This is one of those topics that keeps coming back around time and time again. The link between contrast and resolution. So I thought I would take a few minutes to create some simple illustrations to demonstrate the point.
Best Contrast.
This first image represents a nice high contrast picture. The white background and dark lines have high contrast and as a result you can “see” resolution a long way to the right of the image as indicated by the arrow.
Lower contrast.
Now look at what happens as you slowly reduce the contrast in the image. As the contrast reduces the amount of resolution that you can see reduces. Keep reducing the contrast and the resolution continues to decrease.
Low Contrast.
Eventually if you keep reducing the contrast enough you end up with no resolution as you can no longer differentiate between light and dark.
Now look at what happens when you reduce the resolution by blurring the image, the equivalent of using a less “sharp” lower resolution lens for example. What happens to the black lines? Well the become less dark and start to look grey, the contrast is reducing.
Reduced resolution.
Hopefully these simple images show that contrast and resolution are intrinsically linked. You can’t have one without the other. So when choosing lenses in particular you need to look at not just resolution but also contrast. Contrast in a lens is affected by many things including flare where brighter parts of the scene bleed into darker parts. Flare also comes from light sources that may not be in your shot but the light is still entering the lens, bouncing around inside and reducing contrast as a result. These things often don’t show up if you use just a simple resolution chart. A good lens hood or matte box with flags can be a big help reduce stray light and flare, so in fact a matte box could actually make your pictures sharper. They are not just for pimping up your rig, they really can improve the quality of your images.
The measurement for resolution and contrast is called the MTF or modulation transfer function. This is normally used to measure lens performance and the ability of a lens to pass the light from a scene or test chart to the film or sensor. It takes into account both resolution and contrast so tells you a lot about the lens or imaging systems performance and is normally presented as a graph of contrast levels over a scale of ever increasing resolution.
IMPORTANT PLEASE ENSURE YOU USE THE REVISED SETTINGS UPDATED ON 24th JULY.
After my recent side by side look at the F3 and FS700 and seeing how different the two cameras look, I decided to try to match them a bit better. There will be many shoots where I will use them both together so getting them to look the same is important. I thought this would be a relatively straight forward task, simply dial in the FS700 to match the F3.
Well it wasn’t simple and it ended up taking me several hours to get to the point where I couldn’t get them any closer. The main issues are that the F3, like most of the XDCAM cameras has a yellow colour cast that’s hard to completely remove and the FS700 has quite a blue image and only very limited matrix controls. Initially I started to try to match the FS700 to a standard F3. While I could get the FS700 closer to the F3, I just couldn’t get a near match let alone a complete match. So back to the drawing board.
For my second attempt I decided first to work on getting rid of the yellow/orange cast to the F3 pictures by adjusting the F3’s matrix, at the same time creating a neutral look picture profile with good dynamic range, but one that could be used without grading. This took some extensive matrix tweaks. You will find the full details of my new “STD-REAL” picture profile in the forum by clicking here.
So once I had a neutral starting point on the F3 I then turned to the FS700 which I think is very blue. The matrix settings on the FS700 are quite limited so I wasn’t able to get an exact match to the F3, however the setting I came up with get them close enough for most jobs, it’s not perfect but it will do. I’m quite happy with my new FS700 settings and I think with this profile it produces a very nice image. You can find the full profile settings in the forum by clicking here. Remember you need to use the matching F3 profile in the F3 for the best match. If you want the maximum dynamic range then instead of Cinegamma 1 you should use Cinegamma 4 with the black gamma set to zero. My STD REAL profile for the FS700 is closer to a standard F3 than the default FS700 settings.
More clues as to what’s being announced next Thursday from Sony. Apparently the next big thing is coming! I promise to have news of this here on the 26th.
There is something a little curious about the specs for the sensor in the FS700:
Imager
Exmor Super35 CMOS sensor
Number of pixels
Total pixels approx. 11.6M
Effective pixels in movie shooting (16:9) approx. 8.3M
Effective pixels in still picture shooting (16:9) approx. 8.4M (3:2) approx. 7.1M
Why create a sensor with 11.6 million pixels and then use only 8.3M? It is normal to have some extra pixels that are used for setting black levels etc, but this is a massive difference between the number of actual pixels on the sensor and the number that are used to create the pictures. Where are all the un-used pixels? Given that this is a Super 35mm sensor, the active area used for video is APS-C ish sized, so it’s quite a big sensor already. What would you put a near full frame 35mm 11.6 MP sensor in these days? That’s a low pixel count for a modern large sensor DSLR or stills camera, even the compact NEX7 stills camera has 24MP. If (and this is just random speculation) the FS700 is taking an 8.3MP window out of the middle of the sensor, that makes it a pretty big chip. Another thought is that the FS700 does read the full height and width of the sensor but then uses some pixel skipping only actually reading 8.3MP, but why do that? In stills mode the camera only uses 8.4MP yet with so many extra pixels you could get higher resolution stills. So… why 11.6 MP and what else was this sensor designed for?
