I understand that field curvature makes your picture edges and corners appear out of focus and thus somewhat soft. This is understanable because our lens glass is curved not flat. I'm wondering how this is corrected on better more expensive lenses than cheaper ones? Do ED and Aspheric elements come in to play or are they just for chromatic aberration? I understand that smaller apateures help reduce this so are bigger barrel lenses better? Where do you find it most constant? I find that for me close up shots for example at a building @28mm causes field curvature. I understand also that some lenses do this purposely such as soft focus for portrait work. All lenses have some field curvature just wondering how and why some are less appearent?
Merry X-mas all and happy new year!
#1. "RE: Field Curvature" | In response to Reply # 0frankie Basic MemberSat 23-Dec-00 12:42 PM
Aspherics come in to play mainly for Field Curvature issues. It allows the lens maker to reduce the amount of corrective lenses within the barrel. They do this by grinding a profile of two or more seperate lens shapes into one lens. Part of the lens helps to reroute oblique rays and others work for rays of light entering in other ways.
Lens designs, in order to work, usually have to have a meniscus design in them somewhere, otherwise they'd be standard glass sheets...
So, you could say that an aspheric, at the most basic level could be used as the rear-most element to realign oblique rays so they all focus on the same plane of the film.
On paper, this works magically, but it's a little harder to predict on the ground glass itself.
You also have to factor in the type of glass (crown vs. flint vs. Jena vs. Lanthanum vs. Barium-doped Crown...) It gets really messy.
#2. "RE: Field Curvature" | In response to Reply # 0WAn Basic MemberMon 25-Dec-00 07:33 AM
unfortunately I have no data about lens field curvature, but there is a good link about film curvature:
It is very long but very instructive page. I highly recommend to study it.