Tamron 90 mm f/2.8 Di III Macro VXD
6. Distortion and field of view
Field of view
A rectilinear 90 mm lens on a full frame sensor should provide you an angle of view of 27 deg but the producers in their official specifications state 27 deg and 2 arcminutes so 27.03 deg. Of course we decided to check how big this field really is. In order to do so we took photos of starry sky and saved them as uncorrected JPEG files.Then we transformed the pixel layout (X,Y) from the photo into the equatorial coordinate system (right ascension and declination), which locates a star on a celestial sphere. That way we could determine the field of view of the lens with utter precision and in the right way, so for rays of light coming from infinity. The transformation was based on the locations of 91 stars spread evenly across the frame and the average mesh-fitting error amounted to 34 seconds of arc.
Finally we got a result of 27.81 deg with measuring error amounting to 0.05 deg. The real field of view of the tested lens is a tad wider than the declared one and it means that the real focal length of the Tamron is a tad closer to 87 mm than to 90 mm.
Distortion
In case of the smaller APS-C/DX sensor geometric deformations are completely imperceptible. Our official result amounts to +0.11% so, within the margin of error, in accordance to zero.
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On full frame distortion is a tad higher so you deal here with slight 'barrel' amounting to -0.35%. Of course it is not something you should carp about.
Nikon Z7, 90 mm, JPEG, APS-C/DX | |||
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Nikon Z7, 90 mm, JPEG, FF | |||
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