OK, not really very scientific, but I’m busy on some paying projects at the moment and the weather is very changeable so I only had a short window to do this. I’m still exploring the image quality of the FS700. It is very good, of that there is no doubt, but my benchmark right now is the F3. So I just did a very simple side by side test to look at noise and dynamic range. The scene has about an 11 to 12 stop range if you include the specular highlights and reflections off the silver car bonnet and the brightest clouds. If you take the S-Log clip as the reference the clouds are at about +5.5 stops over nominal middle grey and the darkest part of the image, the black stand holding the chart, is about 6 stops under. I wasn’t looking to actually measure anything here, just get a feel for the differences between the cameras.
The cameras were set to 800 ISO for all the clips, so the FS700 had +6db gain applied for all clips while the F3 had +6db applied for the Cinegamma and standard gamma clips and no additional gain in S-Log. Frame rate was 25fps with a 180 degree – 1/50th shutter. The HDSDi out from the cameras was recorded using ProRes 4:2:2 on an Atomos Samurai.
For the S-Log sample I exposed using the DSC Labs S-Log exposure reference chart (which you can see in all the frames) by placing the cameras centre spot meter over the middle grey and aiming for 38%, however my waveform monitors are telling me the mid grey exposure was actually 35% so I’m about a 1/4 stop under (and need to check why I didn’t get 38%). For the Cinegamma 4 tests I used the histogram to keep peak white at about 95% with similar mid range exposure. In fact the mid grey patch on the DSC S-Log chart is around 38-40% on both cameras which is just a touch low for the cinegammas (I normally aim for 42%-45% with cinegammas). Exposure for the standard 709 gamma was established with the histograms trying to get a reasonable balance between clipped highlights and a reasonable mid range. The result is that the standard gamma shots are under exposed by around 1.5 stops, mid grey is only 33% on the F3 and 30% on the FS700. I would normally aim to put middle grey around 45-48% for Sony’s REC-709 compliant gammas. If this shot included a person or face then I would have been forced to either over expose the sky still further or use some fill lighting or a reflector to bring up the foreground. This is typical of the dilemma you get when trying to expose a scene with a greater range than the camera can deal with, do you overexpose the sky to preserve the mid range or underexpose the mid range to keep the sky. Either way something has to suffer.
While not very scientific I think the clips highlight some interesting differences between the two cameras. The most striking difference is the colour. Both cameras were set to preset white at 5600k with their standard colour matrices. White does appear to be white, but the F3 when not in S-Log is clearly more saturated and has a touch more red and a lot less blue that the FS700, so clearly I’m going to have to do some work on the matrix to get these two cameras to match better. Next thing to note is that the Cinegamma curves are quite different. The FS700 curve has more gain in the mid range which results in brighter upper mid range compared to the F3. The dynamic ranges are very similar, I could have exposed the FS700 about a stop lower to gain a little more highlight room, but this would have resulted in some quite dark mid tones and a little loss of shadow detail. In both cases the cinegammas give a quite appreciable increase in dynamic range over REC-709. I would guess at about a 1.5 to 2 stop improvement in dynamic range gained from using Cinegamma 4 over the Sony REC-709 compliant curves + knee. The S-Log clip from the F3 shows the marked increase in dynamic range that you get when using log. The brightest clouds are about 5.5 stops over middle grey with the peak recording level reaching about 89% which means there are around a further 1.5 stops of unused headroom available.
The FS700 is a little noisier than the F3, no surprises here. It’s not hugely noisier and the noise levels at 400 and 800 ISO are perfectly reasonable for a broadcast production. At a push I would use 1600 (+12db) if I had to, but I think for me at least the comfort zone is 400 – 800 ISO (0db and +6db).
The FS700 images appear to lack a little of the crispness of the F3. This may just be because the standard F3 is a little over sharpened (in my opinion). The FS700 pictures look more like the S-Log F3 which has no added image sharpening and in fact in some respects the Cinegamma FS700 looks more like the S-Log F3 than the Cinegamma 4 F3. From what I’ve seen you probably could figure out a very flat log type picture profile for the FS700, but I’m not sure that this would bring any significant benefit over Cinegamma 4. The extra noise in the shadows that you would get if you bring up the low end with some black stretch (black gamma) would likely limit the usefulness of any slight extra latitude gained. However you look at it the FS700 does appear to be able to cope with around 12 stops (maybe a little more) just by using Cinegamma 4, which is about as much as you want with a 8 bit camera anyway.
