rec-709 has a 6 stop dynamic range (without adding a knee). So between 0% and around 100% we fit 6 stops with white falling at 85-90%. So if we want to record 14 stops where do we fit in the extra 8 stops that S-Log3 offers when we are already using 0 to 100% for 6 stops with 709?? The answer is we shift the range. By putting the 6 stops that 709 can record between around 15% and 68% with white falling at 61% we make room above and below the original 709 range to fit in another 8 stops.

So a difference in image brightness when changing gamma curves does not represent a change in exposure, it represents a change in recording range. The only way to really change the exposure is to change the aperture and shutter speed. It’s really, really important to understand this.

Furthermore your exposure will only ever look visibly correct when the gamma curve of the display device is the same as the capture gamma curve. So if shooting log and viewing on a normal TV or viewfinder that typically has 709 gamma the picture will not look right. So not only are the levels different to those we have become used to with traditional video but the picture looks wrong too.

As more and more exotic (or at least non-standard) gamma curves become common place it’s very important that we learn to think about what exposure really is. It isn’t how bright the image is (although this is related to exposure) it is about letting the appropriate amount of light fall on the sensor. How do we determine the correct amount of light? Well we need to measure it using a waveform scope, zebras etc, BUT you must also know the correct reference levels for the gamma you are using for a white or middle grey target.

You might also like to read this article on understanding log and exposure levels.

Workflow

Notes on Timecode sync with two cameras.

March 19, 2015 alisterchapman Leave a comment

When you want two cameras to have matching timecode you need to synchronise not just the time code but also the frame rates of both cameras. Remember timecode is a counter that counts the frames the camera is recording. If one camera is recording more frames than the other, then even with a timecode cable between both cameras the timecode will drift during long takes. So for perfect timecode sync you must also ensure the frame rates of both cameras is also identical by using Genlock to synchronise the frame rates.

Genlock is only going to make a difference if it is always kept connected. As soon as you disconnect the Genlock the cameras will start to drift. If using genlock first connect the Ref output to the Genlock in. Then while this is still connected connect the TC out to TC in. Both cameras should be set to Free-run timecode with the TC on the master camera set to the time of day or whatever time you wish both cameras to have. If you are not going to keep the genlock cable connected for the duration of the shoot, then don’t bother with it at all, as it will make no difference just connecting it for a few minutes while you sync the TC.

In the case of a Sony camera when the TC out is connected to the TC in of the slave camera, the slave camera will normally display EXT-LK when the timecode signals are locked.

Genlock: Synchronises the precise timing of the frame rates of the cameras. So taking a reference out from one camera and feeding it to the Genlock input of another will cause both cameras to run precisely in sync while the two cameras are still connected together. While connected by genlock the frame counts of both camera (and the timecode counts) will remain in sync. As soon as you remove the genlock sync cable the cameras will start to drift apart. The amount of sync (and timecode) drift will depend on many factors, but with a Sony camera will most likely be in the order of a at least a few seconds a day, sometimes as much as a few minutes.

Timecode: Connecting the TC out of one camera to the TC in of another will cause the time code in the receiving camera to sync to the nearest possible frame number of the sending camera when the receiving camera is set to free run while the camera is in standby. When the TC is disconnected both cameras timecode will continue to count according to the frame rate that the camera is running at. If the cameras are genlocked, then as the frame sync and frame count is the same then so too will be the timecode counts. If the cameras are not genlocked then the timecode counts will drift by the same amount as the sync drift.

Timecode sync only and long takes can be problematic. If the timecodes of two cameras are jam sync’d but there is no genlock then on long takes timecode drift may be apparent. When you press the record button the timecodes of both cameras will normally be in sync, forced into sync by the timecode signal. But when the cameras are rolling the timecode will count the actual frames recorded and ignor the timecode input. So if the cameras are not synchronised via genlock then they may not be in true sync so one camera may be running fractionally faster than the other and as a result in long clips there may be timecode differences as one camera records more frames than the other in the same time period.

PXW-FS7, Review

Tested: Lexar XQD cards in the PXW-FS7 and PMW-F5

March 5, 2015 alisterchapman 36 Comments

IMPORTANT UPDATE: There are two different speeds of S series cards. You should only use the faster E stream cards. You can tell which is which by the part number. QDS64E and QDS32E are second generation fast S series. Any other S series is a slower first generation S card and should be treated as H Class cards.

DO NOT USE THE NEWER 2933x CARDS THESE DO NOT WORK WITH THE FS7.

