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EXCERPT page containing first few paragraphs. 2020-02-21 04:36:04

The Importance of Critical Focus

Today’s high-resolution digital cameras (4 megapixels on up) require exact focus to achieve optimal resolution [see Focus Accuracy at]. Even stopping down to f/8 or f/11 is often inadequate to overcome focusing error, particularly with longer focal length lenses and/or subjects that are relatively close. And for reliable critical sharpness, a tripod is mandatory when shooting at less than 1/2f, where f is the focal length (see The Sharpest Image).

The two crops below (greatly reduced in size—click each for larger versions) were taken with the Canon EOS 5D-IR using the 135mm f/2L USM. The manually-focused shot required a very small focus adjustment, one at the limit of manual dexterity, and it also looked blurred through the viewfinder. Yet the difference in the sharpness of the resulting image is obvious. Stopping down to f/8 or f/11 would recapture much of the sharpness lost by autofocus, but not all. Stopping down is not a panacea for focus error.

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