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Archive 2012 · Strange sharpness behavior with 16-35mm L /2.8 mkii
  
 
shoenberg3
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p.1 #1 · Strange sharpness behavior with 16-35mm L /2.8 mkii


According to the test on PZ (http://www.photozone.de/canon_eos_ff/435-canon_1635_28_5d), for almost all focal lengths, the lens peaks in sharpness at around 5.6 and slowly dips after that -- including not just the center but also the borders.
However, on my copy on the 5D, stopping down to around F9 or so results in sharper results in all areas (including the center, where I focused on!), compared to the 5.6. This is particularly true for the extreme borders, in which at 5.6, is quite unacceptable but at F8+, fairly respectable.

I am wondering what could be wrong or perhaps I am misinterpreting the PZ's test..?



Aug 13, 2012 at 10:52 PM
wickerprints
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p.1 #2 · Strange sharpness behavior with 16-35mm L /2.8 mkii


There are a number of possible explanations, some of which are:

1. The Photozone measuring methodology is flawed.
2. Your measuring methodology is flawed.
3. Neither measuring methodology is flawed, but they differ in some other critical fashion (e.g., subject distance not the same), that leads to different data.
4. The Photozone data do not reflect actual image sharpness as evaluated by you.
5. Your copy of the lens exhibits aberrations or defects that are not sufficiently diminished until f/8 or slower.
6. You are using a 5D, not a 5D Mark II, and hence loss of sharpness due to diffraction is more difficult to observe than lens aberrations at the specified f-numbers.

I'm not able to tell you which of these may be true. Only further investigation on your part will help shed more light on the matter.



Aug 13, 2012 at 11:27 PM
S Dilworth
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p.1 #3 · Strange sharpness behavior with 16-35mm L /2.8 mkii


Photozone's testing procedure accommodates field curvature by refocusing for the corners. The real world, especially with wide-angle lenses and distant subjects, is flat. If the lens has substantial field curvature, as wide-angle zooms usually do, a disparity between Photozone and some real-world use arises.

There are many other possible explanations for your observations, but the above is the most likely and most significant.



Aug 13, 2012 at 11:34 PM
shoenberg3
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p.1 #4 · Strange sharpness behavior with 16-35mm L /2.8 mkii


Thanks for the response.
What puzzles me is that the image is visibly sharper at F/8 even for the center area where I focused on...



Aug 14, 2012 at 01:09 AM
 

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S Dilworth
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p.1 #5 · Strange sharpness behavior with 16-35mm L /2.8 mkii


It's possible you're not focusing accurately enough to take advantage of the large-aperture performance the lens offers. Increased depth of field makes very accurate focus less necessary at smaller apertures.

Wide-angle lenses are hard to focus accurately with phase-detection autofocus. With a tripod, a distant subject, and the lens set to 16 mm, try manually bracketing the focus in tiny increments while shooting at several f-stops per focus distance. This will rule out missed focus and establish which f-stop performs best when the focus is accurate.

Note that the required focus increments are so small that stiction may preclude simply turning the focus ring in tiny movements in one direction, from close to far; instead, you may need to jiggle the ring back and forth to settle on a new focus distance very slightly farther than the previous photo, paying close attention to the distance markings to know where to aim at. I suggested a focal length of 16 mm to minimise the accuracy needed: on zoom lenses the focus ring is effectively "slower" at shorter focal lengths.

Alternatively, borrow a 5D Mark II or similar and test your lens with Live View, which will give you a clear indication of focus.



Aug 14, 2012 at 12:14 PM
RCicala
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p.1 #6 · Strange sharpness behavior with 16-35mm L /2.8 mkii


If I'm reading correctly, photozone tested on a 5D II and you're shooting on a 5D so I would think difference in where diffraction softening kicks in contributes. And all the suggestions above are good.

I would add, while it's not likely, sometimes a decentered lens improves much more as you stop down than a 'normal' copy of the lens would -- the result is it's improving so much as you stop down it more than compensates for the diffractions softening. If the copy you have is bad at f/2.8 this might be a factor, but otherwise I wouldn't consider it.



Aug 14, 2012 at 03:37 PM
shoenberg3
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p.1 #7 · Strange sharpness behavior with 16-35mm L /2.8 mkii


Thanks all for the ideas. I will try out a few more tests to check out some of these hypotheses.


Aug 14, 2012 at 04:13 PM
Photon
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p.1 #8 · Strange sharpness behavior with 16-35mm L /2.8 mkii


How nice to see a thread with three knowledgeable and intelligent people responding, without picking at each other's suggestions!
Shoenberg3, good luck with the tests. If there is a problem, I hope it's not costly to fix. We don't all need to have the very best possible copy of each of our lenses in order to do good photography, but of course it's reasonable to want good performance from our expensive equipment.



Aug 14, 2012 at 04:45 PM





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