I really hate to agree with Godfrey sometimes, especially when he's
being pedantic. But this is so obvious that it's hard to understand why
anyone is left arguing.
Godfrey DiGiorgi wrote:
>On Jul 5, 2006, at 11:41 PM, Toralf Lund wrote:
>
>
>
>>I was talking about the number of different voltage levels that may be
>>output from the sensor itself. That number is not necessarily 4096; it
>>is the analogue-to-digital converter that has 4096 different values.
>>This distinction is far from irrelevant. Unless the actual sensor can
>>produce more than 4096 different levels, your whole exercise is quite
>>pointless at ISO settings much higher than the native rating of the
>>sensor. Heck, at low-light conditions, there is not even point in
>>metering at all, if you are going gamma correct and/or "scale" the
>>data
>>as part of our raw conversion anyway.
>>
>>I think you'll understand why if you consider the following
>>"experiment"
>>for a while:
>>Get a digital camera with a native ISO of 100.
>>Turn the ISO dial to 100, and set shutter/aperture so that you get
>>correct exposure, and start firing away. Check the maximum pixel
>>value.
>>Reduce the amount of light by 50%. Don't update the camera
>>settings. Try
>>again. What is the pixel value range like now?
>>Now switch to ISO 200, and repeat.
>>
>>The big question is whether the last step actually changed the
>>number of
>>different pixel values, or just replaced the range 0,1,2,3...2047 with
>>0,2,4,6...4094. Like I said, think about it for a while...
>>
>>
>
>I've thought about it for long enough ... a year or two ago.
>
>There's no point in considering the A/D conversion as separate from
>the sensor *because you can't do anything about it*. The RAW data
>output is what you can manipulate and represents conceptually a
>linear gamma space. If I look at the output with a proper exposure at
>ISO 200, I get values from 0-4095. If I look at the output at ISO
>1600 with a proper exposure, I also get values from 0-4095. Using a
>standard target and developing a curve fit will demonstrate that the
>dynamic range and gamma change between the ISO settings, but do not
>change the notions of how to evaluate the best exposure for a given
>scene.
>
>With regard to the data you can manipulate, using the strategy of
>providing enough exposure to place Zone IX properly just under the
>saturation threshold will *always* produce the best data to work with
>regardless of ISO because the reason it works is founded in numerical
>operations. If you plot difference in the response curve of the
>sensor at different ISOs, you could also use that information to
>expand or constrain the dynamic range you want to apply to scene
>capture, but that says nothing about proper exposure evaluation ...
>which was the point of my explanation.
>
>I take the tactic that I use the lowest ISO that I can all the time,
>consistent with achieving the focus zone and exposure time desired,
>since I can compress or expand the dynamic range in software with
>little constraint in post processing. The lower ISO setting will
>allow more data and thus greater flexibility in rendering.
>
>Godfrey
>
>
>
>
>
--
When you're worried or in doubt,
Run in circles, (scream and shout).
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