It was brought to my attention at the BVE show last week that Sony XQD cards were in short supply. This is probably due to the run away success of the PXW-FS7. More cameras sold means more media required.

So I decided to test out a Lexar XQD card in my PXW-FS7 and in my PMW-F5 (via a QDA-EX1 SxS to XQD adapter).

The good news is that it appears to work just fine, which shouldn’t really be a surprise as the Lexar cards are bonafide XQD cards. It’s also worth noting at this point that you don’t have to use the latest and greatest, extremely fast “G” series XQD cards from Sony. You can also use the slower H, N and S series cards. Although I personally would stick to just the faster G and S series cards as these can be used for all modes and frame sizes.

G Series – 400MB/S OK All Frame Rates/Modes.

S Series (QDS64E and QDS32E only) – 180MB/S OK All Frame Rates/Modes.

N Series – 125MB/S OK for XAVC-L all frame sizes/rate. XAVC-I HD up to 30fps plus 24fps 3840×2160. OK for Mpeg2, No S&Q, No ProRes or DNxHD.

H Series – 125MB/S OK for XAVC-L HD up to 60fps. No XAVC-I, No 4k/UHD, No S&Q. No ProRes/DNxHD

Lexar 1100x -168MB/S – Not tested, but should be OK, as Sony H series, maybe N series.

Lexar 1333x – 200MB/S – Tested all modes and frame rates.

Lexar have two classes of card a slower 1100x – 168MB/s card and a faster 1333x – 200MB/s card. For my tests I chose the faster 1333x card as this wasn’t much more expensive than the slower card and on paper at least matches or betters the Sony S series cards which can be used for all modes and frame rates. The 1100x card should also work just at least as well as an H series card, maybe N series, but I have not tested one and would recommend testing before use.

I tested the card across a large range of frame rates and resolutions going all the way up to UHD 60fps on the FS7 and SStP on the F5 as well as S&Q all the way to 180fps. I had no errors or other major problems. I did notice in the F5 that the it takes a little longer for the red light above the record slot to return to green at the end of a recording. While the slot light is red you cannot start a new recording so you do need to be aware that you may have a momentary delay before you can record the next clip.

I purchased the Lexar 1333x card via Amazon in the UK and it cost me £127 inc VAT for a 64GB card, which is quite a bit cheaper than a Sony G series card (currently around £220 in VAT). So the Lexar cards offer a perfectly good alternative to the Sony cards at a lower cost with only a slight decrease in off-load speed. As well as the PMW-F5/F55 and PXW-FS7 I see no reason why these cards should not also work in the PMW-Z100, FDR-AX1 or via the QDA-EX1 adapter in the PMW-200, PXW-X160, X180 and X200, but I have not tested this.

I can’t comment on long term reliability as I’ve only had the card a couple of days. I see no reason why the Lexar cards should last as long as the Sony cards. Heck looking at a Sony G series card and the Lexar card side by side the materials appear to be identical, it looks to be exactly the same plastic (even the texturing is the same) and the same brushed metal. The Sony card is marked as made in Japan and the Lexar card as a product of Taiwan.

CARD READERS.

The new Sony G series cards have a different interface to the older Sony XQD cards and the Lexar cards. Currently when you buy a G series card it comes with a USB3 card reader. This reader will ONLY read G series XQD cards.

You can buy USB3 card readers for the other XQD cards for around £25.00. These readers will normally work with any XQD card, including the G series. But when you use a G series card in the non G series readers you get a reduced read speed of up to 168MB/s, which is fast, but not as fast as the dedicated G Series reader.

cameras, PXW-FS7, Scene Files and Luts

Scene Files (Picture Profiles) for the PXW-FS7

February 19, 2015 alisterchapman 30 Comments

One of the great features of the PXW-FS7 is the ability to be able to change the look of the images when shooting in Custom Mode. You can change many settings including the gamma curve, matrix and sharpness setting. The gamma settings change the contrast, the matrix the color and the detail and aperture settings change how sharp the pictures look.

Once you’ve made some changes you can save these settings as a Scene File using the File menu on an SD card.

I am a big fan of Sony’s Hypergammas. There are 6 in the FS7. Hypergamma 3 is very good for getting a nicer highlight roll off when shooting in lower light situations. Hypergamma 4 is good for brighter scenes and Hypergammas 7 and 8 really extends the cameras dynamic range and handles high contrast scenes very well, but can look a little flat so will need some tweaking in post production. In fact all the hypergammas need a bit of a tweak in post as to get the very best from them you should expose your shots about 1 stop darker to keep skin tones etc out of the upper compressed part of the curve and then bring the brightness back up again in post.

