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06-02-2016, 09:03 AM   #31
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The reason most lenses are not at their sharpest after ƒ5.6 on APS-c is diffraction, based on how big the aperture is. The point at which diffraction sets in for both APS-c and full frame is between ƒ5.6 and ƒ8, but in most cases the difference between ƒ5.6 and ƒ8 is minimal. Ansel Adams belonged to the ƒ64 club. SO much smaller apertures are effective on later formats.

What I'm saying is that even with a TC on, ƒ8 should be sharper than anything smaller. Being wide open or not has not much to do with it. As you use a smaller and smaller aperture , the lens would become sharper and sharper were it no for diffraction. ƒ8 should still be the sharpest setting on a 150-450 @ 450mm. But it's all downhill after ƒ5.6, no matter what you do on APS-c. ƒ11 with a TC will suffer from diffraction just as much as ƒ11 without it. If the lens only goes to ƒ8, unless it;s a really crappy lens ƒ8 will be the sharpest setting, ultimately, although because of the narrow DoF at ƒ8 it may not appear to be if a lot of the subject is not on the focal plane.

If you understand how ƒ-stop is calculated you'll understand that the same aperture opening will lead to a different ƒ-stop when the focal length is increase. The same setting that gives you ƒ5.6 on the lens without a TC now gives you f-8 with the TC on, and it's still the sharpest part of the lens, and it behaves no differently than if it were a select on an ƒ2.8 or ƒ4 lens.

That's the theoretical truth. I'd be interested in any empirical test results.


Last edited by normhead; 06-02-2016 at 09:12 AM.
06-02-2016, 09:30 AM   #32
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QuoteOriginally posted by UncleVanya Quote
Not ALL internal focus lenses have focus breathing issues however common it might be for most. The 50-135 Pentax has very little if any focus breathing.
It may be a small effect, but it still has to be there. Internal focus zooms suffer more focus breathing than primes, but even primes show reduced focal length at close focus distances. You can calculate the actual focal length using max magnification and MFD but I don't have the time right now.
06-02-2016, 10:19 AM   #33
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QuoteOriginally posted by audiobomber Quote
It may be a small effect, but it still has to be there. Internal focus zooms suffer more focus breathing than primes, but even primes show reduced focal length at close focus distances. You can calculate the actual focal length using max magnification and MFD but I don't have the time right now.
I have read otherwise but I am willing to learn that my previous sources are wrong. I compared the da* 50-135 and the takumar bayonet 135 f2.5 and I recall little difference in image size whereas the 18-135 had significantly smaller images at close distances at 135mm.

I remember being told that traditional focus lenses can exhibit increases in focal length and that the normal model is to name a Len after the nominal infinity focal length. I am not however well versed in the math of optics.

---------- Post added 06-02-16 at 01:27 PM ----------

QuoteOriginally posted by normhead Quote
The reason most lenses are not at their sharpest after 5.6 on APS-c is diffraction, based on how big the aperture is. The point at which diffraction sets in for both APS-c and full frame is between 5.6 and 8, but in most cases the difference between 5.6 and 8 is minimal. Ansel Adams belonged to the 64 club. SO much smaller apertures are effective on later formats.

What I'm saying is that even with a TC on, 8 should be sharper than anything smaller. Being wide open or not has not much to do with it. As you use a smaller and smaller aperture , the lens would become sharper and sharper were it no for diffraction. 8 should still be the sharpest setting on a 150-450 @ 450mm. But it's all downhill after 5.6, no matter what you do on APS-c.

That's the theoretical truth. I'd be interested in any empirical test results.
Well the only things we differ on is the word "much" in your description and your assertion the f5.6 is an absolute pinnacle for sharpness. I think if you review lens tests you will find some lenses peak at f8 rather than f5.6 on apsc. I'm guessing this is due to labeling but I'm not sure. In any case the other point is that diffraction is also relative to what size you print and view the image at. Even if it is there when pixel peeping it may have no effect on the perceived sharpness of the ultimate image.

Too many people are afraid to stop down when increased depth of field trumps a slight drop in sharpness. I recently did some macro shots at high f stop and I was stunned that the loss of sharpness was far outweighed by the increase in depth of field.
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