Tamron 16-30 mm f/2.8 Di III VXD G2
6. Distortion and field of view
Field of view
A rectilinear 16-30 mm lens on the full frame sensor should give you a range of angles of view from 107.0 to 71.6 degrees and Tamron provides these values exactly in their official specifications. Of course we decided to check these declarations. In order to do so we took photos of a starry sky and saved them as both corrected and 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 were able to measure the field very precisely, and exactly as it should be done, for rays of light coming from infinity.At 16 mm focal length and a corrected JPEG file the transformation was based on the positions of 187 stars spread evenly across the frame. An average mesh-fitting error amounted to 1 minute and 10 seconds of arc. Our final result amounted to 107.44 degrees with a measurement error not exceeding 0.05 of a degree. It's a result only slightly higher than the value stated in official specifications so everything seems to be in perfect order.
In case of the 16 mm focal length and an uncorrected JPEG file the transformation was based on positions of 154 stars spread evenly across the frame. An average mesh-fitting error amounted to 5 minutes and 47 seconds of arc. Our result is 109.20 degrees with a measurement error not exceeding 0.1 of a degree. The spare field of view, amounting to 2.2 deg, will be needed because of the presence of distortion.
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In case of the 30 mm focal length we used only an uncorrected JPEG file and the positions of 160 stars. An average mesh-fitting error decreased to 58 seconds of arc. The angle of view, measured by us, is 70.49 degrees with a measurement error not exceeding 0.03 of a degree. Here the field is narrower than in specifications but it's rather an asset; it means that the effective focal range, offered by this lens, is by less than 1 mm higher than the declared one.
Distortion
You can find out that distortion problems will be here a serious issue very soon. Already on the smaller APS-C/DX sensor you can notice this aberration without any problems. At the 16 mm focal length you deal with significant barrel deformations amounting to -3.82%. After passing to 20 mm this value decreases to -1.78%, and at 24 mm it is -0.40%. At the maximum focal length distortion changes its sign and turns into its barrel variant, amounting to +0.58%.
| Sony A7R IIIa, APS-C, JPEG, 16 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 20 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 24 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 30 mm | |||
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Still full frame means a real challenge. At the widest andgle of view you have to tackle high barrel distortion reaching -4.72%. It is not a result noticeably higher than the one we got on the smaller detector but it stemms from distinct moustache deformations. When you limit your measurements to an area within 1:1 markings barrel distortion increases to a value of -6.29%.
Fortunately here you also notice a swift decrease of deformations with the increase of the focal length. At 20 mm the 'barrel' decreases to a value of -1.36%, officially lower than for the APS-C/DX detector. Once again it is connected to the fact that even at this focal length there is distinct 'moustache' left.
After passing to the longer end of the focal spectrum deformations become zero and then change the sign: at 24 mm you deal with a slight 'pincushion' of +0.67%. Unfortunately with the further lengthening of the focal length distortion increases as well so at 30 mm you get a significantly high level of +2.58%.
The result is such that the distortion of the Tamron 16-30 mm is higher at both ends of the focal range than the distortion of the Sony 16-35 mm f/2.8 GM II, a lens with a wider focal range so more difficult to correct. Of course you should also remember that the Sigma C 16-28 mm f/2.8 DG DN, a lens positioned at the same price point, showed results even a tad higher than the results of the Tamron even though its focal range is narrower.
To sum up you can say that the producers didn't particularly care about distortion correction. They seemed to think that the problem would be solved by a combination of software correction and a tad of spare field of view left on uncorrected files.
| Sony A7R IIIa, FF, JPEG, 16 mm | |||
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| Sony A7R IIIa, FF, JPEG, 20 mm | |||
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| Sony A7R IIIa, FF, JPEG, 24 mm | |||
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| Sony A7R IIIa, FF, JPEG, 30 mm | |||
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In order to be honest we have to show also the distortion level at the widest angle of view after correcting the image. Before the correction images feature a much wider field that the declared one and even the frame after automatic distortion correction is slightly bigger than in official specifications.
A photo below shows an image we got at 16 mm with the function of automatic deformations correction switched on in the camera menu. In this case distortion decreases to a moderate level of -1.56%. Of course if you take a closer look you can notice the fact that software didn't get rid of moustache distortion completely.
| Sony A7R IIIa, FF, CORRECTED JPEG, 16 mm | |||
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At first glance the situation is much better – by and large you get an image with an angle of view on a level of 107 deg with relatively low distortion so in full accordance with the official declarations.Unfortunately the situation is far more complicated. Correction of high distortion levels entails cropping of image and rescaling it to its original resolution by 'pumping' empty pixels into it. We are against such practices and we consistently criticize it because it causes lowering of final image quality.
