Oh, why not ? ... Auto ISO in M

Started Oct 1, 2013 | Discussions thread
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Dennis Forum Pro • Posts: 17,871
Oh, why not ? ... Auto ISO in M

So I just got around to browsing through the now-closed thread "What ISO isn't" and was reading this post by BlueSkies in which he writes:

A fact of life to many (most) of us is that ISO-choice and SNR are related. So, if I want a noise-free image, I choose a low ISO. If I want a fast shuter, I choose a high ISO, and accept more noise.

Now, I'm not intending to restart that discussion, and may be taking this out of context, but I've seen this argument posted before in arguments over whether Auto ISO in M should be provided on cameras.  (Amazing that some people would want others to not have a useful feature because they don't understand it !)  For the record, I'm not saying that the author of this quote takes that stance - just grabbing a quote for the purpose of discussion.

Anyway, what I'm getting at is ... why does it seem reasonable to select ISO to get shutter speed ?  It's kind of like being on a see-saw and saying "I want to go up, so you have to go down".  Sure, you want to know what ISO the camera says is needed according to the way you've metered the scene so you can make intelligent compromises.  But you don't want ISO 800.  You don't want ISO 1250.  You don't want ISO 1600.  You want base ISO and everything else is a compromise that lets you get the exposure settings you want.  So why do so many people insist that setting ISO is the right way to do this ?  That you're somehow "in control" if you select ISO in A mode, but you're giving up control by letting the camera "choose the ISO" for you if you're using Auto ISO in M mode ?

When is any ISO other than base ISO your priority over shutter speed and aperture ?  You can say "I'd rather have ISO 800 than ISO 3200" but you have to have a shutter speed & aperture in mind to know you have to shoot at ISO 800 instead of base ISO in the first place.

All of this applies to digital sensors: I choose a fast shutter, or slow shutter - for the action, I really cannot independently choose a high ISO or low ISO - for the 'brightness'.

Right.  It's a dependent variable.  Which some people just don't get when you bring up Auto ISO in M.  In A mode, the camera calculates shutter speed.  It's the independent variable and it's going to be whatever the meter says it has to be.  In M mode, with Auto ISO, the camera calculates the ISO and it's going to be whatever the meter says it has to be.

The two benefits of Auto ISO in M (or TAv on Pentax) to my mind are:

(1) Mindset.  The seesaw example.  Why change one parameter to get to where you want another to be ?  Why not just change the parameter you know you want ?

(2) Shooting repeatedly at the same exposure.  Great for sports and other events where you might shoot a sequence of shots over many minutes at the same shutter speed and aperture.

Obviously, this all assumes you're basing your exposures on the built in meter, and it's the wrong mode to use at base ISO.  But I bet more photographers would enjoy this mode if they gave it a shot.  Note to Sony: get on the ball and put this in ALL models.  Note to Canon: get on the ball and allow exposure compensation in this mode, else it's little better than not having it.

Again, sorry to use BlueSkies' comments out of context; he may be a fan of Auto ISO in M for all I know.

- Dennis

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