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Delay in Depth of Field by Double-Image Bokeh

The circle of confusion is a useful conceptual model for image definition, but most lenses have multiple aberrations that together produce misshapen blurs and haloes, diverging colors, and often in highly corrected lenses, double images. These “blur circles” are not circular shapes at all, and while the diameter of a circle enclosing the mangled point can be given, it is necessarily large then a computed circle of confusion for a perfect lens, hence real (actual) depth of field is necessarily less than that of theoretical.

Double image bokeh

A lens producing a double image cannot realize useful perceptual gains in real depth of field until the two images converge into one; the circle of confusion is the area that includes both of the double images. It’t not that this area is necessarily larger than a single (non doubled) image, but the effect is certainly unpleasant in bokeh terms. And with most lenses, there seems to be a substantial delay in real gains in depth of field associated with this double image effect.

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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 2.8, 4, 5.6, 8, 11, 16 available in full article
Double image bokeh
Rolleiflex Schneider-Kreuznach AF 50mm f/2.8 Super Angulon PQS

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