Flint-Hill
Visualizing field curvature
Field curvature is an undesirable property of photographic lenses in which the center and borders can't be brought into sharp focus at a single focus setting. Lenses of simple design focus sharply onto a bowl-shaped surface. (See first comment below for an illustration.) They misbehave when we ask them to focus their images on a flat sensor or piece of film.
A lot of smart people, mostly with German and Japanese names, worked from the 1880s to the 1950s to perfect multi-element lenses that could project a sharp image onto a flat surface. We've now come to take flat-field lenses for granted.
I just posted an article at dpreview.com on field curvature with some images that demonstrate how it works. I did the experiments using a Fujian 35mm f/1.7 CCTV lens that exhibits the worst field curvature I've ever encountered. It's a $25 lens that makes dreamy-looking portraits with a sharp center frame. It's a huge bargain if you don't care about the borders being in focus.
Visualizing field curvature
Field curvature is an undesirable property of photographic lenses in which the center and borders can't be brought into sharp focus at a single focus setting. Lenses of simple design focus sharply onto a bowl-shaped surface. (See first comment below for an illustration.) They misbehave when we ask them to focus their images on a flat sensor or piece of film.
A lot of smart people, mostly with German and Japanese names, worked from the 1880s to the 1950s to perfect multi-element lenses that could project a sharp image onto a flat surface. We've now come to take flat-field lenses for granted.
I just posted an article at dpreview.com on field curvature with some images that demonstrate how it works. I did the experiments using a Fujian 35mm f/1.7 CCTV lens that exhibits the worst field curvature I've ever encountered. It's a $25 lens that makes dreamy-looking portraits with a sharp center frame. It's a huge bargain if you don't care about the borders being in focus.