Anyway here are some scene files for you to download and install in the camera.

AC-NEUTRAL-HG3 This is for flatter scenes, it provides a natural look with some yellow/green removed to provide a more neutral look.

AC-NEUTRAL-HG4 This is for brighter or high contrast scenes, it provides a natural look with some yellow/green removed to provide a more neutral look.

AC-FILMLIKE1 A high dynamic range film like look.

AC-FILMLIKE2 A high dynamic range film like look with an increased blue and red response with decreased yellow/green. A little more block-buster like.

AC-VIBRANT-HG3 A vivid matrix with good dynamic range. Good for punchy direct to air images where strong colours are wanted.

AC FS7 Scene Files, set of 5.
If you find these scene files useful, please consider buying me a beer or a coffee. All donations are really appreciated and allow me to spend more time on the blog creating new guides and scene files etc.

PXW-FS7

PXW-FS7. Turn OFF white clip if using Hypergammas!

February 19, 2015 alisterchapman 2 Comments

I’m doing some work on some scene files for the FS7 and one little thing I found is that the default white clip of the camera is set to 105% and if you use HG3, HG4, HG7 or HG8 this means that the camera clips before you reach the near flat top of the hypergamma curves. This results in hard clipping of highlights rather than a more gentle roll-off.

So if using Hypergammas it’s also a good idea to turn off the white clip for the best results.

PMW-F55, PXW-FS5, PXW-FS7, Scene Files and Luts, Uncategorized

Over and Under Exposure LUT’s for the PXW-FS7, PMW-F5 and PMW-F55

February 5, 2015 alisterchapman 14 Comments

Here are two sets of LUT’s for use in post production with the PXW-FS7, PMW-F5 and PMW-F55.

These LUT’s are based around the Sony 709(800) LUT and the Sony LC-709TypeA LUT (Arri Alexa look). But in addition to the base LUT designed for when you shoot at the native ISO there are LUTs for when you shoot at a lower or higher EI.

When you shoot at a high or low EI the resulting footage will be either under or over exposed when you add the standard LUT. These LUT’s include compensation for the under or overexposure giving the best possible translation from SGamut3.cine/S-log3 to rec-709 or the Alexa look and result in pleasing skin tones and a nice mid range with minimal additional grading effort.

If you find these LUT’s useful please consider buying me a coffee or beer.

PXW-FS7, Scene Files and Luts

How to create a User3D LUT for the PXW-FS7 and load it in to the camera

February 5, 2015 alisterchapman 38 Comments

It’s very easy to create your own User3D LUT for the Sony PXW-FS7 using DaVinci Resolve or just about any grading software with LUT export capability. The LUT should be a 17x17x17 or 33x33x33 .cube LUT (this is what Resolve creates by default).

Simply shoot a correctly exposed Slog3 clip at the native ISO. You must use the same Gamut as you intend to use in any other productions that will use this LUT, I recommend SGamut3.cine.

Import the clip in to Resolve and grade it as you wish the final image to look. A very good way to do this is to include a MacBeth chart or DSC Labs OneShot or CDM chart within the test shot. Then use Resolves excellent chart matching tool to get create a starting point for the grade. The big benefit of this approach is that Resolve will provide a very good gamma correction moving your footage from Slog3 to Rec-709. Once you have used the chart to move you into the correct gamma range just tweak and fine tune the image to get your desired look. Then once your happy with your look, right click on the clip in the timeline and “Export LUT”. Resolve will then create a .cube LUT.

Then place the .cube LUT file created by the grading software on an SD card in the PMWF55_F5 folder. You may need to create the following folder structure on the SD card, so first you have a PRIVATE folder, in that there is a SONY folder and so on.

PRIVATE/SONY/PRO/CAMERA/PMWF55_F5

Put the SD card in the camera, then go to the File menu and go to “Monitor 3D LUT” and select “Load SD Card”. The camera will offer you a 1 to 4 destination memory selection, choose 1,2,3 or 4, this is the location where the LUT will be saved. You should then be presented with a list of all the LUT’s on the SD card. Select your chosen LUT to save it from the SD card to the camera.

Once loaded in to the camera when you choose 3D User LUT’s you can select between user LUT memory 1,2,3 or 4. Your LUT will be in the memory you selected when you copied the LUT from the SD card to the camera.

