New G1X II Sample Images

Started Feb 26, 2014 | Discussions thread
MOD Dale Buhanan Veteran Member • Posts: 4,277
Re: why f/6.3? tripod for shot #2?

technic wrote:

Dale Buhanan wrote:

CAcreeks wrote:

I wonder if f/6.3 was chosen to maximize DOF. You would think that diffraction would start to be an issue at that aperture, but the architectural details are very sharp.

The EXIF maker notes say "Image Stabilization = 257." Do any Canon experts know what that means? You would think that the photographer would disable IS if on a tripod.

Remember that this is a large sensor camera, just a little smaller than APS-c. So diffraction does not start on this camera until about f11. F16 would be well into the diffraction region, but f6.3 would not. That would be just fine.

That's just theory. The 15-85IS - which is a somewhat similar lens - already gets a bit soft at f/8 and by f/9 it goes rapidly downhill over most of the zoom range. I would not be surprised to see the same with this lens, but starting around f/6.3 already because of the slightly smaller sensor and larger maximum aperture (= peaks earlier).

Well, I agree with a lot of what you say here technic. You are right about diffraction at f-6.3. But actually the theory says that diffraction is a continuous function that is the reciprocal of f-stop. ie.:

Diff = 1/Aperture.

So that the lowest diffraction occurs with very large aperture lenses. Unfortunately, there is a competing function where lens correction for spherical aberration, coma, and other lens image degrading factors increase with larger f-stops. So that the sweet spot of the lens is where these two factors cross. Lower of course is better -- but also more expensive.

But back to the diffraction term only... In real world performance, the circle of confusion that diffraction causes doesn't optically become objectionable until the COC becomes somewhat greater than the pixel pitch of the sensor. On an APS-c sized sensor with 13 megapixels that puts the point at about f-8.

Since f-6.3 is a larger aperture than f-8 that means that even though there is diffraction at f6.3 that would be objectionable on say a 14 MP small, pixel pitch, 1/1.23 sized sensor, on the G1X-II it is not very noticeable. I shoot at f6.3 and f-7.1 with my 7D frequently and don't really see the softening effect of diffraction until I am with an f-stop higher than f-8. You can see it at f-11 and it is pretty bad by f-16. f-22 is all but unusable... although on my old speed graphec 4X5 f22 was a good aperture for landscapes. f-4 is the sweet spot on my G15.

All this to say that the "diffraction limit" f-stop is a function of sensor size and pixel pitch. And on the G1X-II 13 MP sensor, f-6.3 is not going to give noticeable diffraction.

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kind regards

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