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EXCERPT page containing first few paragraphs. 2023-03-29 18:19:17

Canon Focus Assist (green dot)

This outdoor example using the 21MP Canon 1Ds Mark III shows the severe disagreement between what the eye sees and what the autofocus system sees.

The Zeiss ZE 85mm f/1.4 Planar was used, with the aperture set to f/1.4. The 85/1.4 Planar has considerable uncorrected spherical aberration (by design), thus making focus shift a challenge.

Why the Canon DSLR doesn’t focus accurately

The Canon AF system effectively sees only light rays equivalent to f/5.6.

When an image is made at f/1.4, about 93% of the image-forming light rays are coming from other areas (f/5.6 = 1/16 the area of f/1.4). So the AF system is making a judgment best on only a central ray bundle which is quite different than the ray bundle that will be recorded at f/1.4.

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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.

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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.

Entire frame as shot at f/1.4, along with crops.

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