What is the point of setting an upper limit in Auto-ISO?

Photography often involves compromise. Often we want an aperture that yields sufficient depth of field, a shutter speed that doesn't result in unwanted motion blur, and an exposure high enough to yield no visible image noise.

In many situations, we cannot achieve all three of these goals, and we must compromise on one or more of them. If the camera is in a mode where it is allowed choices in how to compromise, then we can use set various limits to influence how the camera decides.

For instance, if limiting motion blur is important to us, we can set a minimum shutter speed. This tells the camera that when light is low, it should compromise by allowing a shallower depth of field (larger aperture) or more image noise (lower exposure, and a correspondingly higher ISO).

If limiting visible image noise is important, we can set a maximum ISO. This effectively tells the camera that we want to maintain at least a certain minimum exposure. In low light the camera will use compromise with a shallower depth of field (larger aperture) or allow more motion blur (longer shutter speed).

Now, if your personal preference is to maintain sufficient depth of field, and keep motion blur under control, then you may not want to set a maximum ISO value. If this is your preference, than you need to accept low exposures in low light situations.

+1
In a scene with bright cloudy sky, the landscape will be unusable dark for JPEG, unless one of the “compression” modes is being used (e.g., Canon’s HTP mode).

Here are my thoughts after reading other people’s opinions.
AFAIK, fixing unacceptable noise is much easier than fixing unacceptable motion blur. Therefore, it seems that preventing unintended motion blur should have priority.
With an upper ISO bound in A mode, we must monitor the shutter speed, and act once it is lower than acceptable. If we miss the point when ISO bound is reached, the image becomes likely unusable. Without an upper bound, we monitor ISO instead and act once our limit is reached. If we miss that point, the image will be noisier than we want it to be. Often, that is fixable.
Also, using a specific ISO value as a threshold for acceptable noise does not work. Depending on the scene, noise at ISO 3200 may be fine or not.

It seems that you have a higher tolerance for noise than motion blur. This is a reasonable choice. However, there are situations where a photographer's desire to limit image noise may be stronger than their desire to limit motion blur. That's also a reasonable choice. There are many different types of photography (and photographers), and an artistic choice made by one photographer may seem crazy to someone in a completely different situation.

Sometimes image noise detracts more from the aesthetics of an image than motion blur or shallow depth of field.

there is a another dimension to play, DOF ... S/N vs motion blur vs DOF - may be one can accept lesser DOF by wider aperture ( if lens allows ) vs blur by longer exposure ... may be gain in S/N can compensate possibly lesser resolution in some areas of the frame due to wide aperture ( may be not )

And sometimes noise is a good thing:

TMax 3200, Nikon S2.

The noise looks better in the full size image.

And the choice here isn't between guaranteed motion blur and guaranteed higher noise, it's between a higher risk of blur and guaranteed noise.

There are many situations where I would accept a lower keeper rate but of higher quality.

On my camera, I find that noise becomes unacceptable at higher ISOs. (And yes, I'm well aware that ISO doesn't cause noise; low exposure does.) For me, hitting that upper ISO limit (or, more specifically, seeing the image go dark and/or flashing numbers in my EVF) is a reminder that I need to do something differently vis-a-vis exposure, such as bracing the camera so I can set a slower shutter speed.

Aaron

Aaron