9

In this answer it is noted that:

many lenses intended specifically for portraits or fisheye lenses, are designed to have a spherical field of focus

What advantages does having a spherical (or otherwise non-planar) field of focus give a lens, excluding that it's probably cheaper to design/build a lens with less correction?

10

The distinct "look" it provides. In addition to adding more pleasing bokeh blur to background objects near the edges of the frame, it also allows objects on the periphery that are in the same plane as the subject when that plane is perpendicular to the optical axis of the lens to be blurred as well, although not to the same degree that background items can.

Some folks want it, especially in portrait lenses where the edges are usually intentionally blurred. Some folks don't want it and prefer a flat field. You pay your money and you make your choice.

Here is a typical image linked at Flickr. In all honesty field curvature vs. flat field wouldn't matter much with this one because there is nothing near the edge of the frame that is in the flat plane perpendicular to the optical axis of the lens at the subject's distance. Here's another. See how the parts of the bench in front of the model but nearer to the edge of the field of view are as sharp as the point of focus while parts of her arms that are the same distance as her face from the camera aren't quite as sharp? Also this one (where the flowers in the same flat plane as the point of focus in the middle but near the edge of the frame are blurrier) and this one (in which the items at the edges are not as sharp as the items in the center, even though all are in the same flat plane that is parallel to the camera's sensor).

Notice that in this one even the raindrops that are the same distance from the camera as the subject are blurred away. In this one the railing in the lower left that is nearer than the main subject is in focus and demonstrates the shape of the front focal plane that is a portion of a sphere.

In contrast, notice how the EF 100mm f/2.8 L IS Macro maintains a flat field of focus all the way to the edge of the frame. It is demonstrated in this one by the tree along the left edge. The subjects hair on the right in this one shows the flat field. With this one, the flat field of the EF 100mm f/2.8 L IS Macro made the shoulder in the lower left of the frame as sharp as the subjects face. If a lens with field curvature, such as the EF 85mm f/1.2 L II had been used instead, the shoulder would have been slightly softened and perhaps would distract less from the subject's face. The same could be said for this one.

Another advantage of using a lens with field curvature is that there is less error in focusing distance when using a 'focus and recompose' technique if the camera is rotated around the optical center of the lens. Most of the error usually introduced with 'focus and recompose' is due to the center of rotation being at the center of the photographer rather than the center of the lens. But even when the camera is rotated around the optical center of the lens, there is more error with a lens having a flatter focal plane than one with field curvature that matches the focal length of the lens.


Update: The two photos included in this question are perfect examples of the types of shots for which a lens with uncorrected field curvature would be useful.

enter image description here

  • I appreciate it's tricky to demonstrate, but is there any way to show the look and how it differs from what a flat field of focus would give? – Philip Kendall Sep 22 '16 at 14:50
  • Since I don't own either the EF50mm f/1.2 L, the EF 85mm f/1.2 L, or the EF 85mm f/1.2 L II I don't own the rights to any images taken using those lenses that I am free to share. I've rented 2 of 3 of them at times in the past but due to the agreements under which the the images I shot with those lenses were produced I don't feel comfortable using them here sans a model/client release. The links will have to do. I'll try to dig up some links to comparable flat field lenses and add them as well. – Michael C Sep 23 '16 at 3:11
  • Additional links included to show the different characteristics of flat field versus field curvature. The number of links used should insure that not all of them will be dead in the near future. – Michael C Sep 23 '16 at 3:48
  • 'Most of the error usually introduced with 'focus and recompose' is due to the center of rotation being at the center of the photographer rather than the center of the lens'. Why do you think this? If I draw both in a diagram, the error due to the flat focal plane is many times bigger. – Orbit Jan 5 at 22:38
  • @Orbit Only a pinhole camera has a perfectly flat focal plane. The best "flat field" lenses that are highly corrected for field curvature have a field of focus shaped more like a lasagna noodle. lensrentals.com/blog/2016/09/fun-with-field-of-focus-part-1 and lensrentals.com/blog/2016/11/… – Michael C Jan 5 at 23:12
5

Theoretically, if you "focus and recompose" to keep a specific point of interest in focus - the eyes in a portrait, for example - only a spherical field of focus will keep the focus where you intended. If the field of focus is flat, recomposing will cause the focus point to move behind the point of interest. The further away from the initial point of focus the camera axis moves - the greater the focus error.

