Disappointing image quality, especially for chroma keying

Paul Curtis · November 25th, 2013, 06:51 AM

Bit late to this but i've done keying with the 700.

My observations:

You need to record 4:2:2 out of the camera, a shuttle costs next to nothing and you'd be insane to not do this.

You only care about edges, most of the green screen would be garbaged out, so focus on those areas.

Be very very careful with lenses and apertures. In the spirit of garbage in and out, poor lenses can add aberrations that are way harder than colour sampling to remove. Also be very mindful of diffraction at wider stops. Diffraction (this is what i'm calling it) can made the RGB parts of the light wrap differently around the edges - so the blue channel edge will be different to R and G, it's very obvious when wide open and lead into some pretty hairy keying.

Keying is an art form, there's no such thing as a single button keyer unless you are super careful with how you shoot and what you are looking for at the end. I work in Nuke with all manner of different keying approaches for different shots, different subjects and conditions. There is no one keyer to rule them all and invariably keying means using combinations of approaches.

hth
Paul

Leonard Levy · November 25th, 2013, 01:15 PM

Paul, I may be wrong but I think diffraction is something that happens at very closed f stops due to light bending around the small opening. Sounds like what you are talking about is basic chromatic aberration which is highest at wide open stops and will vary with the quality of the lens.
Its just nomenclature though. You're point is well taken.

Paul Curtis · November 26th, 2013, 03:53 AM

Quote:

Originally Posted by Leonard Levy

Paul, I may be wrong but I think diffraction is something that happens at very closed f stops due to light bending around the small opening. Sounds like what you are talking about is basic chromatic aberration which is highest at wide open stops and will vary with the quality of the lens.
Its just nomenclature though. You're point is well taken.

You're right. I'm calling it diffraction because it's what the light does - it's diffracting around the edges of the subject (not lens). Each wavelength is being spread out differently. So i'm probably going to confuse people who think of diffraction as something that happens with a lens closed down but i think i'm using the scientific version of the word (perhaps incorrectly!)

I guess it's similar to the effect of waving your hand in front of a lens when it is wide open and focus is in the distance, you'll see the background image distort as the light bends around your fingers. Cool effect, but not for post people...

It's one of those things that i see rarely mentioned but for some shots and some lenses it's really quite apparent and a real pain. I've seen samples of Epic footage that shows the same problem, so it's physics based not sensor.

cheers
Paul

November 25th, 2013, 06:51 AM	#46
Paul Curtis Major Player Join Date: Mar 2005 Location: canterbury Posts: 411	Re: Disappointing image quality, especially for chroma keying Bit late to this but i've done keying with the 700. My observations: You need to record 4:2:2 out of the camera, a shuttle costs next to nothing and you'd be insane to not do this. You only care about edges, most of the green screen would be garbaged out, so focus on those areas. Be very very careful with lenses and apertures. In the spirit of garbage in and out, poor lenses can add aberrations that are way harder than colour sampling to remove. Also be very mindful of diffraction at wider stops. Diffraction (this is what i'm calling it) can made the RGB parts of the light wrap differently around the edges - so the blue channel edge will be different to R and G, it's very obvious when wide open and lead into some pretty hairy keying. Keying is an art form, there's no such thing as a single button keyer unless you are super careful with how you shoot and what you are looking for at the end. I work in Nuke with all manner of different keying approaches for different shots, different subjects and conditions. There is no one keyer to rule them all and invariably keying means using combinations of approaches. hth Paul

November 25th, 2013, 01:15 PM	#47
Leonard Levy Trustee Join Date: Feb 2006 Location: USA Posts: 1,684	Re: Disappointing image quality, especially for chroma keying Paul, I may be wrong but I think diffraction is something that happens at very closed f stops due to light bending around the small opening. Sounds like what you are talking about is basic chromatic aberration which is highest at wide open stops and will vary with the quality of the lens. Its just nomenclature though. You're point is well taken.