What do you call lenses that can focus like this?

[Brian K]

So in your opinion the DOF of a lens is achieved by curving the field of focus? And once you have a flat field lens the DOF is not there anymore? Good grief...

Where did you get that from?

What i'm saying is that if focus follows distance and not just a flat plane regardless of distance, then the use of distance for a DOF scale as the means to determine DOF is flawed, as a DOF scale would then only be reliable at determining DOF on a flat plane and not by the same distances of any other subjects within the photograph.

In the diagram I include, points A and B are the same distance, point C is obviously further away. With a non flat field optimized lens, I would assume that point B should be in focus because it is the same distance from the camera as point A. If it is not in focus then the use of a DOF scale on a lens is a flawed tool.

With a flat field lens, I would assume that the plane of focus has been flattened to allow point C to remain in focus and point B would go out of focus.

[Stephane]

Petzval lens for portrait (@f/4): easy to see the curved field...
Is that a good example?

[jnantz]

frank

enlarger lenses are corrected for flat field and copy work.
not many of the tominon lenses will cover a full sheet of film
except doing close up/copy work, except for the 127 ...
it covers without problem and is flat field in a copal press shutter
that can also be removed from the shutter
and you can replace it with a g-claron, also flat field.

have fun !
john

[Helen Bach]

I agree with everyone.

Field curvature and correction for astigmatism are related - simple lenses can be corrected for astigmatism, but that causes field curvature. Lens designers try to correct for both, of course. They may not achieve perfection! Hence a lens that does have correction for field curvature may have residual curvature, but the plane in focus will not be spherical (with the front nodal point as the centre of the sphere) at all focus distances.

Correction for field curvature is likely to be distance-dependent, because the relative differences between on-axis and extra-axial object-side distances changes with distance (ie it's relatively greater at short distances than long). This means that the plane in focus can be backward curving at long distances and inward-curving at short distances. (It can even be wavy, in fact.)

It's best to learn the characteristics of your lenses at the distances you use them - this comes naturally.

Errors using 'focus and recompose' (which supposes a spherical field) are fairly obvious at wide apertures to users of small and medium format rangefinders.

Best,
Helen

[Steve Barber]

I don't know nothin bout no flat field focus. Maybe he do, but don't go blamin me iffn he got it wrong:

http://web.archive.org/web/200902022...r.com/myth.htm

[aduncanson]

Please pardon my ardor, but of course most photographic lenses are corrected to image a subject plane onto the film plane. Remember the terms used in expressing the Scheimflug Principal, "subject plane", "lens plane" & "image plane"? Thankfully they are not "subject sphere" & "image sphere" - what a nightmare that would be.

There are exceptions, Petzvals perhaps, (I have no experience), very inexpensive lenses, (like the Holga's), lenses designed to image a curved subject, like a cardboard mounted 35mm slide or the face of a CRT, onto a flat screen or film. Also, many wide angle lens designs have a significantly curved field. As in the example provided by Rick "Who usually has something worthwhile to say" Denny. the subject field of these lenses is often convex from the camera's perspective.

I remember buying my first super wide angle lens, a 90mm/6.3 Komura (for a princely sum since this was quite a while ago.) It came in the house and as soon as I could I slapped it on to a camera and pointed it out the window toward the row of trees at my property line. I was sickened by the lack of corner sharpness. Later I read somewhere about the practice of "focusing in" often required with wide angle lenses and I made peace with my Komura.

Since Rick mentioned never seeing this on his Super Angulons, I just checked my 47/8 SA on a 6x9cm camera and could not detect the phenomenon on the ground glass. That does not mean it does not exist.

- Alan

[engl]

That Komura 90/6.3 is a retrofocus design, one of very few such lenses for large format. Non-retrofocus wides, which is the norm for large format lenses, suffer much less from field curvature.

[Michael Clark]

Very nice thread, thanks people for the replies.Thanks for posting Frank, nice cartoon also.

Mike

[GPS]

That Komura 90/6.3 is a retrofocus design, one of very few such lenses for large format. Non-retrofocus wides, which is the norm for large format lenses, suffer much less from field curvature.

In the light of the recent discussion of retrofocus LF lenses I have to ask you - what exactly do you mean when saying - LF retrofocus design lenses? What order of retrofocus do you have in mind?
LF lenses with a "slight" retrofocus effect (several cm) are very common, which contradicts what you say. "True" LF retrofocus lenses (using the scholarly definition) are non existent - which contradicts you too.
Non retrofocus wides suffering less from field curvature? Isn't it just the opposite that the negative front element reduces considerably the field curvature of the lens..?

[Colin Robertson]

Frank,
http://www.rokkorfiles.com/24mm%20VFC.htm
just for fun