Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
Dennis Forum Pro • Posts: 17,534
Don't worry about ISO !

I didn't read the whole thread; I saw other mention of Auto ISO including M mode and I saw that you have a D7000.

So here's the thing.  Thinking about ISO is thinking about exposure from the wrong direction.  It's placing emphasis on something that should be secondary.

What causes noise in a photo ?  The noise you see is the result of low signal:noise ratio.  The noise is already there; how much of it you see depends on how much signal is present.  In other words: the more light you put on the sensor, the less noise.

So instead of worrying about whether you should shoot this scene at ISO 800 or that scene at ISO 1600, you should concentrate on what shutter speed and aperture you want to/have to use.

Let's say you're shooting an indoor sporting event.  You know from experience that you want 1/500s.  And you know from experience that your f/2.8 zoom is soft wide open so you want f/3.2.  So you put the camera in M mode, set 1/500s and f/3.2 and let the camera *CALCULATE* the ISO in Auto ISO mode.  (I emphasized calculate, because too many people get hung up on the issue of control and worry about what ISO the camera will "choose" when in fact the camera isn't going to "choose" anything ... it's going to calculate the ISO based on the meter reading).  So the ISO gets set to whatever the ISO *has to be* in order to use the shutter speed and aperture you want to use.

What if the ISO is too high ?  Well, it can't be too high.  It is what it has to be.  What if you'd rather drop down to 1/250s ?  In practice, you might find that you have a minimum shutter speed and max aperture that you'd strongly prefer to shoot with, but might be willing to compromise a little if the ISO gets up around 6400 (or wherever your threshold lies).  So a half press of the shutter shows you the calculated ISO in the VF and a quick turn of the front or rear dial adjusts shutter speed or aperture.  No problem.

But the basic idea is that you set your shutter speed and aperture with the intent of putting as much light on the sensor as possible (assuming low noise is the goal ... if you have aesthetic reasons for setting them differently, then that's another issue ... and in bright light situations, you obviously don't want to overexpose at base ISO).  If you do that, you don't need to think about ISO; it just falls into place.  On the other hand, setting the ISO manually requires constant monitoring of your exposure settings to see if they're acceptable, if you have to raise it to get acceptable settings, or if you want to lower it to avoid unnecessary noise.

I switched from Sony to Nikon over a year ago in large part to have this feature available, and it's been liberating.  I initially wanted it for sports and school events, but now use it pretty much any time I'm shooting above base ISO.

- Dennis

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