Field Curvature: Rokinon 135mm f2 FF lens + Viltrox 0.71x booster

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Larry Rexley Contributing Member • Posts: 615
Field Curvature: Rokinon 135mm f2 FF lens + Viltrox 0.71x booster
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Today I tested a Rokinon 135mm f2 lens (FF, EF mount) with the Viltrox 0.71x (Canon) EF - EOS M2 speed booster.

The results are so interesting I'm posting them here.

I've always felt that the Viltrox speed booster introduced significant field curvature to boosted lenses, which didn't fully settle down until about f4 - f5.6 or so. Wider than that and images had 'soft' corners if the corner detail was at the same distance as the center subject.

With the Rokinon 135mm lens at f2 lens with the Viltrox booster, I found that most of the frame from the center is nicely sharp, and interestingly, the corners can also be nicely sharp if the detail is around the same distance or even closer than the center subject. Even more interesting is that there's a small zone between the center and the edges that is slightly unsharp and shows some chromatic aberration.

The depth of field of the Rokinon is very thin at f2.0 --- the optical properties and field curvature of the Rokinon seem to complement the Viltrox booster. In other words, they play well together.

This settles down when the Rokinon lens is set to f4 on the barrel, where the whole image is sharp at the same focus distance.

The result is that the Rokinon is usable (for some images) with the booster at f2.0, resulting in a ~100mm f1.4 combination! I have tried the booster with many lenses, and haven't seen another lens that gives sharp edges and corners until they're stopped down to at least f4.

Here are four uncropped, full-res images which show this. All were shot with the 135mm Rokinon f2.0 lens wide open at f2.0, mounted on the Canon M6ii with the Viltrox 0.71x EF - EOS M2 speed booster. All processed with DxO PL5, images sharpened using unsharp masking, CA applied, Deep Prime de-noise at luminance 20. No distortion or vignetting corrections applied, and it appears none were needed.

Note that each image has detail at many different distances, and due to the extremely shallow f1.4 depth of field, it isn't expected that everything will be sharp, of course. What is surprising is images with any sharp edge/corner detail with the speed booster at f1.4 equivalent.

Other testing showed that with my Kenko 1.5x SHQ (MC5) and Kiron 2x MC7 teleconverters, I could get sharp results with good contrast across the frame when the Rokinon 135mm f2 lens was stopped down to f4 and narrower. Wider than that the corners were slightly smudgy and showed uncorrectable CA.

 Larry Rexley's gear list:Larry Rexley's gear list
Nikon D700 Canon EOS M6 II Canon EOS M50 II Canon EF 50mm F1.8 II Nikon AF-S DX Nikkor 35mm F1.8G +7 more
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