Samyang AF 85 mm f/1.8 P FE
5. Chromatic and spherical aberration
Chromatic aberration
Perhaps the Samyang 1.8/85 P doesn't bowl us over with the complexity of its optical system but it certainly can impress with as many as three low dispersion glass elements. Their presence is one of the reason why longitudinal chromatic aberration is corrected so well. Photos below show very clearly that colouring of images far away from the depth of field are slight, noticeable only in the areas nearest to the maximum relative aperture. |
A high number of ED lenses didn't help with reducing lateral chromatic aberration and a graph below shows it very well.
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This aberration increases very quickly on stopping down the aperture. By f/1.8 and f/2.0 it is still low, then it enters medium range and up from f/8.0 you deal with high values. It is clear that the lateral chromatic aberration is an important factor that prevents the Samyang from spreading its wings on the edge of the frame.
A graph below, showing a comparison between the tested lens and its direct rivals, proves that this aberration limits its performance in a significant way.
Other 1.8/85 mirrorless lenses didn't experience any problems in this category so the Samyang remains a shameful exception and deserves to be criticized for it.
| A7R IIIa, RAW, f/1.8 | A7R IIIa, RAW, f/16.0 |
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Spherical aberration
In phirst photos of this chapter you can notice a focus shift effect but it is a tad specific. While passing from f/1.8 to f/2.5 you don't notice anything worrisome, only stopping down fromf/2.5 to f/3.5 shows a slight shift of depth of field toward greater distances. This performance explains why the Samyang was able to defeat its rivals at the maximum relative aperture and then it lost its advantage on stopping down the aperture to near f/2.8.Slight influence of spherical aberration remains visible also in the light spread of out-of-focus circles we reached before and behind the focus. Both circles show a slightly accented rim but in the case of the second one it is far brighter.
| A7R IIIa, f/1.8, before | A7R IIIa, f/1.8, after |
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