Iso Sos!

[wtexasgal]

I have read the first few chapters of Understanding Exposure, and thumbed through some of the others...

I understand that exposure is a function of aperture, shutter speed and ISO. I also understand how aperture determines DOF, and shutter speed can capture motion (freeze or pan)....

Generally speaking, when do you modify ISO? It appears the author shoots majority of shots at 100/125.

When do you elect to kick up your ISO? Do you do so when you are happy with aperture and shutter speed but cannot achieve exposure with a lower ISO? (Low light, such as indoor, sunsets...)

What is your "default" ISO? Do most use auto-ISO?

[Daniel Bates]

I usually know what shutter speed and aperture I need to create a picture, and I vary the ISO accordingly. If I'm shooting landscapes, I leave it at minimum (100 or 200); at indoor sports games, I usually have to push it to maximum (1600 or 3200).

[Dusty]

Ditto what Daniel said. I shoot outdoors mostly so it is rare for me not to use the lowest possible ISO (100). However, if I'm trying to stop action I'll bump it up too whatever I need to to get the shot.

[Daniel M]

You are basically correct in that you must raise the ISO when you cannot achieve the desired exposure with the current aperature and shutter speed.

As an example - say you are in Av mode (aperture priority) and have the aperture set to where you want but when metering for the correct exposure you see that your shutter speed is to slow to hand hold the camera for the shot without motion blur occuring. You then raise the ISO in order for the shutter speed to increase for an acceptable level for the correct exposure.

I use the lowest ISO I can for the particular kind of shot I am taking. You must remember that the higher the ISO the more noise you will generate in your picture.

[kenw]

think of the ISO as "sensitivity". For the same shutter, aperture, etc, a higher ISO will be more "sensitive" to the light than a lower one.

[JPalmer]

Really...it's a mathematical formula.

X + Y + Z = photo

If you change any one of the variables (ISO, shutter speed or aperture), you will affect the result. They must all work together.

You modify the ISO based on the situation AND the desired shutter speed/aperture combination.

For example...if you're outside in the bright sunlight....you would shoot with a very low ISO speed (100-200), then adjust your shutter speed or aperture for proper exposure.

Indoors, the same ISO speed won't be able to produce the same shutter speed and aperture combination.....then, your choices are to either bump up the ISO speed or make changes to the shutter speed/aperture.

Either way...it's pretty much just as important a thing to learn as any of the other basics.