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EXCERPT page containing first few paragraphs. 2023-02-04 06:30:54

Zeiss 35/2 Biogon at f/2
(typical messy point spread)

Point Spread Function

The point spread function describes what happens to a point of light imaged by the lens.

Especially with fast (bright aperture) lenses, source points of light bear little resemblance to a point when imaged by the lens at wider aperture, such as f/1.4 or f/2, the effects generally increasing in off-center areas.

Even very well corrected lenses have trouble with a plethora of aberrations, which all combine to produce a blob that is far from the original point.

While the examples make it easy to see the flaws, with normal photographs the misshapen blobs simply overlap to reduce contrast and detail, and are not otherwise directly visible. Night shooting reveals such behavior.


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  • 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.

Zeiss 35/2 Biogon @ f/2 (left), and stopped down to f/5.6

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