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Old August 30th, 2010, 04:23 PM   #1
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"clean" isos and focal length applications

Hi guys. Two questions I've been thinking about, and haven't gotten much from searching on here or googling.

So, first, I've come across this claim several times that on the 5D, the "real" iso numbers are the ones that are multiples of 160. . .so 160, 320, 640, 1250, etc. The numbers in between those are allegedly interpolated from the "real" numbers and are therefore prone to more noise or something. The 160 multiples are supposedly cleaner. Anyone have any insight into whether this is BS?

Another question I had involves lens focal lengths as the apply to common usage in the world of TV/film. I've never worked with a 35mm sensored cam before, and I'm about to shoot a short film with the 5D.

We have several lenses available (thanks to a generous girlfriend): 24-105 zoom, 50mm prime, and a 75-300 zoom.

I'm basically curious as to what are common focal lengths for different types of shots. Obviously some of this is up to artistry and one can shoot CUs with a wide lens for several reasons, or compress a WS with a telephoto lens some reasnon, but I'm assuming there are more typical ways a 50mm lens is used vs a 24mm lens. Can anyone enlighten me? Thanks.
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Old August 30th, 2010, 06:44 PM   #2
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That whole ISO thing was based on people who didn't know what they were doing. The native ISOs are the ones you would expect (i.e., 100, 200, 400, etc.).
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Old August 30th, 2010, 06:49 PM   #3
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Originally Posted by Josh Bass View Post
So, first, I've come across this claim several times that on the 5D, the "real" iso numbers are the ones that are multiples of 160. . .so 160, 320, 640, 1250, etc. The numbers in between those are allegedly interpolated from the "real" numbers and are therefore prone to more noise or something. The 160 multiples are supposedly cleaner. Anyone have any insight into whether this is BS?
Well, the answer is different when you're talking about timelapse video (raw stills) vs. regular video (h.264).

For raw stills, the 200, 400, 800, 1600 ISO settings are "real" in that they do not have any in-camera multiplication nonsense applied to the raw files. 160, 320, etc. are "fake" in the sense that they start out the same as the "real" ISO settings, but Canon applies a simple digital "pull" to them 1/3 stop darker. In a well-designed system this would result in slight quantization error. But fortunately (or unfortunately, depending on how you look at it), Canon's system is not well-designed, so the extra 2-3 bits that normally go to waste serve to hide this nonsense. For the other "fake" ISO settings (125, 250, etc.), Canon increases brightness 1/3 stop with digital gain. This has two results: first it would again result in slight quantization error if Canon had tighter tolerances, but again is cancelled out by their mistakes in other areas. Second, it reduces dynamic range by 1/3 stop, as it is the highlights that get pushed off the edge in the linear digital push.

Now, for video, everything above still occurs, but we now have to consider the raw conversion and post processing that happens in-camera. For the "real" ISO settings (200, 400, etc.), Canon inexplicably sets the white point of the JPEG at 1/3 stop less than the actual white point of the raw files. This means 1/3 stop of highlights are clipped for no reason. For the "fake" ISO settings (160, 320), Canon leaves the white point the same (as if it was still 200, 400), so that 1/3 stop comes into usage. EDIT: Since the raw data is pulled down 1/3 stop, this equals out to the same highlight headroom, the net result is 1/3 stop cleaner shadows.

In summary: for raw stills, 200, 400, 800, etc. are just as good as 160, 320, 640, etc. For video, 160, 320, 640, etc. are not braindamaged like 200, 400, 800, etc., so they are (slightly) better.

Hope that helps.

Last edited by Daniel Browning; August 30th, 2010 at 11:04 PM.
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Old August 30th, 2010, 07:28 PM   #4
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How intriguing. Thanks.
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Old August 30th, 2010, 09:03 PM   #5
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I use 160, 320, 640, etcetera... religiously.

If you took a clip at 320 and then took a clip at 400 and showed me both back to back, I bet I wouldn't be able to tell.

For what it's worth.
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Old August 30th, 2010, 10:30 PM   #6
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They did some testing during those amazing Zacuto episodes and concluded the same thing, that 160 and the multiples of it are the best for video.
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Old August 30th, 2010, 10:34 PM   #7
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Sometimes things that don't show up on a computer screen/monitor/TV manifest themselves quite nastily on larger screens, say at a festival screening. So if you guys say it's so, I'll take it as doctrine.
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Old August 31st, 2010, 12:28 AM   #8
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Sometimes things that don't show up on a computer screen/monitor/TV manifest themselves quite nastily on larger screens, say at a festival screening. So if you guys say it's so, I'll take it as doctrine.
I've watched our short from last year, shot on a 5D on a 40ft screen. I learned about this 160, 320 thing in the middle of shooting. So we used both.

I'll tell you this. If you light your scenes properly, you'll have a DEVIL of a time picking up noise even on a large screen. We shot up to 640 ISO, but I'd say that anything below 400 is the safe zone for the big screen.
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Old August 31st, 2010, 12:42 AM   #9
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I'll stick with the multiples thing. Unfortunately we have a few outdoor night/street shots where I'd like to see the backgrounds, so I may go up to (gasp) 1250 or (gasp harder) 1600 on those with the 1.8 prime. If we get noise we get noise.
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Old August 31st, 2010, 12:48 AM   #10
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I'll stick with the multiples thing. Unfortunately we have a few outdoor night/street shots where I'd like to see the backgrounds, so I may go up to (gasp) 1250 or (gasp harder) 1600 on those with the 1.8 prime. If we get noise we get noise.
Believe me, I did a shot for that movie outdoors about 8pm. Sunset was about 5:45. We shot that at ISO 640 and it was noise free, and the background was fine. Go shoot some tests in similar conditions from 320 up to 1600 and see what you think. Blow it up 400% and see what you think, because that's what its going to be like for people sitting in the front of the theater.
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Old August 31st, 2010, 01:12 AM   #11
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Well, for the night stuff, I think the ISO chooses me rather than the other way around, right? If I'm already open to 1.8 and I need it bright enough to see the backgrounds, I need to raise the ISO to whatever it takes to get there. Otherwise I would think the idea would be to shoot as low as you can, right?
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Old August 31st, 2010, 01:30 AM   #12
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Yea, that's pretty much it. But I'd experiment with adding that last stop in post, versus adding it in the camera on production day. Do some tests and see which you like better.
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Old August 31st, 2010, 02:18 AM   #13
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Sorry I'm unclear. . .do you mean going no higher than 800 and gaining up in FCP vs shooting at 1250/1600?
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Old August 31st, 2010, 04:15 AM   #14
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Sorry I'm unclear. . .do you mean going no higher than 800 and gaining up in FCP vs shooting at 1250/1600?
Yes, that is precisely what I mean.
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Old August 31st, 2010, 04:26 AM   #15
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I did a little test. 800 doesn't look half bad, actually. At least on my Panasonic/Viera Plasma. . .played through the cam via HDMI. I don't know if that's a good way to judge.

Still curious about the focal length thing.
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