Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
mike703 Veteran Member • Posts: 6,686
Re: Are there any "rules" for using ISO?

GreenMountainGirl wrote:

This is what I do, but still tend to get some underexposures.  The photos are taken in early evening when the light is falling away.  For instance, took some photos at f/5.6, 1/250sec,  ISO 320 which turned out OK.  But when I raised to f/16 and 1/500sec, ISO 800 the picture was underexposed.  I think I was using shutter priority, but not sure.  Another underexposed photo was taken at f/5.3, 1/250sec, ISO 640, also probably shutter priority.

This is a bit weird.  If f/5.6 and 1/250 sec at ISO 320 was OK, how did you end up with f/16 and 1/500 sec and ISO 800, which is about 3 stops less exposure?  If course that pic is massively underexposed... but the camera wouldn't have let you do that unless you were in manual mode and ignoring the frantic warning somewhere that the picture would be underexposed.  f/16 is a rather ridiculous aperture to be using in low light (it's a tiny hole that doesn't let much light in and would only normally used in very bright light) and does suggest that you are slightly confused...

If balancing shutter speed, ISO and aperture worries you, don't use manual mode (yet).  Use one of the auto modes - the camera is very good at calculating the correct exposure so let the camera do its thing.  Probably the simplest and most widely used auto mode is aperture priority (Av).  Select this and, as you are shooting in fading light, set your lens to its widest aperture (smallest f-number... so f/4 is a much bigger hole than f/16 and lets in a lot more light).  See what shutter speed you get.  WIth a 300mm lens hand-held ideally you want 1/500 second or quicker, so raise the ISO until you get 1/500 sec.  Now you're good to go.

Don't worry if you have to raise the ISO to 800, 1600, 3200... as others have said your camera's sensor is very good and even at ISO3200 you can get clean looking pics using the noise reduction in your processing software.  ISO6400 is usable if necessary - noise can be cleaned up (there are some amazing noise-reduction programs out there), but blur due to a too-slow shutter speed ruins the shot.

If your camera has an auto-ISO mode, an easy way to get the same effect is to set widest aperture on the lens (smallest f-number); 1/500 sec shutter speed; and let the ISO do what it wants.  If it really is too dim and the ISO is trying to go off scale (above 6400) you'll have to bring the shutter speed down to maybe 1/250 sec but only do that as a last resort.

Good luck

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