Tamron 35-150 mm f/2-2.8 Di III VXD
6. Distortion and field of view
Field of view
In their specification Tamron states that the offered range of angles of view changes from 63°26' at 35 mm to 16°25' at 150 mm. We checked these declarations by taking photos of starry sky and transforming 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 were able to measure the field very precisely as it should be done, for rays of light coming from infinity.In case of the 35 mm focal length we used 176 stars spread evenly on the photo and transformation error reached a bit over one minute of arc. The measured field of view amounted to 62.36 deg so 62°22' with measuring error not exceeding 0.05 of a degree. Such a field is slightly narrower than the value from official declarations.
In case of the 150 mm focal length we used 79 stars for our transformation and the mesh fitting error amounted to just 3 seconds of arc. The field of view measured by us amounted to 16.17 deg (so 16°10') with the measuring error on a level of 0.05 deg. In this case the field is also narrower than in specifications. Still we admit that on the longer end it almost doesn't matter at all.
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In case of the 35 mm focal length a field narrower by over one degree than in official declarations does matter. Why? Firstly, in a lens of this type we care about a wide angle of view; secondly usually in these situation the field is wider than declared in order to provide some spare space for distortion correction. As we are going to show in a moment, the distortion level at 35 mm is not high but it also is not zero so its correction takes one or two degrees from the field of view. As a result the effective focal length will be here much closer to 37 mm than these officially declared 35 mm.
Distortion
In case of the smaller APS-C sensor distortion doesn't give us any serious reasons to complain. At the widest angle of view you deal with slight barrel deformations of -0.68%. With the increase of the focal length they become zero very quickly and they turn into pincushion variant. At 50 mm distortion reaches +0.90%, at 70 mm it increases to +1.26%, at 105 mm to +1.40%, and at 150 mm it drops slightly to +1.36%. All results we got are lower than 1.5% in absolute values – it means distortion doesn't cause here noticeable problems.
| Sony A7R IIIa, APS-C, JPEG, 35 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 50 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 70 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 105 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 150 mm | |||
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The situation changes dramatically when you progress to full frame. At 35 mm distortion amounts to -0.82% and it is a result just slightly different from the result of the smaller detector. Still, that was achieved by introducing a minimal amount of moustache distortion.
After passing to higher focal lengths there are no moustache distortion anymore but we also observe a distinct 'explosion' of the pincushion variant. At 50 mm it reaches a noticeable level of +2.29%, and at longer focal lengths we land near high values - at 70 mm we measured a result of +2.93%, at 105 mm it was +3.16%, and at 150 mm we got +3.17%.
A bit pity that the designers of the tested lens gave up on distortion correction because its correction limits the offered range of angles of view which, even before the correction, was in reality a bit lower than declared. As a result after making such an adjustment you are going to deal with a lens starting from 37 mm and ending a tad above 150 mm, not a 35-150 mm class instrument.
| Sony A7R IIIa, FF, JPEG, 35 mm | |||
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| Sony A7R IIIa, FF, JPEG, 50 mm | |||
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| Sony A7R IIIa, FF, JPEG, 70 mm | |||
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| Sony A7R IIIa, FF, JPEG, 105 mm | |||
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| Sony A7R IIIa, FF, JPEG, 150 mm | |||
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