Spherical aberration and leica.

[luke_neher]

I'm going to be brief, hopefully. I assume you know what spherical aberration is and its effects on

focus.

I use an m6 and a cron DR. Now, assuming that the DR is corrected for SA, i should get a strait line of

focus across the image, roughly at least. Meaning that things are in focus if their distance from the

imaginary perpendicular lines running out of the sides of my lens is the same distance i focused for.

This said:

i have persons face that i want sharp at f 2.8

I also want them aligned in the left of frame.

Say that their face is 5.5 meters from the lens

And also say that the straight line of focus (that i talked about earlier) that would intersect with their

face is 5 meters away.

Where do i focus? 5 meters, or 5.5 meters. This is assuming my lens is corrected for SA.

Please shed some light.

This becomes a problem with rangefinders as i can only focus in the center, then recompose, i cant

compose, then twist till they are sharp, as i could with an slr.

IF you need more explanation, i could do a diagram, but this is for people who know optics, and i tried

to be concise.

thanks

[ronald_moravec1]

If I understand you want to focus in the center, swing the camera so the subject is now off center, and have it remain in focus.

 

No it will not remain in focus with a Leica or any camera with center focus aid because the distance from camera to subject changes with the swing.

 

Imagine a camera at waist level and tipped up to focus on a face and then tipped down so the lens axis is now horizontal. The distance changes to the face. It would have been better to focus on the subjects belt and not recompose.

 

This is not spherical aberation. SA occures when different colors do noto all focus on the same plane at the same time. Lenses corrected for two colors are called anistigmats and if all three colors are corrected to top standards, Leica calls them APO or apochromatic.

[winfried_buechsenschuetz1]

What you are talking about are NOT spherical aberrations. Spherical abberations appear due to the spherical shape of (usual) lens elements which is NOT perfect from an optical designer's point of view. Sph.aberrations will NOT cause a non-flat field of view but unsharpness when the lens aperture is opened wide.

 

Concerning field flatness, this refers to planes perpendicular to the lens axis. Everything which is in a plane with a certain distance to the lens which is perpendicular to the lens axis should be sharp. Of course the actual distance of the object depends on the angle the rays coming from this object have with the optical axis but optical calculations always refer to planes perpendicular to the axis.

[charles_stobbs3]

Many (most?) camera lenses have a curved field of focus, that is if you focus at 5 meters everything on an imaginary string 5 meters long from the center of the lens will be in focus. There are flat field lenses, enlarger lenses and architectural lenses for example. You have to know what the characteristics of your lens are.

[helenbach]

"Many (most?) camera lenses have a curved field of focus, that is if you focus at 5 meters everything on an imaginary string 5 meters long from the center of the lens will be in focus."

 

Based on that statement I guess that you haven't wasted any of your time taking pictures of resolution charts or walls.

[winfried_buechsenschuetz1]

It is true that there is more or less field curvature with most lenses. But you never know to which direction this curvature stretches. Anyway all lens designers try to keep the field as flat as possible. Flat-field lenses are better corrected for field curvature but you have to make other compromises, mainly concerning lens power (you probably will not find flat-field lenses with f/1.8).

[frank uhlig]

If you want to have the person's eyes in focus after center focusing and then swinging their image to the corner, center focus a bit before their eyes, such as the tip of the nose, chin. Do a bit of trigonometry and you will see that the difference is not 50cm at all, just a few cm at most.

 

This natural phenom has nothing to do with any words in the title of this post, by the way. It happens for Canon, Contax, Nikon, Oly, ... cameras and is not due to barrel distortion, nor spherical distortinon etc ... It occurs due to the laws of physics and trigonometry which are not too well kept around here, I am afraid ... Sorry.

[vahe_sahakian]

"This is not spherical aberation. SA occures when different colors do noto all focus on the same plane at the same time."

 

There are two major form of aberrations with fast lenses, one is sperical the other is chromatic.

In spherical aberration the central portin of the lens will focus at a slightly longer focal length compared to the edges, the effect can be minimized but not totally eliminated in faster lenses, your best be is to try the new Elmar 50/2.8M, that lens due to its simpler and symmetrical design will give you much better results.

In chromatic aberration different colors focus at a different distance from the focal plane, this is where apo designs come closer to perefection compared to non-apo lenses, the effects of chromatic aberrations are much more pronounced in very long telephotos and that is where the use of ED glasses helps overcome the problem.

[pico]

<i>Based on that statement I guess that you haven't wasted any of your time taking

pictures of resolution charts or walls.</i><p>

Gawd, Bach, you crack me up! The day will come when some critic and museum

gatekeeper will feature a military rez target just to confound the rest of us.<p>

Mr. Neher - centre focus on your subject and then swing the lens aside and I will bet that

your focus will be so very close at 5.5 metres that the difference will not be greater than

the usual focusing error most people make and never notice.<p>

If there is an appreciable error not due to focusing technique, then nudge the focus ever

so little closer - in other words, bracket focus.<p>

I doubt you will go wrong, or waste more time than you have spent speculating about the

problem.<p>

[charles_stobbs3]

If you have to compromise a flat field on the film plane is more important than a flat field plane.

