ISO 200 vs ISO 400

Using my D70, set on ISO 200, I shot some pictures yesterday in dappled shade (the sun coming down thru the leaves on the trees). Since the available light was somewhat limited, I got shutter speeds of 1/15, 1/30, etc. What is wrong with setting the ISO at 400? What do you sacrifice (if anything)? Does it really matter if you set the ISO at 200 for sunlight, and 400 for shade? Does that affect quality? I am confused about this. What real purpose does the ISO on a digital camera serve?

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Originally Posted by Paul in OKC

Using my D70, set on ISO 200, I shot some pictures yesterday in dappled shade (the sun coming down thru the leaves on the trees). Since the available light was somewhat limited, I got shutter speeds of 1/15, 1/30, etc. What is wrong with setting the ISO at 400? What do you sacrifice (if anything)? Does it really matter if you set the ISO at 200 for sunlight, and 400 for shade? Does that affect quality? I am confused about this. What real purpose does the ISO on a digital camera serve?

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ISO sttings on a digital camera set the sensitivity of the sensor. There are no ISO standards for digital, so the term is really just marketing.

You can. however, think about digital ISO in terms of film. For some lighting situations you want a "higher speed", for others, a "lower speed." But ISO is just one of many settings that are involved with exposure. This means that the simple answer you are looking for doesn't exist!

To use ISO effectively, you have to learn a lot about exposure. I would set the camera at a single ISO and use sunny 16 to determine exposure without a meter. After a few months, keep the ISO the same but start looking at how the meter changes exposures based on the scene. Then, finally, start adjusting the ISO value to see how it interacts with all the other parameters.

Using a higher ISO will also cause an increase in noise. That's the trade off. Faster available shutter speeds for for noise.