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Tamron 500mm f/8 SP macro (Adaptall-to-Nikon) Review / Test Report - Analysis |
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Lens Reviews -
Nikon / Nikkor (APS-C)
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Page 2 of 3
Distortions
In practical terms the Tamron mirrors produces only a negligible degree of pincushion
distortions (~0.3%).
The chart above has a real-world size of about 120x80cm.
Vignetting
As mentioned mirror lenses offer no variable aperture so the vignetting test is
limited to one reading - the Tamron produces only a slight amount of vignetting at f/8.
MTF (resolution)
Well, not much to see here as well. It was fairly difficult to obtain valid test data.
Besides being a 500mm lens (thus requiring a camera-to-chart distance of 30m) the camera-lens
combination is very light-weight and prone to shaking. Anyway - resolution-wise the
Tamron is capable to produce "good" results. Subjectively the lens is quite a bit softer
at close-focus distances.
Below is a simplified summary of the formal findings. The chart shows in line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
The chart is limited to the visually relevant LW/PH range of [750, 2250].
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations.
Chromatic Aberrations (CAs)
As already mentioned above the mirror design results in a very low degree of CAs
(color shadows at harsh contrast transitions) which is not field-relevant.
Bokeh (out-of-focus blur)
A major argument against mirror lenses is the quality of the bokeh. The secondary
mirror causes a donut-like effect on out-of-focus highlights. The effect is minimized
or absent in long distance images because of the bigger depth-of-field (thus minimal
out-of-focus blur) or if you can manage to avoid out-of-focus highlights.
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