[Leica] Auto ISO Explained and Illustrated

[kanner at acm.org (Herbert Kanner)]

I hope I'm not flogging a dead horse. This really arose as a consequence of 
my asking about camera shake with 90mm lens on an M9.

Let me start by quoting from memory something that Dr. Ted wrote quite a 
long time ago. He said that once he got an M7 he stopped using previous M's 
and used aperture preferred, thus being able to concentrate on the subject 
and not wasting time on getting correct exposure.

As part of the reaction to my query about camera shake, Bob Adler suggested 
that I try Auto ISO and I began a series of experiments in the hope of 
figuring out how it worked.

In Aperture Preferred, you set the ISO, you pick the aperture, and the 
camera automatically gives you a correct exposure by choosing the shutter 
speed. But suppose you are in action situations where you are not as 
concerned with picking the aperture as with picking the shutter speed. You 
would like to have Shutter Speed Preferred, but Leica does not do that. 
Well, you have an even better world: choose both the aperture and shutter 
speed and let Leica give you automatic exposure by setting the ISO if you at 
the same time choose Auto ISO.

I created a set of test exposures. They are all labeled with what they are. 
There are three sets: 1. All seven stops from f/2 to f/16 in three groups: 
Aperture Preferred at ISO 160, Auto ISO with Aperture Preferred, and Auto 
ISO with shutter at 1/500.

My conclusion was that the combination of Aperture Preferred and Auto ISO 
was pretty useless for me. It must have been intended for Point and Shoot 
folks.

Here are two glitches in my examples, one of them inconsequential. When you 
look at the EXIF, you will see focal length 35mm. I forgot to manually set 
it to 90; I was using a 90mm lens. The only effect of this was to set the 
minimum shutter speed in the last group to 1/35 since I picked minimum 
shutter speed to be lens related: 1/f. This had no effect on these examples. 
The other glitch was that apparently Auto ISO does not take ISO below 160, a 
fact I discovered in this experiment; the Leica Manual does not mention it. 
So the shots at f/2 and f/2.8 at 1/500 are overexposed.

I should have posted each shot as a separate picture. Instead, I thought an 
album would be more convenient for viewers. I entered each shot in sequence, 
and it looks like the album software shuffled them into some random order. 
However, every shot is fully labeled.

Here are the links:

http://gallery.leica-users.org/v/herbk1/Auto+ISO+Test/?g2_page=1
http://gallery.leica-users.org/v/herbk1/Auto+ISO+Test/?g2_page=2

Finally, I must say that the section of the Leica Manual on Auto ISO is 
sadly deficient. Oh, and I almost forgot to mention: exposure compensation 
works with Auto ISO.

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

Question authority and the authorities will question you.