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Blur Caused by Lens Optics
Optical blur is caused by a lens problem— the lens itself is decentered or misaligned in some way. Typically, this results in one side or corner of the image being “soft” as compared to the others.
The key factor to look for is symmetry: all lenses whose elements are accurately ground and properly assembled will show symmetric edge and corner sharpness, across both focus and zooming operations. However, low quality lenses can suffer from lack of precision in the gearing, etc, such that a lens might show asymmetry from that aspect of quality. Even high quality (expensive!) zooms can be at fault.
Normal optical blur
Optical blur is expected with most lenses due to uncorrected aberrations, which are most pronounced wide-open. A highly-corrected lens minimizes these aberrations, whereas a less well-corrected lens requires more stopping down to achieve similar control. A 50mm f/1.4 lens generally needs to be stopped down to f/4 to reduce all aberrations to a minimum. The sum total of aberrations is the point spread function.
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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.
Nikon 300mm f/2.8G VR II — this is NOT focus error