[bobatkins]

You actually need to focus CLOSER, not farther away and the amount depends on the angle of view of the lens. The wider the lens the greater the correction and the closer the subject is to the lens the more focus movement you will need (obviously if the subject is at infinity, you need no correction).

 

You also mean field curvature, not Spherical Aberration.

 

The simple solution to this problem is to use an SLR. There are many things that rangefinders aren't good at. This is one of them.

[bobatkins]

"a flat field on the film plane is more important than a flat field plane"

 

I have no idea what this means. All lenses have a flat field at the film plane if you allow the field plane to be curved, and vice versa. Fixing either one as flat defines the shape of the other.

[canfred]

Hi Luke , this here will help .

http://www.vanwalree.com/optics.html

[Rob F.]

To Summarize:

 

1. When the sharpest focus lies along a curved surface, and not in a flat plane, that is called curvature of field.

 

2. While curvature of field and spherical abberation are both among the seven basic (Seidel) abberations, they are two separate things. This is not to say that spherical abberation could not cause the subject area at the edge of the frame to be softer in focus. It certainly could, if the lens is being used at a wide aperture. SA increases at wide apertures, and it does the most harm toward the edges and corners.

 

2a. The DR Summicron indeed is not as sharp at the edges, wide open, as it is nearer the center--although it's very, very good for a lens of its speed. Luke, you might have better luck at f/4, or f/5.6.

 

3. Lenses designed for use in the three-dimensional world (as opposed to those meant for copying/reproducing material that lies on a plane surface) may not be highly corrected for field curvature. By accepting some field curvature, the designer can improve some other aspect of lens performance that he/she may consider more important.

 

3a. I haven't notice enough curvature of field in my DR Summicron to make me aware of it.

 

 

4. Frank Uhlig noted that trigonometry is involved in the issue of whether to focus on a subject area at the edge of the frame, and then recompose by swinging the camera back to center. I agree: specifically, it causes cosine error, which can actually reduce the sharpness of focus for a lens with a reasonably flat field. It could cause the plane of sharp focus to lie farther from the camera than intended. This must be what Bob Atkins meant when he said you need to focus closer. I would say, just focus on the person in the middle of your group lineup.

 

Good thread! Good contributions!

[kelly_flanigan1]

This is why ancient books had one shooting group shots in an arc.

[MTC Photography]

Luke, the question about how handle off center focus, see thread

<a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=002cCK">off center focus</a>

[MTC Photography]

See also <a href="http://www3.sympatico.ca/mientronics/public_html/Leicafile/recompose.html">Leica M & R Recompose Focus Guide</a>

[james_itard1]

I remember one or two great old LHSA articles by Dick Gilcreast, in which he talked about alternating generations of Leica 35mm lenses. Some were corrected for curvature of field and others that were corrected for other aberrations instead--designers had to make hard choice between one or the other.

 

Here is section from one of the articles(orginally cited by Ray Moth's in this forum in 2001):

 

From Legendary Leica Lenses: The 50mm f/1.4 Summilux" By Dick Gilcreast, published in Volume 31 No. 3, 1998 of "The Viewfinder", quarterly journal of the Leica Historical Society of America:

 

"The [50] Summilux underwent a complete but unannounced redesign apparently in 1962 (Lager) in which the layout of the lens was changed. Still seven elements in 5 groups, the two separate thin corrector elements at the back became a single thick cemented pair while the second and third elements in front of the diaphragm were separated with an air space. Performance of the new lens was quite different, indicating a change in Leitz's basic design parameters. The lens was now quite sharp and contrasty wide open, but this change was brought about at the expense of some increased field curvature and a little less sharpness at the extreme edges of the frame at the intermediate apertures around f/4 and f/5.6. The lens was now more suited to low light wide open work, but less so for the amateur's landscape motifs."

[pico]

Would a picture help? In the case below, the photographer is the blue dot. If he focuses at 5.5 meters 90 degrees to his left, 45 degrees or ANY degree, the subject will be in focus at 5.5 meters when he pivots the camera.

 

No?

[Rob F.]

Pico, your idea would work if Luke arranged all his subjects around the semicircle, <i>and if the lens had a curvature of field that matched the semicircle</i>. But: <i>if the lens has a flat field, and if the photographer arranges his subjects along a straight line, and then focuses on a person at the extreme left or right, and then recomposes to center the camera on the middle person, </i>then the camera will have been focused to a greater distance than it should have been. And where should it have been focused? To 5.5 meters, just as before. The moral: <i>if your subjects are in a flat plane,</i> don't swing the camera over to the edge of the picture area to focus. Focus instead with the camera axis perpendicular to the flat plane of your subject.

[pico]

I read and hear you.

 

If his subject is just to his left or right and whithin his finder and he center focuses on it,

then turns his camera so that the subject is on the edge of the field, the the subect will still

be in focus.