Especially with high-resolution digital cameras, full-image sharpness is limited to a narrow zone even when stopped well down, demanding careful attention to detail for best results.
My recent PHOTO Techniques articles have explored how diffraction, focus shift, and field curvature all can lead to blur. The focus shift and field curvature issues can be mitigated by increasing the depth of field, so long as diffraction is held at bay by not stopping down too far.
Depth of field means “the zone of reasonably sharp focus,” with the term “sharp” being both arbitrary and ambiguous. The word “zone” is truer to reality, as “depth” implies a fixed-thickness layer of sharpness at a fixed distance, which is generally not the case once f ield curvature and other aberrations are taken into account. Furthermore, lenses are complex physical objects that have build variances from design specifications that can at best only approach the theoretically best performance. Lenses also vary in rendition, so that two lenses at the same aperture can show strikingly different real-world sharpness.
Circle of confusion
The circle of confusion refers to the blur produced by an out-of-focus point of light. Wide open, there is lens-barrel vignetting causing a partial eclipse of the blur circle (the “cat’s eye” effect). It’s one reason why off-center detail can be sharper than expected—the blur “circle” is smaller than it is at the center.
shaped more or less in a planar fashion; a tall tree sticking too far out of that plane would be blurred due to the tilt. The distant trees in Figure 6 are close to the tilted plane of focus, which extends from the foreground to Yosemite’s Lempert Dome in the distance. They remain sharp, and the whole image is sharp at ƒ/2.8. That’s the power of tilt. Figure 5 shows the test scene at ƒ/8; stopping down would never match the results with tilt.