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Tokina AF 10-17mm f/3.5-4.5 AT-X DX Fisheye - Review / Test Report - Analysis |
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Lens Reviews -
Nikon / Nikkor (APS-C)
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Page 2 of 3
Distortion
The 10mm fisheye effect of the 10-17mm AT-X cannot be measured via the Imatest toolkit but the
reference images below should give you an idea about the degree of barrel distortion
(at 17mm: ~10%) - remember that the lens is supposed to do just that. This is not a design problem.
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Move the mouse cursor over the focal length text marks below to observe the respective distortion
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| 10mm |
17mm |
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The chart above has a real-world size of about 120x80cm.
Vignetting
Sorry - no vignetting figures this time. I'm absolutely clueless how to produce an evenly
lit plane for a lens with a field-of-view of 180 degrees ... 8-)
MTF (resolution)
Testing a fisheye lens is a bit experimental. The Imatest toolkit freaked out regarding
all the wild curves at the image borders at the 10mm setting. The 17mm MTFs
should be quite accurate whereas the border resolution figures at 10mm should be only
taken as a very rough guidance - the outer regions were not testable!
Keeping this in mind the Tokina Fisheye produced very good results in the lab. At 10mm the
center performance is already very high at wide-open aperture and the borders follow not
far behind. Stopping down produces only marginally sharper results. The local sweet spot
is at f/8. At 17mm there's a very slight performance drop - not in the center but the
borders suffer a bit at f/4.5. The situation is better at f/5.6 where the border quality
recovers to very good levels.
Please note that the MTF results are not directly comparable across the different systems!
Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations
Chromatic Aberrations (CAs)
Lateral CAs (color shadows at harsh contrast transitions) can be quite a problem with
the 10-17mm Fisheye. At 10mm the lens produces a quite excessive amount of border
CAs (~2.5 pixels) whereas the situation isn't quite as bad at 17mm (~1.3 pixels).
Please note that lateral CAs are correctable via imaging tools.
Purple Fringing
Lateral CAs may not be good but another aspect is purple fringing - a color blooming
effect at extreme contrast transitions. Here's a 100% crop which should illustrate the
potential degree of the problem.
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