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Testing for distortion

Testing a lens for distortion is easy: photograph any building or structure with sufficient angular lines to make it obvious what is going on.

  1. Align the camera squarely (at 90°) to the subject. You really only need the top 2/3 or so of the frame, so ignore the bottom portion.
  2. Shoot one frame at f/8 (for uniform illumination).
  3. Open the image in Photoshop or similar, and examine. Drag a ruler line to assess how the distortion presents in milder cases. For severe cases a rule is not needed, but for mild distortion it can be helpful.

The amount of distortion that is acceptable varies by application. Most users will find distortion up to about 1.5% to be very acceptable. At 2% and beyond, it becomes quite noticeable with the “wrong” subject. Zeiss and Leica provide distortion graphs, Nikon and Canon do not, but you can find disortion graphs at other sites, such as dxomark.com.

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Diglloyd Making Sharp Images articulates years of best practices and how-to, painstakingly learned over a decade of camera and lens evaluation.

Save yourself those years of trial and error by jump-starting your photographic technical execution when making the image. The best lens or camera is handicapped if the photographer fails to master perfect shot discipline. High-resolution digital cameras are unforgiving of errors, at least if one wants the best possible results.

  • 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.

Pronounced barrel distortion with Nikon 16-35mm f/4 VR
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