Lee Jones wrote:
>> Your examples show up your film background... Image sensor's
>> native speed may not be in the 100 / 200 / 400 sequence.  Higher
>> & lower speeds are created by applying gain & anti-gain via the
>> electronics to match common & expected ISO speeds.

Olin Lathrop wrote:
> Yes, I understand that.  However, I was measuring the camera to
> learn how to use it most effectively in different circumstances.
> For that purpose the whole process is a black box.  It really
> doesn't matter *how* the camera achieves what it does, only *what*
> it can be counted on to achieve in various conditions.

*how* may matter if it directly leads to better or worse noise.
But currently we must infer the how from the results.

> Also, setting the ISO a little off from whatever the native value
> is isn't going to make a whole lot of difference.  The further off
> you get, the more post processing is applied.  If the native value
> is 160 and I set it to 200, I seriously doubt I'll be able to see
> the difference in quality the 1/3 F-stop post processing correction
> will add.

Some sequences I've seen show that 1/3 F-stop -- at certain points
-- does make a visual difference in noise.

Possible cause (from others, not my idea but sounds reasonable)
is that process inside camera has 2 phases.  Gain stage operates
in whole stops.  Separate ISO loss stage operates in fractions of
an F-stop, follows gain stage, and introduces more noise than gain
stage.  For example, assume sensor's native sensitivity is ISO 160.
Going up to ISO 640 takes 2 stops of gain.  Going to ISO 400 is
done using 2 stops of gain followed by 2/3 stop of speed loss.
If that's true, you are better off (noise wise) using ISO 640
than using ISO 400.

Harder to evaluate this on your D3S or other full frame sensor
digital SLR body because the sensors are now very low noise.

Easier to see some of these issues arise when testing an APS-C
size sensor in a mid-range camera.  Original work that found the
non-intuitive results was done by someone with a Canon EOS 7D
taking photos over the entire ISO range with lens cap on to
maximize noise visibility.

> Did you look at the pictures I posted?

Yes.

> You can see the quality changes very little for whole F-stop
> increments in the 200 to 1600 range.

And image quality from higher end Nikon & Canon bodies is great.
When pressed in low light, I'm now using ISO 3200 and getting
roughly the same noise level as a couple of body generations
back when used at ISO 800.

Because of lenses owned, I use Canon bodies.

In your other post, as I recall, Russell uses Minolta lenses
and bodies and migrated to Sony after they bought Minolta.

						Lee
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