NOTE: If you use a Mac to copy the LUT to the SD card the Mac will place some junk files on the SD card with almost exactly the same name as the LUT. The camera will see this junk file and display it in the list of LUT files. The junk file will have a “._” before the LUT name. Don’t try to load these junk files by mistake, they won’t work. Make sure you scroll down the LUT list to the real LUT file when you try to load the LUT’s in to the camera.

cinematography, PMW-F55, PXW-FS7, Uncategorized

Using the High/Low key function on the PXW-FS7, PMW-F55 and PMW-F5.

February 5, 2015 alisterchapman 6 Comments

A very useful feature not well documented on the FS7, F5 and F55 cameras (and the F65 too) is the High/Low Key feature.

The High/Low Key function works by changing the brightness range of the image displayed in the viewfinder, this is very useful when shooting in the Cine-EI Mode and using a LUT to help judge your exposure.

My preferred LUT for exposure assessment is the 709(800) LUT. As this LUT is compatible with the gamma curve used in most TV’s and monitors it provides a nice contrasty image with what I would call “normal” brightness levels (middle grey at 42%, white at 90%, skin tones around 60-70%). So if you expose via the 709(800) LUT so that the pictures look right on the screen or in the viewfinder then your S-Log recordings will also be correctly exposed.

But the 709(800) LUT, like most LUT’s cannot show the full 14 stop capture range of the the S-Log recordings. So sometimes you might see an image via the LUT that looks correctly exposed but the highlights might look clipped or blown out as they are beyond the range of what the LUT can show as in the image below where the sky looks blown out. This is where the High/Low Key function comes in to play.

Normal range 709(800) as seen in the viewfinder. Is the sky over exposed or not in the Slog recording? It's hard to tell. — Normal range 709(800) as seen in the viewfinder. Is the sky over exposed or not in the Slog recording? It’s hard to tell.

To access the function you have to assign High/Low Key to one of the cameras assignable buttons. Once assigned to a button on the first press of the button the viewfinder or monitor image will show the High Key parts of the shot. To do this the VF or monitor picture is made darker so that you can “see” into the full highlight capture range. “High Key” will be displayed in the top left hand corner of the viewfinder. As you can see in the image below we can now see that the sky is not blown out, so we know the S-log recording will be OK.

The High Key function darkens the LUT range and we can see that the sky is not over exposed so the Slog recording will be not be clipped or over exposed. — The High Key function darkens the LUT image so we can view the High Key range of the Slog recordings . We can see that the sky is not over exposed so the Slog recording will be not be clipped or over exposed.

The second press of the button shows the Low Key (darker) parts of the scene. This is done by making the image much brighter so you can “see” into the shadows better and the entire under exposure range of what is being recorded is shown. “Low Key” is displayed in the top left of the viewfinder screen.

The Low Key function brightens the LUT image so we can see whats going on in the deepest shadows and dark areas of the Slog capture range. — The Low Key function brightens the LUT image so we can see whats going on in the deepest shadows and dark areas (Low Key areas) of the Slog capture range.

The third press of the button returns the image to the normal range that the LUT can show.

So by using the High / Low Key function you can see the entire range that the camera is capturing, check for over exposure or under exposure issues without having to turn the LUT on or off. This is a really useful function that I recommend you take advantage of when shooting with CineEI and LUT’s. However do remember to make sure you are back to the standard view range when setting your exposure level.

Uncategorized

Time to catch up!

February 3, 2015 alisterchapman 7 Comments

No, I haven’t given up on XDCAM-USER. Just been rather busy with work.

Shooting with the new Sony PXW-X200 in Iceland.

My new year started off with a trip to Iceland for a workshop and to try out the new Sony PXW-X200 camcorder. The weather was really abysmal. I had fully expected it to be cold and snowy, but I didn’t count on the extreme storms and constant gale force winds that we had. The temperature wasn’t too bad, only around -10c but the constant 50km/h plus winds and driving snow made shooting difficult to say the least. I had a PXW-FS7, A7s and a PXW-X200 in Iceland and they all worked flawlessly in the harsh conditions.

While filming the geysers with the FS7 an unexpected change of wind direction blew the water from the erupting geyser all over me and the un-protected FS7. I was drenched and so was the camera, water was literally running off the camera. Thankfully the FS7 has some very good weather sealing and no harm was done to the camera. I spent the rest of the day freezing cold thanks to some very wet clothes.

