[Leica] Auto ISO

[kanner at acm.org (Herbert Kanner)]

I thought a few words about how this function operates on Leica, at least on 
the M9, might be useful to y'all, since the Leica Manual is not the clearest 
on this subject. 

Let's start with an understanding of the relationship between the three 
parameters: ISO, f-number, and shutter speed. We are used to thinking of 
exposure having one degree of freedom for a correct exposure. That is 
because we are used, from years of experience with film, of thinking of ISO 
being a constant. You can't change ISO in the middle of a roll of film. So, 
for any situation, there is one degree of freedom for a "correct" exposure: 
you change shutter speed, you have to change aperture, and vice versa. 
Hence, for the little all-electronic cameras where both the aperture and 
shutter are under computer control, you can choose the aperture (aperture 
preferred), or you can chose the shutter speed (shutter speed preferred) and 
the camera choses the one you didn't chose. You all know this; I'm just 
being pedantic.  Oh, then these idiot cameras have "programmed" mode where 
the camera chooses both based on some arcane set of rules. That's for bozos.

Now, lets look at the Leica. The camera can control the shutter, but it 
can't control the aperture. So the only automatic mode would appear to be 
aperture preferred. Ah, but the ISO is under the control of the camera's 
computer. It is now a third variable: it can be changed at any time. So, 
Leica in its wisdom invented Auto ISO. Now we have two degrees of freedom. 
That is, we can pick the values of any two: say ISO and aperture, and now 
the shutter speed is determined. Thus, on the Leica, we now have a way of 
doing shutter speed preferred: set the shutter to the speed you want, set 
the aperture to the f-number you want, and the camera will pick an ISO that 
gives the correct exposure. So, what happens if you set Auto ISO and 
aperture preferred on the M9? You will be in s situation similar to program 
mode in a point and shoot. The camera will chose both the shutter speed and 
the ISO value. I took a few shots at three consecutive stops on the dial, 
and the shutter speed sat at 1/150, perhaps not what I would want with a 
90mm.

If you set a shutter speed and an f stop with Auto ISO, everything works 
fine as long as the ISO that gives "correct" exposure is in the available 
ISO range. And you can use exposure compensation. What you lose is any 
information about exposure in the viewfinder. What information could that 
be? The ISO the camera selects, of course.

One warning: the little dot and triangles used for manual exposure setting 
seem to be meaningless with Auto ISO: just ignore them.

Herbert Kanner
kanner at acm.org
650-326-8204

Question authority and the authorities will question you.