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ISO speed vs. Levels adjustment

There are arguments that the ISO speed value doesn't affect the operation of the digital camera sensor, but it is only used afterwards in the camera processor to adjust the measured values (by multiplying them by some ISO-speed specific constant etc). If this is so, does the ISO speed affect the amount of information recorded in a RAW file? This question is examined in the following pictures.

The first and last rows are the same picture photographed in ISO 800 and in ISO 100. The middle row is the ISO 100 picture adjusted in Photoshop so that it looks as much as possible like the ISO 800 picture. All pictures are shown before and after USM sharpening (Photoshop UnSharpMask filter with options 100%/1px/0). The pictures are processed in Photoshop in 16 bits, so no detail should have been lost.

The camera is 5D.

ISO 800 jpg jpg
ISO 100
jpg jpg
ISO 100)
jpg jpg

Conclusion: ISO should be used like when using film.

This can also be deduced from the fact that if ISO were only implemented by simple multiplication, if would probably be possible to set its value afterwards in the RAW->JPEG conversion phase.


© Erkki Vanninen