Why bother changing base ISO?

Started Jul 18, 2007 | Discussions thread
cycle61 Senior Member • Posts: 1,550
Re: To the Left to the Right

Stan-o-Stan wrote:

Hi Nick,

thanks for your responce. I followed up to 'analog amplification'.
Could you please explain? I thought the whole process was digital
from camera to computer?

Stan,

The very first step, where the signal from the sensor is amplified as it offloads into the converter, is the only analog step in the process. This is where you benefit from amplifying this signal to use the whole 4096 steps of tonal range available from 12 bit digital. The analysis linked to previously rates the D200 sensor "well" capacity at around 32,000 electrons per photosite, so working with that signal gives you potentially that many tonal steps. Once the A/D converter does its thing, you're down to 4096, and if the analog signal is too weak to use the whole digital range, you end up with potentially far fewer levels than that. I think the important concept is "unity gain", which seems to be at ISO800 for the D200. My test shots I mentioned earlier were in bright daylight, at f/8 and 1/1250th second, for a proper exposure at ISO 800. I then turned the ISO down to 400, 200, and 100 without changing anything else. Results: ISO 800 is visibly noisy on the D200 (we knew that already) ISO 400, 200, and 100 look similar once boosted up to match, but ISO 100 actually looked the worst, being the most underexposed. I'm going to have to try again including 1600 and 3200. By what I understand, you benefit up to ISO 800 by the analog amplification, but beyond that there are more levels than electrons, and there's no benefit to boosting ISO further.
Test shots coming up in an hour or so...
--
-Nick Davis
Please feel free to critique anything I post. I'm here to learn.

My galleries, such as they may be...
http://www.pbase.com/cycle61

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