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Flaky autofocus

Focusing Overview

Accurate focus is the dominant factor in whether an image is sharp or not. It’s only when spot-on focus is achieved that the megapixels and lens quality become relevant, so accurate focus should be a key concern. Stopping down is not a fix for sloppy focus.

Certain subjects demand pinpoint focus

Consider for example, a portrait, where the eyes are all-important: sharp eyebrows or nose hairs fail us; the focus must be precisely on the eye, on the surface of the eye, not slightly in front of it or behind it, because “acceptable blur” means no blur when it comes to eyes. Yet a similar-sized non-portrait subject might have no single point of attention that is so critical as an eye, so the demands are somewhat lower for focus accuracy there— it all depends.

Stopping down is not adequate

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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.