Sony Carl Zeiss Sonnar T* FE 55 mm f/1.8 ZA review - Coma, astigmatism and bokeh

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Sony Carl Zeiss Sonnar T* FE 55 mm f/1.8 ZA

27 August 2016

Arkadiusz Olech

7. Coma, astigmatism and bokeh

Coma and other aberrations of higher order which deform points of light in the frame corners are corrected well only for the APS-C sensor area. In the corners of full frame the deformations of diode images are distinct even on stopping down the aperture by one full step. Still you have to state clearly that the image remains better than that provided by 1.8/50 Canon and Nikkor constructions. Their deformations were far more noticeable.

Center, f/1.8	Corner APS-C, f/1.8	Corner FF, f/1.8

Center, f/2.5	Corner APS-C, f/2.5	Corner FF, f/2.5

Starry sky proves to be even more ruthless challenge for the tested lens. Only after applying the f/3.5 aperture the stars in the frame corners become point-like; still even after stopping down the lens so significantly the images of very bright stars feature residual coma effects. It tallies well with our conclusions from the resolution chapter where we suggested it might be necessary to stop down to near f/4.0 in order to get fully useful images across the whole frame.

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A7R II, JPEG, f/1.8, lewy górny Corner kadru 1:1

Sony Carl Zeiss Sonnar T* FE 55 mm f/1.8 ZA - Coma, astigmatism and bokeh

A7R II, JPEG, f/2.5, lewy górny Corner kadru 1:1

A7R II, JPEG, f/3.5, lewy górny Corner kadru 1:1

The photos of the sky, downloadable from sample shots chapter, show another aberration mentioned before, the longitudinal chromatic aberration to be more precise.

The optical construction of the lens deals with the astigmatism well. That aberration, understood as an average difference between horizontal and vertical MTF50 function values amounts to 5.6% so a level considered by us as low.

It would be difficult to criticize any out-of-focus areas, generated by the lens – they are very pleasing to the eye. The appearance of defocused light circles proves that much: they feature no significant local extremes, even the rim which often appears on stopping down the lens, is of very low intensity.

Still mechanical vignetting should be mentioned at this point. The circle in the frame corner is noticeably truncated. What’s more, that truncation remains perfectly visible even on stopping down the lens by two full aperture stops.

Center, f/1.8	Corner APS-C, f/1.8	Corner FF, f/1.8

Center, f/2.5	Corner APS-C, f/2.5	Corner FF, f/2.5

Center, f/3.5	Corner APS-C, f/3.5	Corner FF, f/3.5