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Sony NXCAM NEX-FS700 CineAlta
4K EXMOR sensor with SDI, slow-motion recording.


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Old February 12th, 2013, 08:53 PM   #1
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Disappointing image quality, especially for chroma keying

Today I did some green screen tests and I am surprised how badly the image coming from the FS700 is suited for chroma keying. The recorded chroma-data is just of pretty low quality. I guess even my iPhone provides better footage for chroma keying, it's just amazing especially for the price point of the FS700.
Here are two examples (100% crops):
http://dominik.ws/fs700_chroma_key_test1.png
http://dominik.ws/fs700_chroma_key_test2.png
They are in lossless PNG-format and no further compression was added, so you can try yourself to key them. I have used AE's KeyLight.
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Old February 12th, 2013, 09:28 PM   #2
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Re: Disappointing image quality, especially for chroma keying

It looks like you may have a couple of issues with your setup looking at the images. For one your shutter speed is too low. You want to minimize motion blur when keying. This is a time when you want to break the 2x frame rate rule. Use something in the range of 125 and see if your key improves.

In the image of the woman in particular it looks like the focus on the person is soft. You don't want to shoot a green screen with a wide aperture. Stop down to f/8 at least and light appropriately. Keep the gain below 6db.

When you make errors in setup and operation you create more challenges to pull a good key since the codec is 4:2:0. You will get better performance if you use an external 4:2:2 recorder.

It looks like you have some issues with chroma subsampling on high contrast edges too. A 4:2:2 recorder will certainly help there.
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Old February 12th, 2013, 11:19 PM   #3
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Re: Disappointing image quality, especially for chroma keying

The focus was spot on. The softness comes from the low resolution the FS700 delivers. Especially the chroma-channels are far more soft and artifacting than what is possible with 4:2:0-sampling.
In this picture I converted the pictures in Photoshop to Lab-color and extracted and auto-contrasted the a- and b-channels:
http://dominik.ws/fs700_chroma_channels.jpg
Look for instance at the huge artifacts at the right edge of her dress. This is not concerned with 4:2:0-sampling but rather with abysmal debayering. This is so bad that even after down-conversion to 720p the artifacts are clearly visible. So concerning the chroma-information you should call the FS700 rather a SD- than a HD-camera.
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Old February 13th, 2013, 12:35 AM   #4
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Re: Disappointing image quality, especially for chroma keying

That is odd. I'm able to get pristine keys out of the 700. I have an outboard 422 recorder but have found it isn'tt necessary unless the background is not properly lit. Looking at your images, it does appear soft, but that is either focus, a missed setting, or a very inferior lens. Also, the backdrop is very poorly lit and the color levels look washed out. I'll post an example of the camera's output using the internal codec when I get back to the studio.
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Old February 13th, 2013, 08:06 AM   #5
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Re: Disappointing image quality, especially for chroma keying

Quote:
Originally Posted by Will Salley View Post
That is odd. I'm able to get pristine keys out of the 700.
I would really like to see your results.
Quote:
Originally Posted by Will Salley View Post
Looking at your images, it does appear soft, but that is either focus, a missed setting, or a very inferior lens.
I've used here the Canon EF 24-70mm f/2.8L in conjunction with the Metabones Speed Booster. Yes, I set the detail-level to -7 to achieve a more natural look. Any higher setting just makes it look more like video in my opinion, but no additional detail is revealed.
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Also, the backdrop is very poorly lit and the color levels look washed out.
The colors look washed out because I used the cine2-gamma and set the exposure as high as possible to get the maximal signal/noise-ratio. But it doesn't matter if I lower the exposure, change the gamma or crank up the detail-value, the chroma-channels stay blurry and artifacty.
The cheaper PMW-EX1 from 2008 (which is in many ways much more professional and capable) for example gives me much better results at chroma-keying (also with internal 4:2:0 recording). I can only hope that there will be soon a 4k-recording possibility for the FS700 which makes the whole setup not more expensive than a Canon 1D C.
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Old February 13th, 2013, 08:31 AM   #6
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Re: Disappointing image quality, especially for chroma keying

If you have an EX1 that is what you should be using to shoot green screen for sure. That is a totally different animal.

An EX1 with a 4:2:2 recorder is an EXCELLENT machine for that task.
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Old February 13th, 2013, 10:08 AM   #7
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Re: Disappointing image quality, especially for chroma keying

I can't imagine why the sharpness is so poor = in fact, SD DV looks sharper than that? The other thing is that the green colour looks light green, and nowhere near how it ought to look for a successful key? Something is very wrong. Have you got a shot from the camera shooting in ordinary light? Is that sharp and reasonably saturated? The lack of sharpness and odd green colour are working against you too!
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Old February 13th, 2013, 10:15 AM   #8
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Re: Disappointing image quality, especially for chroma keying

http://www.dingodigital.net/Public/GS-keyex2.pct

http://www.dingodigital.net/Public/GS-keyedex.pct
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Old February 13th, 2013, 10:19 AM   #9
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Re: Disappointing image quality, especially for chroma keying

Was it shot in slow motion?
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Old February 13th, 2013, 10:27 AM   #10
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Re: Disappointing image quality, especially for chroma keying

I could certainly work with that Will.
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Old February 13th, 2013, 10:41 AM   #11
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Re: Disappointing image quality, especially for chroma keying

Chromakeying, especially with compressed video, is the one situation that calls for detail in the camera. The camera can add the edge, pre-compression, and if you find it too crisp inside the key, you can soften after the key.

It also helps to add a bit of saturation to the green channel, in camera.

Finally, if you have the room in your studio, add a 1/2 plus green gel on your cyc lights (or whatever lights you're using to light the backdrop). If you do this, the lights can't have any spill on the foreground. The less variation in the green, the better the key.

I have a PP dialed in just for greenscreening - now if I could just name it.
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Old February 13th, 2013, 08:44 PM   #12
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Re: Disappointing image quality, especially for chroma keying

This was not shot on a FS700. It looks like MPEG2-compression. Which camera was it?

Quote:
Originally Posted by Sergiu Macarescu View Post
Was it shot in slow motion?
No, it was not shot in slow-motion.

Quote:
Originally Posted by Will Salley View Post
I have a PP dialed in just for greenscreening - now if I could just name it.
Can you post the parameter-values of it?
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Old February 20th, 2013, 10:56 PM   #13
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Re: Disappointing image quality, especially for chroma keying

Dominik, you should check the infinity focus setting on your SpeedBooster. It's a known issue - that thing easily gets out of whack, and screws up focus on attached lenses. I had the same problem with softness when using the SpeedBooster, and after some adjustments and trial and error today, it is MUCH sharper, and behaving like it should. This camera is absolutely sharp, as I knew from using the non-Speed Booster adapter successfully. Check your adapter, mate.

Metabones - Infinity adjustment (Speed Booster? only)

Oh, and I should add that I'm using new Canon EF lenses in the L series. So, my lenses are just fine.
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Old February 21st, 2013, 03:57 AM   #14
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Re: Disappointing image quality, especially for chroma keying

The speed booster introduces additional chromatic aberration. The 24-70 L series lens is a poor performer in the CA arena already, there's already been a lot of discussion about this. I'd suggest your setup is pretty much a worst case scenario for getting a decent key.
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Old February 21st, 2013, 07:50 AM   #15
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Re: Disappointing image quality, especially for chroma keying

Quote:
Originally Posted by Alister Chapman View Post
I'd suggest your setup is pretty much a worst case scenario for getting a decent key.
The chromatic errors created by the fs700 are orders of magnitude larger than those coming from the optics. Using a better lens would be a drop in the ocean.
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