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06-16-2009, 11:44 AM   #1
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Does ISO affect Image Sharpness?

I think the simple answer is NO, not directly.

But of course its more complicated than that. This is the testing i did:

1. About a month ago, i took my K10, DA300, a 1.4 Tamron converter, a solid tripod, and focused on a Real Estate sign about a 1/4 mile away. I tried a number of settings to see what sharpness i could get by varying exposure and aperture. Later i concluded i could see the difference in letter sharpness between ISO 100 and ISO 200 with the K10. PP software was Lightroom V2.3.

2. After i got my K20, i repeated the test, using ISO from 100 to 800 and later examination showed no difference in the sign's letter sharpness between the different ISO.

For these tests, my normal routine was to use the 2 sec delay to get the mirror out of the way, and/or turned off the SR since i had it on the tripod.

Having the camera and lens on a tripod, should largely negate the benefit from using faster shutter speeds. But varying the aperture during these tests could affect resolution. In hind sight, i should have maintained aperture constant while varying shutter speed to compensate for ISO variations. So the question is probably better phrased to be:

While on a tripod and maintaining constant aperture, do ISO changes affect image sharpness?

With these clarifications, i think the answer is still NO, except that if excessive noise is developed during PP, then of course if noise is bad enough, it could affect the final image.

So even if my test methods are not good, are my conclusions correct on this :-)

06-16-2009, 12:40 PM   #2
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What was the aperture range?

I think the answer to your question is probably yes with an "if" or no with a "but". It's possible to use the ISO to drop the shutter to where motion becomes a factor (ie, sign blowing on a windy day), noise becomes noticeable as a lack of sharpness, or the aperture is open wide enough to be soft.
06-16-2009, 07:08 PM   #3
Ash
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Not by an appreciable amount - to a point.
Exceeding a critical ISO threshold decreases detail exponentially thereafter.
Check out the DxO website for measurbating-level test results on ISO and resolution:
K20D

Interestingly, the K10D had a slight edge overall to the K20D on their tests...
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