A classic assertion is that a 15mm lens has “huge depth of field”. Well, sort of. But not on a Nikon D800E or other high-res digital camera. Not to my standards.
In bygone days where film curled and warped and had significant thickness and accepted acute ray angles, and resolving power was often limited by grain, and perception of sharpness was enhanced by acutance without actually being all that sharp, one could get away with such stuff with a straight face. But a critical eye with a 10X loupe revealed the truth, as I often found with my Linhof Technorama 617 SIII with its 72mm and 90mm lenses (very wide angle).
Enter high resolution digital. Even with a highly corrected lens such as the Zeiss 15mm f/2.8 Distagon, the choice of focus has a large influence on total sharpness and often with an asymmetric tradeoff: gain a lot and lose a little, or gain a little and lose a lot. Better to think through what matters most in the image and bias to that rather than apply fixed “1/3 in” or similar disaster rules.
Coming soon are two studies that shown near/mid/far focus and how this affects the sharpness throughout the frame. I’ll be posting these in Guide to Zeiss, but also Making Sharp Images: the studies are relevant to using the Zeiss 15/2.8 Distagon in the field but also in general to all lenses (field technique).