In article ,
McLeod wrote:
On 11 Jul 2005 08:30:09 -0700, "
wrote:

wrote:

Does anyone understand what are the mechanics behind the ISO settings
for RAW images?

Yes, I understand them. At least for the Canon 1D Mark II: there are
three buttons, a finger wheel, and a thumb wheel. Maybe the Nikon D70s
has the Telepathic Adaptor which can lock onto the mind of the
photographer, thus dispensing with these crude mechanical implements?

It depends on the camera. The Nikon D70 has an "AUTO-ISO" mode
(selected from the menu), in which, first the camera attempts to select
an appropriate shutter speed and aperture at the base ISO of 200, and if
that does not achieve the desired effect, it then increments the ISO
until a "reasonable" combination of shutter speed and aperture are
possible -- or until it gives up at the top ISO.

Menu entry 21 allows you to select a "floor" shutter speed, from
1/60 of a second down to 30 seconds, depending on what your needs are.
Yes, a higher floor would be nice with longer lenses, but with longer
focal lengths you are somewhat more likely to be using a Nikon VR lens
("Vibration Reduction" the equivalent of Cannon's IS "Image
Stabilization").

I think the poster was asking a more generalized question than you
responded to, but hey, who am I to stop you from making a jerk of
yourself.

Usually the lowest ISO setting of your digital camera is going to be
where you get the best picture with the lowest noise. Higher settings
are amplified, I believe, so noise is increased.

They are only in increments of 100, 200, 400 to remain reciprocal with
shutter speed and aperture settings.

Here, again, this is camera dependent. It may be so with your
Cannons, though I don't know, as I've not used one. However, the Nikon
D70 allows ISOs in the following steps:

200, 250, 320
400, 500, 640
800, 1000, 1250
1600

Rather finer steps than those which you listed, but the D70
adjusts exposures in 1/3 stop increments.

As for the mechanism of the actual ISO changes, it is
accomplished by one or more variable gain stages between the analog
sensor cells and the A/D converter(s) which feed the digital signals
into the camera's buffer memory. I don't know the count of A/D
converters, but I would expect multiple ones, each allocated to a
subgroup of sensors, so the A/D conversion could be accoplished at least
partially in parallel, to speed up the process, and thus to clear the
camera's sensor for the next shot more quickly.

Enjoy,
DoN.
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