For the rest of the trip I made sure that the PXW-FS7 was safely wrapped up inside it’s Camrade Wetsuit. The Camrade wetsuit is made from a very nice rustle free fabric that is completely waterproof. There is a large clear panel on the left side so you can see all the buttons and switches. There is a separate cover for the viewfinder that allows it to be used with or without the monocular extension tube. It has plenty of space for different lenses, mounting options and the extension unit.

Shooting in the cold and wind of Iceland with the PXW-X200

Most of my time in Iceland though was used to get a good feel for the new PXW-X200 camcorder. This is a replacement for the well regarded PMW-200 and adds some very nice new features such as a 17x zoom lens, new sensors, multiple codec choices including XDCAM Mepg2 of course, but more excitingly XAVC-I and XAVC-L bringing 10 bit high quality recording to a very nice camera front end. If that’s not enough you an even record any of the codecs that have a bit rate lower than 50Mbps on SDXC cards, including Mpeg2 HD 422 (the original XDCAM codec) and HD XAVC-I. To top it all the camera has a full range of wireless options including ftp file transfer and in the future streaming over WiFi or via a 3G or 4G USB dongle.

I’m currently putting together a video on the PXW-X200 and it’s a real delight grading the 10 bit XAVC footage, it holds together much better than the older 8 bit XDCAM footage from the PMW-200.

The second half of January saw me in Norway for my annual Northern Lights expeditions. Once again we encountered some pretty extreme temperatures. The last day of the first group saw -39c and the first day of the second group saw -38c. Most of the time the temperature was below -20c. We saw the Northern Lights most nights but they weren’t very bright. However towards the end of the second groups trip we had some nice Aurora displays and I was able to video these in real time with the A7s. I have to say the A7s never ceases to amaze me. It really is incredibly sensitive and it’s pocket size means you can take it just about anywhere.

For my Norway trip I was also loaned an Atomos Shogun to use with the A7s to enable 4K recording. The Shogun is a nice piece of kit and as it uses off the shelf SSD’s the recording media is pretty low cost. The screen is very good giving very clear and accurate pictures. I did have some problems with it in the cold, when the temperature dropped below -15c it would stop working and present a message saying that the firmware wasn’t correctly installed. As soon as it warmed up above -15c it would start working correctly again. To be fair -15c is well below the design specifications and not many people use kit in that kind of weather so this really shouldn’t be a big deal for most people.

The nice clear screen of the Atomos Shogun.

Overall I don’t think the Shogun is as robustly constructed as the Convergent Design 7Q, the Shogun feels very plasticky, but it is a lower cost package than the 7Q and the recorded images do look great. When I get time I’ll be putting together some of the A7s and Shogun footage from the Norway trip and writing more about my experiences with the Shogun.

Another toy I have been playing with recently is the NextoDI NSB25 storage bridge. This is a stand-alone backup and storage device that can transfer data from almost every type of solid state recording media to a pair of internally housed (but easily changed) hard drives as well as a 3rd hard drive connected via USB3. You can make individual copies of the media or duplicated copies of the media across all the drives for added security. The NSB-25 can play back the majority of video formats on it’s built in screen or you can plug in a TV or monitor via HDMI for large screen play back. The unit is about the size of a house brick and has it’s own internal battery. If you need a robust backup solution for larger projects or for bigger media files such as 4K or even raw files you should take a look at the NSB-25.

That’s it for now. I’m off to Norway again next week, but there will be lots of new stuff coming to the web site in the coming weeks so keep checking back.

Uncategorized

Apple Pro Video Formats 2.0 – MXF in Quick Time, XAVC in FCP7???

December 20, 2014 alisterchapman 8 Comments

So, Apple have released an update to the Pro Video Formats available in quicktime. http://support.apple.com/kb/DL1396?viewlocale=en_US&locale=en_US

The main thrust of this update appears to be to include MXF support in quicktime, including native support for XAVC in FCP7 and Quicktime7! I honestly never thought this would happen so I’m somewhat surprised by this. But it’s good news for FCP7 users and XAVC shooters in general. It does beg the question now as to whether you need the ProRes options for the FS7 or F5/F55.

Using Quicktime Player 7 you can play back XAVC MXF’s on a Mac computer, even in 4K, so VLC may no longer be required and the playback is smooth even with 4K clips. I’m not sure why but Quicktime Player 10 does not recognise 4K XAVC clips at all and HD XAVC clips get transcoded to .mov first. So download and install QT Player 7 for XAVC playback.

Currently it looks like the support is mainly for XAVC-I.