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Blur by Focus Shift
Focus shift is explained in detail in the Focus Shift section.
Focus shift results from a “smeared” bundle of light rays. When the lens is stopped down, the peripheral rays are blocked, so the remaining rays appear to shift focus rearwards behind the apparent best focus at wide open apertures. See the case study of the Nikon 14-24 for an example.
Your best means to counteract the side-effects of focus shift is to focus at the appropriate aperture using Live View, or carefully bias the focus just a bit to account for the shift (tricky to get just right).
Even some experts insist that focus shift is a minor issue— apparently having a cognitive commitment that depth of field is sufficient to neutralize it. That is simply not true.
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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.
