[Leica] Auto ISO

[steve.barbour at gmail.com (Steve Barbour)]

On Oct 2, 2013, at 9:41 AM, Herbert Kanner <kanner at acm.org> wrote:

> The sense for me would be when I'm using ny 90 mm lens actively, want 
> automatic exposure, but want to select (for all shots) a sufficiently high 
> shutter speed.


ie you'd like flexibility in setting the slowest shutter speed, as a lens 
dependent variable...but of course.  


s



> 
> Herbert Kanner
> kanner at acm.org
> 650-326-8204
> 
> Question authority and the authorities will question you.
> 
> 
> 
> 
> On Oct 2, 2013, at 7:58 AM, Leowesson <leowesson at gmail.com> wrote:
> 
>> I think I'm the only naysayer on this, but I would never use auto ISO.  
>> It makes no sense to me!
>> 
>> Leo Wesson
>> www.leowesson.com
>> 
>>> On Oct 1, 2013, at 23:30, 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 a
> t
>> 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.
>>> 
>>> 
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> Leica Users Group.
>>> See http://leica-users.org/mailman/listinfo/lug for more information
>> 
>> _______________________________________________
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> 
> 
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