Quote:
Originally Posted by inkista
Pretty sure I've got it right. Think of it this way:
If I have a good exposure at iso 100, f/8, 1/60 second, and I flip the iso to 1600, but leave the aperture and shutter speed where they are, I'll have overexposed the picture by four stops. Higher iso => higher sensitivity => grabbing more light.
Higher/faster the shutter speed, the less light you get.
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Well, yes, but I don't know anyone that would increase the ISO and leave A and S unchanged. Unless there wasn't enough light in the scene to properly expose the scene before... let's say in a low, available, light situation. High ISO allows you to get by with less light.
Normally, when you think of the sensor, it is an analog device that is sensitive to light, when the ISO is increased, the actual sensitivity of the sensor doesn't physically change, but what happens is the A/D converter makes do with less S in the S/N processing. Hence the N (noise) sometimes overpowers the S (signal).
Now, if you over-expose the sensor, by using a high ISO, yet leaving the A and S at their old levels, the picture will just be blown. If you hold A and S unchanged, the amount of light hitting the sensor does not change - how it is interpreted by the A/D converter does change though, and will blow all the pixel sites.
Now, if you are doing a HDR composition, this could be extremely useful thing to do. But in an ordinary shot, blown or over-exposed, areas are not what I would like to see.