What do you call lenses that can focus like this?

[Frank Petronio]

I'm optically ignorant and have never studied any of this (no silly photo school for know-it-alls), so I am not going to be able to use the correct terminology. But I think most lenses must focus in an arc, everything being in focus at the same radius distance out from the center of the lens. Right? Example in figure 1.

But has anyone ever made a lens that focuses on a flat field, like figure 2?

I think the practical application would be if you were using very wide lenses or close up lenses wide open, where you might lose focus on the edges of some large flat plane, like a tower, shot vertically? Or crowd scenes, horizontally.

[rdenney]

Actually, most lenses are corrected for a flat field. Few aren't--maybe a Petzval or something similar. Not all do so perfectly, of course, but that's a design objective for all modern lenses. Lenses intended for copy work will be most carefully corrected for flat field and zero geometric distortion.

Rick "infinity is flat" Denney

[Michael Clark]

Actually, most lenses are corrected for a flat field. Few aren't--maybe a Petzval or something similar. Not all do so perfectly, of course, but that's a design objective for all modern lenses. Lenses intended for copy work will be most carefully corrected for flat field and zero geometric distortion.

Rick "infinity is flat" Denney

Rick,are you saying that the center field of view of the lens when focused at say 2 1/2 ft the outer field of view will be in focus at 3 1/2 feet? as in Franks example.

Mike

[Frank Petronio]

Oh jeez... Almost 30 years of this and never ran up against it.

OK so if most lenses are flat-field, then when I use my rangefinder to shoot a portrait, and my eye and camera are at waist level, if I use my rangefinder to focus straight ahead, centered upon my subject's belly button - belt buckle area, the lens might be 5-feet away....

And then when I tilt my camera and rangefinder up, to focus on the subject's eyes about 24-inches higher, the distance between the lens and the rangefinder might be about 6-feet apart.

So if just focus on the belt buckle - 5-feet away - I can trust that the person's eyes, 6-feet away, will also be in critical focus, even if I use a fast lens wide-open, for shallow depth of field?

Or should I focus on the eyes, 6-feet away? Even if the camera is pointed and centered on the belt buckle?

I know, a ground glass and loupe solves this dilemma. So does Polaroid. But it's late.....

[Michael Clark]

Never thought about it till I saw your drawings Frank.

[Oren Grad]

When you're working at close range with fast lenses used flat out, focus-and-recompose can indeed be a problem.

The belt buckle should be fine - if it's really in the same plane as the eyes, and if the lens really does have a flat field. In fact, many small-format lenses have enough residual field curvature to matter. You really do have to get to know your specific lenses. Yes, easier to run a bunch of tests to figure it out if the lens is on a digital camera.

Science fiction comes to life - if you have $$$$$ to burn, the latest generation of Hasselblads will take care of the problem for you:

http://press.hasselblad.com/media/19...cus_phocus.pdf

[genotypewriter]

But has anyone ever made a lens that focuses on a flat field, like figure 2?

If you're referring only to the flatness of the field, it's like what others are saying... most lenses are like that. This is more important for lenses used in projectors than for photography (explained below). You might also want to look up objective plane flatness/curvature vs. image plane flatness/curvature.

I think the practical application would be if you were using very wide lenses or close up lenses wide open, where you might lose focus on the edges of some large flat plane, like a tower, shot vertically? Or crowd scenes, horizontally.

I don't think you'd shoot a tower with a close up (close focus only) lens. Shooting a crowd with a close focus lens might get you a restraining order or two

The curvature of field matters very little in everyday photography. Say, if you're shooting something like tower, you're going to be relatively far away from it (i.e. relative to the focal length) to fit it in the shot. And then you will have a lot of DOF and using a wide aperture won't really provide much benefit to the shot (other than perhaps an aesthetic effect)... so you can stop the lens down... and then you get even more DOF. So at the end of the day, the rate at which DOF increases, as the subject distance increases, helps in minimising the negative effects of field curvature.

The exception to this is in astrophotography where field flatness has to be specifically corrected for in telescopes. Why DOF doesn't take care of things here is a bit beyond... me. Perhaps it has to do with extreme magnifications. Haven't really looked in to it, to be honest.

Coming back to Earth, field curvature can mostly be a problem if you're shooting something really up close without a good close focus/macro lens that is optimised for such distances. E.g. when you're taking a picture of a coin or a stamp at a high magnification using an ordinary lens, etc.

GTW

[rdenney]

Rick,are you saying that the center field of view of the lens when focused at say 2 1/2 ft the outer field of view will be in focus at 3 1/2 feet? as in Franks example.

Yup.

Rick "noting that large-format lenses tend to be better corrected for field flatness than are strongly retrofocus lenses for SLRs" Denney

[Frank Petronio]

Thanks. I realized that in the real world it wasn't impacting my normal photography very often but perhaps with the rangefinder/portrait example it has? Because I didn't know any better after 28 years of shooting?

[rdenney]

And then when I tilt my camera and rangefinder up, to focus on the subject's eyes about 24-inches higher, the distance between the lens and the rangefinder might be about 6-feet apart.

Just as with a view camera, the focus plane intersects the lens board plane and the image plane. When the image and lens planes are parallel, as they are in a fixed-body camera, the focus plane will also be parallel. So, if the image plane is parallel to the plane containing the eyes and belt buckle, and if the lens is well corrected for flatness, both will be in focus. If you tilt the camera (image plane), then maybe not.

The only lens I've owned that shows a strong field curvature is a 45mm Mir 26--a Ukrainian medium-format wide-angle lens. At wide apertures, the corners are fuzzy--unless there is something in subject field in the right place.



Notice how the branches in the upper corners are in focus, even though the focus plane is on the nearby iron fence? That's field curvature. It gets better on this lens as you stop down. I've never seen a hint of that on my Super Angulons or other LF lenses.

Rick "who posted that picture before having a calibrated color management" Denney