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EXCERPT page containing first few paragraphs. 2023-02-04 06:54:32

Sigma 35/1.4 DG HSM A1

Lens Skew (Nikon D800E, Sigma 35/1.4)

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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  • Eases into photographic challenges with an introductory section.
  • Covers aspects of digital sensor technology that relate to getting the best image quality.
  • Technique section discusses every aspect of making a sharp image handheld or on a tripod.
  • Depth of field and how to bypass depth of field limitations via focus stacking.
  • Optical aberrations: what they are, what they look like, and what to do about them.
  • MTF, field curvature, focus shift: insight into the limitations of lab tests and why imaging performance is far more complex than it appears.
  • Optical aberrations: what they are, what they look like, and what to do about them.
  • How to test a lens for a “bad sample”.

Intrigued? See Focusing Zeiss DSLR Lenses For Peak Performance, PART ONE: The Challenges, or (one topic of many) field curvature.

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

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