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Back in “old” days, ISO was determined by the film you used in your camera.  The lower the number, the lower the film’s sensitivity to light and the finer the grain in the final photo.   When you made the decision to use 100, 200, or 400 ISO film, you were pretty much stuck with it until you finished the roll.

With today’s digital cameras, we can set the ISO differently for each and every picture if we want.  As with film, ISO determines your camera sensor’s sensitivity to light.  The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain.  Higher ISO settings are generally used in darker situations to get faster shutter speeds (for example: an indoor wedding in a dimly lit chapel), however the cost is noisier shots.  What do I mean by noise?  Here is an example:

(Photos courtesy of Digital Photography School)

Notice how the grain is so much smoother in the photo on the left.  This was shot on ISO setting 100.  The photo on the right was set to ISO 3200.  The grain is so much choppier, and is the darkest part, we see uneven lines.

Most people keep their cameras on auto ISO or set to the lowest setting – probably 100 or 200.  Cameras that allow you change the ISO, also give you more options when setting your aperture and shutter speed.  For example, let’s say you set your camera to ISO 100, and in Aperture shooting mode set it to f/5.6 (as wide as your lens can go).  Your camera automatically chooses the speed as 1/8 sec.  It’s a bit dark, maybe twilight, and you are trying to catch a decent shot of your evening primroses.  You are shooting without a tripod and you know that at 1/8 of sec., your speed is too slow and you will most likely have a blurry picture.  If you raise the ISO to 800 though, you can shoot at the same aperture and you will notice that your speed has changed to 1/60 sec. – very acceptable.  The ability to change the ISO on the fly, gives you more options in your other two settings, speed and aperture.

So, when deciding which ISO setting to use, you might want to take into consideration the lighting, whether you have a tripod, and how fast you will need your shutter speed (is your subject moving or stationary).

Situations where you might want to set your ISO higher are

Check your manual for your specific camera to see how to change your ISO and do some experimenting today!