When using film cameras you need to chose the rating of the film you buy. It is possible to up or down rate them a little, but they are quite limited in changes you can make. Digital cameras have more flexibility in the choice of ISO number as it can be changed in the camera's settings.
The higher ISO numbers enable faster shot speed / more depth of field. It is also useful in low light when a shot may not be possible using the ambient conditions. The trade off is more grain in the final image. I have taken a few samples with a digital camera, A Canon EOS 20D at the 100 and 1600 ISO ratings.
|The ISO 100 picture clearly has less depth of field, the leaf is blurred.||The ISO 1600 has better depth of field. Other than that the two pictures look identical|
Again, the 100 is on the left and the 1600 on the right.
Looking close at the two pictures you can see the noise (grain) which is generated by the 1600 rating. The colour tone is much smoother in the left hand image.
Overall though? The two pictures above look very similar, it is only for very high quality production that the noise in the 1600 rated picture would be an issue.
|The two ratings again. The depth of field advantage for the 1600 picture is staggering. It allows f18 at 1/125 compared with f4.5 at 1/125. The close-ups below illustrate the tradeoffs. This is extracted form the mid tones behind the forewing, clearly the 1600 is very noisy, but which is the better picture?|
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