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

The op asked what is the point in setting an upper limit for ISO in Auto ISO.

I simply posted why I set Max iso to the highest native ISO value.

I know. I'm asking about something else you said: "The Auto iso is over cautious when setting iso above base when using evaluative/matrix metering."

Because when setting ISO above base it very often leaves 1/2 stop, if not more, highlight headroom according to the camera's histogram.

I rarely see auto ISO set an ISO above base resulting in highlights being close to clipping.

As noted, I merely observed the result of a test - anyone is free to think up their own reasons for setting limits.

Different behavior when setting iso above base than at base would indicate that when light is halved in M mode, iso wouldn’t double from base iso. Yet when I suggested that, you said “of course not.”

You were talking about stopping down the aperture by 1 stop and I made it clear I was talking about the situation where the aperture and shutter speed are constant after I set the optimal exposure**.

I am just describing my experience with Auto ISO on my 90D after I have set the optimal exposure**. If you do not believe what I observe that is fine 🙂

* exposure - amount of light that struck the sensor per unit area while the shutter was open
** optimal exposure - the maximum exposure* within dof and motion blur requirements without clipping important highlights.
*** under exposed - more exposure* could have been added with the DOF and blur constraints still being met without clipping important highlights.

Is the resulting OOC JPEG brightness/lightness any different than manual ISO in Av, Tv, or P modes, or do you feel that there is some special behavior regarding brightness/lightness for auto-ISO in M mode? I always assume that it is same, because "EC" just targets brightness/lightness regardless of which parameters are automated.

I shoot only raw and not raw + jpeg but I assume the sooc jpegs would look very similar to what I see on the camera's lcd screen.

I doubt very much that the ISO value the Auto ISO sets after I set aperture and shutter speed to suit dof and blur would be very different to what I would have set manually to zero/centre the camara's meter after having set the same aperture and shutter speed for the scene.

The only thing I would do differently if setting ISO manually is that if I had highlight headroom after having set the optimal exposure** then I might raise ISO to push the histogram data closer to the right but without clipping highlights.

* exposure - amount of light that struck the sensor per unit area while the shutter was open
** optimal exposure - the maximum exposure* within dof and motion blur requirements without clipping important highlights.
*** under exposed - more exposure* could have been added with the DOF and blur constraints still being met without clipping important highlights.

It's not a question of not believing you. It's a question of my understanding what you are saying your camera does.

You said: "The Auto iso is over cautious when setting iso above base when using evaluative/matrix metering."

I don't have your camera, so I can't test it.

I think it would be great if auto-ISO left more headroom at higher ISOs than it does at base ISO, but I've never seen a camera work that way, and I don't know what the camera would do with the JPEG preview images in that case.

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