[Leica] Auto ISO

[john at mcmaster.co.nz (John McMaster)]

I cannot see how it would? There I am putting an exposure in for a 
silhouette and the camera thinks "that's underexposed" so ups the ASA 3 or 4 
stops....

john

> -----Original Message-----
> 
> I'll have to read again, but I don't think AutoISO works on pure manual
> mode...
> Would you double check also?
> Thanks Herb,
> Bob
> 
> 
> On Tue, Oct 1, 2013 at 9:30 PM, Herbert Kanner <kanner at acm.org> wrote:
> 
> > 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
> >