Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
Virginia Bill
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Re: Are there any "rules" for using ISO?
In reply to GreenMountainGirl, Apr 3, 2013

GreenMountainGirl wrote:

I have been experimenting with low-light images of moving subjects (mostly deer) and getting mixed results.  Using aperture-priority produces slower shutter speeds, and raising ISO does not seem to work.  Using shutter-priority works better, but still often have to make exposure adjustments in LR4.  Most of these photos are at/close to 300mm, so have not noticed any significant effect on DOF, just need to get a better idea of ISO (and exposure compensation) because there often isn't much time for "test photos"!

Susan

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GreenMountainGirl

What's missing here is information about your metering mode. Shutter, aperture, and ISO are fundamentally and reciprocally linked; if you change one you will either change the amount of light hitting your sensor (in manual mode) or your camera will change another setting to maintain what it regards as accurate exposure (in any automatic metering mode).

For animals with a telephoto, I'd use spot metering on the animal; any other mode might react to foliage, sky, or ground enough to require adjustments in postprocessing.

You need to understand what the meter is trying to tell you. It's calibrated to produce a standard grey tone from the light your subject is reflecting into your lens. If your viewfinder contains very bright material (snow, sky, white sand are the classic examples) the meter will underexpose. If your viewfinder contains a lot of dark material (a tree line in low light for example), the meter will overexpose. Most metering modes use various averaging or weighting algorithms (usually called matrix metering) to produce pleasing results. You can also choose a mode favoring the central area of the viewfinder so that something like 60% of the reading comes from the central area. Spot metering takes its reading only from the very center of the viewfinder.

For deer at 300mm I'd use manual mode, high shutter (1/500 or faster), and spot metering. If changing aperture didn't produce enough light, I'd raise ISO.

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