[philbaum]

I'm thankful to all of you for the dialog this question has produced, but procyon's response is the kind of answer I am looking for - most of the responses seem to assume there is a noticeable difference between middle-value ISO speeds. Yours is the first to explicitly state that you have observed a difference and under what circumstances.

Does anybody else have similar experiences to share on this topic?

I shoot rehearsal pictures for a local theatre org. Their stage is often very black, lit up by various kinds of lighting where they want it. If one has the camera on center weight or matrix, i find that the auto ISO will often result in higher ISO levels than needed. On the other hand, if one carefully spot meters the lit up actor faces, you will get better pics of their faces because they are often standing where spots have been focused. But its often difficult to use spot metering for fast action. So i suspect that many folks like to set iso manually. Unfortunately at a theatre, they have automatic programs that will alter light levels as the play progresses and one can find manual levels are suddenly not yielding hte results you thought they would, and several shots may be unusable.

Having a camera like the K5, where its fairly intuitive on how to alter settings on short notice, is a very important feature for special events.

[Mike Cash]

I adjust ISO, and here's why:

I shoot Takumar lenses, which means I usually shoot in Av mode. The bulk of my shots are taken from my truck and it is quite often the case that one side of the street is in bright sunshine while the other is in complete shadow. I sometimes shoot stationary objects, sometimes people walking, trotting, running, or zipping along on a bicycle.

With such different lighting conditions and such different considerations for shutter speed, I find the easiest and quickest way to put the shutter speed about where I want it is to adjust the ISO with the rear wheel, which I have set to make 1EV steps. I need the speed of fewer clicks and the ability to easily keep track of ISO in my head as I seldom look at the bottom of the viewfinder. I don't need the precision, slowness, or potential for getting lost that 1/2EV or 1/3EV steps would bring.

Not a common set of circumstances, but you asked why anyone would go off of auto ISO and that is one real-world scenario based where it is a practical solution to a problem.