Interestingly I spotted some moire and aliasing from both cameras. The dreaded roof tiles of the houses opposite (a tough test for many cameras) strike again and the F3 is showing a little coloured moire across the roof tops while the FS700 is showing some aliasing on the grill of the silver car. I’m not concerned by either. Yes it would be nice if it wasn’t there, but I’ve shot hundreds of hours with my F3 and it’s very rare to find any shots that are unusable or problematic due to aliasing.
If you want to download the actual footage please use the link below. The file contains a single clip made up of about 5 seconds each of the FS700 with Cinegamma 4 and standard settings, the F3 with cinegamma 4, standard settings and S-log. The clip is a direct copy of the original ProRes 422 files recorded from the HDSDi outputs of the cameras. 10 bit for the F3 and 8 bit for the FS700. The file size is 150 MB and 10 seconds long in total. Lets hope the file sharing service works as advertised!
If you find the clips useful please consider making a small donation using the button below. You can choose the amount you wish to donate. Even small donations of £1 are greatly appreciated and go towards the costs of writing articles and providing further sample clips.
Here are some jpeg frame grabs. Remember you can click on each image to see it larger and once you’ve clicked though to the larger image there should be a link just above it for the full size 1920×1080 original.
PMW-F3 S-LogPMW-F3 Cinegamma 4NEX FS700 Cinegamma 4PMW F3 Standard settingsNEX FS700 standard settings.
Here’s a few clips shot with the FS700 from last weekends Royal International Air Tattoo. Check out the DHL 767 just hanging in the sky, also notice the almost complete lack of skew on the propellors.
UPDATED WITH NEW FRAME GRABS FROM STROBE LIGHT AT BOTTOM.
One of the things that did concern me slightly about the FS700 was how would the sensor behave in Super slow Mo. The sensor is a CMOS sensor, so I expected it to exhibit rolling shutter artefacts, which it it does indeed do when in standard shooting modes and S&Q motion. It’s not bad, but you can make the pictures skew and when you try to shooting something like a spinning propellor you can get some weird effects, especially at higher shutter speeds. However when you switch the camera to Super Slow Mo the rolling shutter effects appear to go away. I was able to shoot propellors, do fast pans, shake the camera about etc and there was little sign of the usual rolling shutter artefacts.
FS700 25P 1/100
Just take a look at these two frame grabs. One shot done at 25P with a 1/100th shutter, the other done at 100fps with a 1/100th shutter, so in both cases the shutter speed is the same, so you would expect the rolling shutter artefacts to be the same, but clearly they are not. In standard mode the fan exhibits a typically lop sided, asymmetrical look and the fan blades appear curved, the upper and lower fan blade both bent towards the right of the frame. But in Super Slow Mo mode the fan blades are straighter and the fan is a lot more symmetrical with noticeably less bias towards the right, notice in particular the differences in the lower fan blade.
FS700 Super Slow Mo 1/100th shutter
You can tell the shutter periods are the same as the amount of motion blur and spreading of the fan blades is near identical, so it’s not a shutter speed difference, this is clearly a sensor scan difference. This is very interesting and requires further investigation as it suggests that the sensor read out process is different in the high speed mode. It is probably just a significantly faster scan rate, but it could also possibly be a global shutter of some kind. It’s just a shame that you can’t access this read out mode for normal shooting.
UPDATE:
FS700 Flash band at 25fps 1/100th shutter.
Here are a couple more frame grabs done with the strobe focussing flash from a Canon DSLR. In both cases the shutter speed is 1/100th of a second so you would expect the width of the “Flash Band” to be the same. The narrower the band, the slower the sensors scan speed. These frame grabs suggest the scan speed is around twice as fast when in Super Slow Mo. It’s not a global shutter, but certainly a nice improvement. This is 100% repeatable.
FS700 Flash band in Super SlowMo 100fps, 1/100th shutter
You can take advantage of this for normal speed shooting by setting the camera to SSM and recording the SDior HDMI feed to an external recorder.
Speculation: There is a little more aliasing when shooting in SSM. Is there some line slipping going on perhaps during SSM? This would allow a faster scan speed as fewer lines of pixels are read and thus might account for both the slight aliasing increase and the faster read out speed.
Many of you will have heard about my involvement in the recent production of a film about one of Duran Duran’s concerts last December. Well it’s out on Blu-Ray and DVD now and I think it looks might fine. Directed by Gavin Elder, Produced by James Tonkin it was a great pleasure to work with Den Lennie to help create a special picture profile for the F3’s used to shoot the concert. Here is a track from the Blu-Ray to give you a taste of the look of the video. Please click through and play it full screen in HD for the full effect.
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