Article Index

Lab Tests

Please note that MTF50 results for APS-C and Full-Frame cameras are not cross-comparable despite the same normalized [0:1] range used to report results for both types of cameras.

 

Resolution: The lens showed mixed performance results in the controlled environment setting. The biggest controversy was the border performance. On an APS-C type Canon Digital Rebel, the lens showed the best corner image quality, but only because of the crop sensor that does not cover the entire imaging circle of the lens. While not particularly sharp at f/3.5, the images taken with the APS-C body did produce higher MTF50 numbers then on either of the full frame bodies used in the tests. To achieve a good resolution across the frame, you really need to stop down the lens to f/8, although f/5.6 should produce fairly  decent results as well. Generally, as one might expect, higher resolution sensors put much more stress on the lens, which is visible from the MTF charts, and so if you're after consistent performance across the frame and the entire aperture range, then you'd better use a smaller sensor camera.

 

Nikon D3 (12Mp)

Nikon D3x (24Mp)

Canon XTi (10Mp)

Canon 5D (12Mp)

Canon 5DMk2 (21Mp)

 

Chromatic Aberration: The lens showed moderate amount of lateral color aberration. CA is more or less uder control in the center, where it ranges from ~0.8px on the wide angle (Nikon D3 and D3x) to a measly ~0.3px at f/11. Border CA however, is where the problem lies - here color fringing hovers at ~1.2px with wide open aperture, and gradually drops to somewhat more manageable ~0.8px by f/11. With these levels, you are likely to see some color fringing in real life photos, particularly at wider apertures.

 

Nikon D3 (12Mp)

Nikon D3x (24Mp)

Canon XTi (10Mp)

Canon 5D (12Mp)

Canon 5DMk2 (21Mp)

 

Distortion: As one might expect from an ultra wide angle, Voigtlander Color Skopar ASPH 20mm f/3.5SLII shows a fairly noticeable barrel distortion. At ~2.1%, distortion will be visible in general type photography, particularly around borders area. Just use PTLens or Adobe Photoshop's distortion correction during post-processing.

 

Distortion

 

Vignetting: Yep, the lens vignettes, but this should not  be  a shock for you. All wide angles vignette to some extent. Given its rather slow maximum aperture, Voigtlander Color Skopar ASPH 20mm f /3.5 shows moderate amount of light fall-off at f/3.5 on full frame cameras. Here, vignetting reaches ~1.1EV, but then drops to a more manageable ~0.6EV by f/5.6. On an APS-C body the light fall-off is still visible but to a much lesser degree - ~0.8EV at f/3.5 and less then ~0.4EV at f/5.6. 

 

Vignetting

 

Chart Crops: Here are 100% crops,  comparing image borders at f/3.5 and f/8

 

Nikon D3 (12Mp)

Nikon D3x (24Mp)

Canon XTi (10Mp)

Canon 5D (12Mp)

Canon 5DMk2 (21Mp)