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Internal Dust 'Eyeball' Bokeh

Some lenses will replicate a shape into two or more parallel shapes. Looking at this type of background can make one go cross-eyed! And it makes the image look very busy, though the effect can sometimes add an interesting liveliness to the image. Whether or not it’s visible depends on the degree of enlargement.

Example — Leica 75mm f/2.4 Summarit-M

The “eyes” (small concentric rings within the large blur circles) are some kind of wave front disturbance most likely caused by a transparent dust particle sitting on some inner surface, so they cannot be removed or cleaned (the lens was brand-new).

Observe that the “eyes” do not change with stopping down and a diffraction ring pattern becomes even more noticeable. But as the lens diaphragm closes down, the portions of the lens with the dust particles is blocked, and the eyes disappear (at least for this crop).

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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.

Aperture series 2.4,2.8,4,5.6,8,11,16 available in full article
Wavefront disturbance
f2.4 @ 1/30 sec, ISO 200; 2015-03-15 12:00:36
LEICA M (Typ 240) + Summarit-M 1:2.4/75 @ 75mm

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