Samyang AF 24-70 mm f/2.8 FE
7. Coma, astigmatism and bokeh
Also the corner of the APS-C sensor in the middle of the focal range is problematic. In case of a full frame lens you should demand a perfect correction of coma in the area covering the APS-C sensor. Meanwhile Samyang shows here noticeable comma. It explains – up to a point – the problems with image quality on the edge of the frame the tested lens experienced in the 45-70 mm range.
| Center, 24 mm, f/2.8 | Corner APS-C, 24 mm, f/2.8 | Corner FF, 24 mm, f/2.8 |
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| Center, 42 mm, f/2.8 | Corner APS-C, 42 mm, f/2.8 | Corner FF, 42 mm, f/2.8 |
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| Center, 70 mm, f/2.8 | Corner APS-C, 70 mm, f/2.8 | Corner FF, 70 mm, f/2.8 |
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Astigmatism, understood as an average difference between vertical and horizontal MTF50 function values, amounted to 4.8%, so right on the borderline between very low and low level. If you look at it closely and divide it between particluar focal lengths then at 24 mm you deal with a value of 3.8%, at 45 mm the result is 4.0% and at 70 mm you see the highest value, that of 6.5%.
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Defocused circles of light don't look perfectly well. You can have reservations concerning slight onion ring bokeh, visible inside the circles, that most probably stemms from employing as many as three aspherical elements. Additionally, a bit brighter rim makes itself felt, as its intesity increases on stopping down. It would be difficult not to notice the influence of mechanical vignetting; by f/2.8 it removes over 1/3 of the whole surface of the circle.
| Center, 70 mm, f/2.8 | Corner APS-C, 70 mm, f/2.8 | Corner FF, 70 mm, f/2.8 |
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| Center, 70 mm, f/4.0 | Corner APS-C, 70 mm, f/4.0 | Corner FF, 70 mm, f/4.0 |
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Center, 70 mm, f/5.6 | Corner APS-C, 70 mm, f/5.6 | Corner FF, 70 mm, f/5.6 |
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