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EXCERPT page containing first few paragraphs. 2022-05-17 11:01:55

Sigma 35/1.4 DG HSM A1

Lens Skew (Nikon D800E)

Shot with the 36-megapixel Nikon D800E at ISO 100.

This scene is exceptionally useful for evaluating lens errors particularly an asymmetric sharpness left to right— call this “skew”. It is of course not that simple; the skew can be corner to corner or top to bottom, but the overall oddity becomes apparent by using a planar target at distance like this.

A left/right or similar asymmetry is unfortunately very, very common with all brands, even vaunted Leica optics. And generally “within specifications” unless it’s substantial.

Detecting skew

Depth of field can be a confounding factor even at f/1.4 (at least at this distance), meaning that a particular choice of focus can help to hide a problem by balancing/centering the skewed zone of sharpness such that there appears to be little difference left to right.In reality the focus can be such that the left (or right) is blurred slightly, but for different reasons (out of focus versus optical non-planarity, e.g. skew of the plane of focus slightly).

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Diglloyd DAP is DSLR-oriented, but also contains workflow and other topics. Much of the focus is on Canon and Nikon but also Pentax and Pentax medium format.

Special emphasis is placed on lens evaluation, focusing on Canon and Nikon and Sigma lenses, but with a few others like Rokinon/Samyang.

  • Make better images by learning how to get the best results right away.
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  • Optical quality analysis of field curvature, focus shift, sharpness, flare, distortion, and performance in the field.

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Variants Focus @ center, Focus @ left, Focus @ right available in full article

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