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EXCERPT page containing first few paragraphs. 2023-02-04 08:00:10

Testing for Field Curvature

To test a lens for field curvature, think “center / mid /edges / corners”.

Although in theory field curvature should be most easily noticed wide open (where there is the least depth of field), the presence of lens aberrations often confuses the issue, so it can be helpful to stop down 1-2 stops so as to reduce the influence of other lens aberrations and vignetting, looking for the zone of focus.

Start with the MTF chart if available

The MTF graph below is at f/2.8— two stops down from wide open, and it still shows quite a “wave”, due to field curvature. It means that at f/2.8, you will not take a sharp picture of a group of people lined up, or anything like the front of a building! You will have to stop down more.

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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.

Aperture series 1.4, 2.8, 5.6 available in full article
MTF at f/2.8 for Leica 35mm f/1.4 Summilux-M ASPH 2010 design

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