Whether this effect would be discernible in an image would depend on distance to subject, depth of field, focal length, lens quality, etc., etc.

error in focus with flat focal plane and focus recompose

In this example the camera is initially pointed at the point of interest (along the red axis) and focused. The camera is then recomposed along the green axis. At this point, with a spherical focal plane (blue), the focus at the point of interest would be "correct". With a flat focal plane (grey), the focus point would move behind the point of interest by the distance marked in yellow.

  • 2
    What you say is correct, but I doubt that's a primary consideration for why most shooters who choose to use such a lens do so. With multi-point AF systems that cover much of the FoV and live view focusing that covers the entire frame, focus and recompose is no longer needed to near the extent it was when manual focus aids were only in the center of the viewfinder. – Michael C Sep 23 '16 at 3:14
  • 2
    Agree with Michael. More than the multi-point AF comment, there's the problem that the exact shape of this curvature varies wildly between lenses and the chances of the trigonometry actually matching up for this to work are astronomical. – Lee Saxon Sep 23 '16 at 11:53
  • Thanks for confirming my answer is correct @MichaelClark – dav1dsm1th Sep 23 '16 at 15:20
  • 2
    @dav1dsm1th How many lenses have the truly spherical field of focus which is required for focus+ recompose to work? – Philip Kendall Sep 23 '16 at 20:36
  • 1
    @dav1dsm1th Well, just because they're not perfectly flat doesn't mean they're perfectly spherical either. They're usually an irregular wavy shape somewhere in between. But please continue to think my (perhaps incorrectly perceived by you) stamp of approval regarding how well your answer actually answers the question that was asked is such a valuable badge of honor. – Michael C Dec 16 '16 at 20:47
1

Fisheye lenses are their own can of worms because they also feature extensive intentional optical distortion, but as to portraits the answer is as simple as depth of field.

If the plane of focus curves toward the camera (away from the background) toward the edges of the image, then the background will be thrown further out of focus, giving more isolation to the subject.

Of course the limitations are (a) obviously that this can be a severe disadvantage for many other types of photography and (b) more specifically, that you have to put your subject in the dead center of the frame (though most cameras these days have plenty of resolution for photographing the subject dead center and just cropping however you want).

  • It also means objects on the periphery that are in the same plane as the subject when that plane is perpendicular to the optical axis of the lens can be blurred as well, although not to the same degree that background items can. – Michael C Sep 23 '16 at 3:17
  • 1
    That's a good detail to add; that can actually be more important. Unless you have severe space constraints you've probably got your background at least somewhat out of the focus plane. Closer objects - at middle distances or even at/near the subject's focus plane - are probably an even greater risk at distracting viewer focus. – Lee Saxon Sep 23 '16 at 11:44
  • 1
    With objects that are slightly nearer than the subject, the field curvature brings them into focus whereas a flat field lens would leave them slightly blurred. You're trading a flat plane for a portion of a sphere. At some distance any objects on the edge at that distance will be in focus. – Michael C Sep 23 '16 at 11:46
-5

The typical camera lens is optimized to obtain a superior rendering of the different planes of the image, rather than being optimized to image a single plane. In other words, image a 3 dimensional vista and project that image on the flat surface of film or digital image sensor. Conversely, the graphics industry uses “process” lenses for copy work. A process lens is optimized to image flat to flat. The enlarger lens and the micro lens are also optimized to image flat to flat. In the case of the macro, most subjects for close-up imaging are shallow as to their depth. We are talking coins, postage stamps and the like. Thus the macro is likely optimized flat to flat. On this subject, a standard camera lens is often limited as to how close it can image. We mitigate this by using extension tubes or rings or bellows. These attachments allow close focusing. Additionally, we can reverse the standard camera lens, pointing the back side at shallow subjects. This technique yields improved crispness. This is because the back focus of the standard lens is optimized to work on a flat surface.

  • I'm struggling to find the bit in this infodump which explains the advantage of a curved field of focus. I know what extension tubes, rings and bellows are for, but that's just not relevant here. – Philip Kendall Sep 22 '16 at 15:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.