The Nikon DSLR trick with Auto ISO and Manual mode

December 31, 2009

Nikon DSLRs have a reasonably smart automatic ISO mode, where you set your minimum shutter speed and maximum ISO and when the camera has hit the minimum shutter speed it starts raising the ISO. They are also famous (or infamous in some quarters) for not turning off Auto ISO if you go into Manual mode, contrary to what you might expect.

(What happens in this semi-Manual mode is that the camera works out its idea of the correct exposure and then attempts to get there purely by changing the ISO.)

I actually sort of like this, because it enables a trick: it essentially turns Manual mode into a combined Aperture+Shutter priority mode, and in turn what this does is give you a convenient way to vary auto ISO's minimum shutter speed as conditions change:

  • if I am shooting braced or with better support than expected, I can switch to M and drop the shutter speed down to lower the ISO.
  • if I switch from one end of a zoom to the other I can either drop or raise the shutter speed as necessary (depending on how I set my minimum shutter speed).
  • if I am suddenly taking pictures of action or something else where I want a fast shutter speed, I can increase the shutter speed without moving from my preferred (or necessary) aperture.

(Life would be somewhat simpler if Auto ISO also let us pick a minimum aperture; even though I can shoot a 50mm f/1.8 wide open, I often don't want to and I'd rather raise the ISO a bit and be at, say, f/2.8.)

Using Manual mode this way means that you really want to be able to control exposure compensation, and in turn this probably makes this trick unusable on bodies with only a single control wheel (where you lose access to exposure compensation in M mode).

The one thing that I really have to remember when doing this is to pay attention to the ISO and to the exposure meter, because the camera can overexpose if you push it. Generally if I'm doing this I want the ISO to always be above base ISO; the ISO going to base ISO when I'm at a comfortable shutter speed is a sign that I should switch to another exposure mode, because M mode probably isn't getting me anything useful.

Written on 31 December 2009.
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Last modified: Thu Dec 31 21:59:10 2009
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