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Old August 12th, 2004, 09:44 AM   #1
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ASA equivalent for digital sensors

I figured I'd start this as a new thread since it's a pretty specific topic and might be better not to muck up the main camera thread.

I'm able to control the SI-1300 camera very accurately now, but I need to be able to put a nice UI in front of it. In particular, I'm looking at the camera's gain settings. I don't really want to put "Gain: 61" in front of a user -- I'd much rather show something more natural.

My questions (and please forgive my ignorance on the subject):
  • Would specific gain settings on a sensor correspond to ASA/ISO ratings for film?
  • How would I determine what the equivalent ASA/ISO is for various gain settings?
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Old August 12th, 2004, 04:27 PM   #2
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An easy way to figure out the ISO is to take an 18% grey card and light it at 2000lux. See what F-stop is needed to make the 18% grey card show up at 50% grey on screen when the image is properly normalized (i.e. gamma corrected). ISO 640 is F13 at 2000lux, so you can work backwords/forwards from that point as a reference. There are probably more "scientific" ways of doing this, but this is a handy starting point for an approximate ISO figure.

I believe to really find the true ISO of a sensor, you have to first find the noise floor, and then work backwords from there over the entire dynamic range of the image. I've never done that before though, and the F-stop/Lux measurement has usually been fine for a rough "guestimate".
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Old August 12th, 2004, 07:04 PM   #3
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Jason is right, another way would be to use a photometer, get the gray card to show as 50 grey after normalize (gamma 2.2 for PC, gamma 1.8 for Mac) and use the photometer to show you what ASA you need for that light and that f-stop.
To exactly detect the grey value I suggest you to use a waveform monitor.Many editing applets has built in waveforms that would be good for this...

BTW you should select the lowest gain that gives you good results, and then increase it and check that increment for its ASA equivalent to match a known film stock sensitivity.
Like 50, 64, 100, 200, 250, 320, 500, 800,
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Old August 12th, 2004, 08:21 PM   #4
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There are numerous discussions on this a search should turn up several threads. Film responds in a fairly linear response under most normal exposures. Digital, however, does not, and ASA (or ISO) is only an approximation at best.
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Old August 12th, 2004, 10:29 PM   #5
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We know, but ask Panavision, Arri and Dalsa why the give ASA equivalents for their cameras ;)
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Old August 13th, 2004, 08:12 AM   #6
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Thanks for the input, everyone! Once I get a little farther along I will try these methods and see if I can get a good idea of what the ISO/ASA levels look like.
Quote:
Jeff Donald wrote:
There are numerous discussions on this a search should turn up several threads. Film responds in a fairly linear response under most normal exposures. Digital, however, does not, and ASA (or ISO) is only an approximation at best.
Good point, but from a user-interface point of view, it's still better than trying to explain "Gain."
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Old August 13th, 2004, 02:03 PM   #7
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Getting an ASA reading is going to be coupled with the T-stop of the lens. That has to be factored in there, at least mentioning what lens was used.

As far as grain/noise, I'd say that you have to look at actual film images of a specific ASA, and compare that to what the electronic version of grain( noise) is looking like aesthetically. On first look noise looks like grain except for the fact that it doesn't clump together like grain does.
There are definite metrics for RMS grain, if you really want to get into it.

-Les
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Old August 13th, 2004, 02:31 PM   #8
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Quote:
Les Dit wrote:
There are definite metrics for RMS grain, if you really want to get into it.
Now that sounds like a fun way to spend a week! :-)

Seriously, thanks for the input, Les. Once I get a few tests run I'll post my results and you guys can take potshots at them.
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Old August 15th, 2004, 03:40 PM   #9
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I would prefer to have gain value option, I'm a computer person so something that I could directly relate would be better (though brightness increases by 2 (a stop every 6 b). So maybe we should have 3 modes for the camera Cinetographer (ASA/ISO or fstop etc), amature (n "stops" (as in 1 stop 2 stop etc) etc), and straight (gain db etc). With the user able to change the representation feild by feild if they wish.

The camera would start up in default Cinetographer mode, maybe the setup software could have a page to give a chioce between the three modes for a default. The user then can flick between them at will.


About auto lenses:

Now most of this stuff is manual, with no way of measuring the aperature, zoom and focus settings. Somebody posted a controll unit for a Canon auto SLR lense, the CCTV lense might also have something. The other thing that could be done is to set up sensors able to be fitted to any lense, that read the rotating rings of the lense accurately (a pain to workout at first but worth it). Going beyond this you could use it combined with screw wheels and motors (or just soliniods) to provide automatic focus, iris and zoom (one of my design things), or with thumb wheel, follow focus. Such a hardware system would be incorporated in a lense adaptor or casing.

Actually consider these stats that could be flicked up on screen, the performance curve for each colour at a set gain. The range at each gain for each colour. The minium and maxium of the range for each colour. The user sees the normal veiw and just flicks to the alternative veiw to check what to expect at that setting. You could even have a amount of niose indication on the performance graph (maybe from blue, green, orange, red, yellow colours). With manufactureres submited values (SI) and the ability for the user to determine value. The performance curves could be used in conjunction with zebra like paterns for each colour. Just a quality sort of thing, that not even a movie camera may have.
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Old August 15th, 2004, 09:10 PM   #10
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Quote:
Wayne Morellini wrote:
So maybe we should have 3 modes for the camera Cinetographer (ASA/ISO or fstop etc), amature (n "stops" (as in 1 stop 2 stop etc) etc), and straight (gain db etc). With the user able to change the representation feild by feild if they wish.
That's a good idea, Wayne. It will probably be just Gain anyway in the first version, unless/until I have the time to figure out the ISO/stuff with decent accuracy.
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Old August 16th, 2004, 02:17 PM   #11
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I think that there are two levels of control for gain a user would want. To start, an absolute level. The typical film maker can start with an ASA/ISO rating to match the requirements. From then on, most of the gain adjustments are relative - either sliders or up/down buttons would work.

Wayne: could you point me towards the control unit for Canon lenses? I couldn't find it after searching. I developed one at another company but that was proprietary info and I can't use it.
Thanks,
Steve
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Old August 18th, 2004, 02:28 PM   #12
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Quote:
Steve Nordhauser wrote:
could you point me towards the control unit for Canon lenses?
I found this link: EF232 Lens Controller for Canon EOS Lenses. Is that what you're looking for?
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