Tamron 17-70 mm f/2.8 Di III-A VC RXD
6. Distortion
At the 17 mm focal length you notice huge barrel distortion of −3.33%. The level is perhaps not very high, after all in constructions with similar focal range we've observed weaker performance, but such a result was reached only with the help of slight moustache distortion. If you limit your measurements to an area outlined by 1:1 markings distortion level increases to −3.75%.
It is interesting that this aberration drops very quickly with increase of the focal length, changes the sign, and already at the 24 mm focal length you deal with slight pincushion of +0.81%.
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Unfortunately distortion increases quickly as well, at 35 mm amounting to +2.50% and at 50 mm to as much as +2.88%. At the maximum focal length the aberration decreases slightly – our measurements showed a value of +2.71%.
The overall image in this category is perhaps not especially rosy but it's obvious the producers tried to limit distortion at the widest angle of view and they succeeded to do so; anyway these values are better than the result of e.g. the Sigma C 17–70 mm f/2.8–4.0 DC Macro OS HSM, the Pentax smc DA 17–70 mm f/4.0 AL [IF] SDM, or the Nikkor AF-S DX 16–80 mm f/2.8–4E ED VR. Still the price they had to pay was rather high – they had to introduce moustache deformations and, at longer focal lengths, they didn't manage to avoid significant levels of pincushion distortion anyway. For comparison sake you might add that distortion levels of the Sigma C 17-70 mm f/2.8-4.0 DC Macro OS HSM never exceeded a level of +1.0% at any of its longer focal lengths.
It's worth noticing the fact that the Tamron, unlike the Fujinon XF 16–80 mm f/4 R OIS WR, didn't give up completely correction of distortion with optics. The Fujjinon at 16 mm showed distortion of −6.39% and at the longer end of its focal spectrum the pincushion variant could reach +3.5% and higher.
| Sony A7R III, APS-C, JPEG, 17 mm | |||
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| Sony A7R III, APS-C, JPEG, 24 mm | |||
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| Sony A7R III, APS-C, JPEG, 35 mm | |||
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| Sony A7R III, APS-C, JPEG, 50 mm | |||
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| Sony A7R III, APS-C, JPEG, 70 mm | |||
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