This is a test with the Canon G9 set to full zoom. These are 100% crops, of a TV antenna atop an apartment building. The goal is to determine the ideal aperture for best resolution. These pictures were taken with JPEG, and as a result there are annoying sharpening halos, as discussed in my previous post; to get maximum image quality from the G9, you have to shoot RAW and do your own sharpening.
First, we examine the results with the antenna in the middle of the photo:
| F4.8 | F6.3 | F8.0 |
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I can’t tell the difference between F4.8 (which is wide open at this focal length) and F6.3. But there is a deterioration in quality at F8.0 because of diffraction effects.
Now, let’s examine the results when the same antenna is at the upper left corner of the photo:
| F4.8 | F5.6 |
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The corner is much fuzzier than the center of the photo, and chromatic aberration (CA) is also visible. But all things considered, it's not so bad. I've seen much worse CA on my Olympus E-500 DSLR with expensive lenses that cost more than the entire G9 camera. (But to be fair here, the CA on the Olympus shows up at wide angle focal lengths. I would get no CA at this telephoto focal length.)
We also see a clear benefit when we stop down from F4.8 to F5.6. Why didn’t I stop down to F6.3 like in the first set of shots? I should have, but I wasn’t paying careful enough attention to this project. F6.3 would probably be even better than F5.6.
CONCLUSION: for maximum resolution from center to corner, stop down to F6.3, but do not use a smaller aperture than F6.3 otherwise you will lose resolution throughout the entire frame because of diffraction.
UPDATE: further testing has revealed that F5.6, and not F6.3, is the optimal F-stop.
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