"Plane" of Focus

Does a lens bring a plane of points into focus or the points at a given distance into focus?

<p>

Sorry if this has been mentioned elsewhere. Jim

Is that lawn out front of your house a single object, "lawn", or is

it individual blades of grass?

Jim:

<p>

The answer to your question is "Yes", depending on the lens, so I'll

try a simple explanation... A lens with field "curvature" will bring

a series of points at a given distance, usually defined by a

spherical or elliptical radius from a given point somewhere behind

the lens, all into focus on the film plane -- a spherical section of

focus if you will. A "flat-field" lens brings all points on a plane

that is perpendicular to the lens and whose center is at a given

distance from the lens in focus on the film plane -- what we think of

as a true "plane of focus".

Thanks, good answer. Now what do we know about Leica lenses, flat

field or curved. The elliptical curvature surprises me by the way.

The answer is again "yes". While I do not consider myself a Leica

lens guru, I understand that some Leica lenses exhibit curvature, and

some are very well corrected for field flatness, however when

compared to the "norm" Leica lenses tend to be very well corrected.

Speaking more from my general experience with lenses, I have found

when shooting three-dimensional subjects, curvature rarely generates

any detectable problem in the final image. However for copy work, or

when shooting any large, flat surface, curvature can become more

noticable, and hence detract from the intended image. Again,

generally speaking, most macro lenses are well corrected for

curvature as well as barrel and pincushion distortion, while faster

lenses are designed to perform best when focussed from about 20X

their focal length to infinity and generally exhibit more of these

characteristics. (And we have not even begun discussing chromatic

abberation!) So, as is usual with anything regarding optics, there

are trade-offs one must weigh and then base their decisions on their

own particular intended use.

Usually the wider aperture lenses have more problems with flat field,

and also shorter focal lengths. I know the 3.5 50mm Elmar is a good

flat field lens, as they were also used as enlarging lenses.

<h3>Field Curvature and Petzval Sum </h3>

The image of a plane from any lens, even a thin lens has a natural

tendency to curved inward, because oblique object is further from the

center of lens then axial object, hence naturally focus closer,

resulting in curved film plane

<p> Hungarian mathemantican Joseph Petzval (1807-1891) found a

mathematical formula to represent this field curvature, the Petzval

sum. <p>

<pre>

(n'-n)

Petzval curvature = sum of --------------

nn'r

</pre>

<p>

Where n' and n' are the refractive index on both sides of a lens

surface, r is the curvature of lens surface.

Various methods are used by lens designer to reduce or elimiate

this field curvature, such as combine high reflective crown glass

with low reflective flint glass.

<p> Petzval sum is independent upon thickness of lens nor airspace,

however there are many other abberations beside field curvature,

changing sum parameters may reduce Petzval sum and increase others

therefore, most lens still has some residue field curvature, except

flat field lenses.

<P> Some camera delibrately to curved the film plane, for example

the famous Complan lens designed by Arthur Seibert, ex-Leitz lens

designer, for the Minox camera used curved film plane to compenstate

for curvature of field, and with the field curvature of field taken

cared of, all the lens parameter can be used solely to reduce other

abberations, resulting in a ultra sharp lens

While I still have you, these wide voigtlanders are said to be not

fisheye. Is the fisheyeness of lens related to this flat field

business?

Jim, fisheye is ultra wide angle lens with barrel distortion, it is

not flat field lens.

Macro lenses are flat field lenses, because these lenses is often used

for photography very flat objects, such as documents, drawings etc

and must have edge to edge sharp focus, therefore the image of the

documents must all be focused on the film plane, with negligible

field

cuvature

A Summicron 50mm lens has a typical Petzval curvature of about

860 mm

Martin:

<p>

To add a bit more confusion, as I understand it the "fisheye" has no

barrel distortion -- it is actually rendering the 3d wide-angle image

correctly onto the smaller 2d film. (And was originally used for

atmospheric study, as it has essentially no light falloff.)

The "rectilinear" wide angles actually have pincushion distortion

designed into them to correct for this optical phenomenon.

While on this subject (which has nothing to do with the original

post): If you project your non-fisheye slides into a screen deeply

curved across its width, which you can make by curving a piece of

poster board in front of the projector, you will see a semi-fisheye

effect in that horizontal lines will have a strong curvature, bulging

toward the top and bottom edges. Now, if you were to project into

the interior of a portion of a sphere (a dome), you would get the

full fisheye effect. Of course, you wouldn't be able to hold focus.

Now enter Omnimax, which does project a rectilinear image into a

dome. As I understand it, they correct the projected image by

projecting through a fisheye lens. Why a fisheye? I think because

light rays traveling backwards (from the film to the screen) would

deconstruct a fisheye image, if there were one on the film. But

since the image on film is rectilinear, the result (I think) is a

projected anti-fisheye effect. When this lands on the dome screen,

the lens thus apparently cancels the fisheye effect of the dome (?).

When sitting in the center of the theater, though, I see outward

bending of verticals, like trees, at the left and right sides of the

screen. So I think the correction isn't perfect.

<p>

Oh, I almost forgot. I understand that the fisheye Omnimax lens was

made by E. Leitz, Canada.

<p>

So what? Nothing. It just seemed like the best chance I would get

to tell about this.

Thanks for your responses. I feel better about my confusion in these

matters. These are not matters that normally concern me when taking

pictures but this nonfisheye business about the new ultra wides

interested (still interests) me. Jim

Jack, fisheye lens has tremendous amount of barrel distorotion.

<p> A 150 degree type I fisheye lens has 65 % barrel distortion.

<p> A 150 degree type II fisheye lens has even more distortion,

74 % barrel distortion

<p> In contrast, a standard lens has less then 1% distortion

Jim,

<p>

Some lenses - especially macros, as Martin has mentioned - are

designed to have a flat field and to bring a plane of points into

focus. The Leica macro lenses work equally well at close range or

infinity.

<p>

On the other hand, long telephotos with relatively few elements, such

as the 400mm f/6.8 Telyt-R, have significant curvature of field. For

the uses to which these lenses are typically put, however, such as a

sharply focused subject against an out-of-focus background, this does

not matter. Also, some lenses that are very well corrected for

infinity can exhibit curvature of field at close range, e.g. the

180mm f/3.4 APO-Telyt-R.

<p>

Regards, Ray

35mm camera's film surface is not abosolutely flat, because there is

a standard 50 micron deep "film channel", thus 35mm film has a

natural tendency to curve inwards. The curl of film has a curvature

of approximately 3 m. This slight curvature compensates some what

field, curvature of lenses. Even macro lens has no need to absolutely

eliminate field curvature. <p> Chrome mounted in slide also has a

natural curvature.

>>

35mm camera's film surface is not abosolutely flat, because there is

a standard 50 micron deep "film channel", thus 35mm film has a

natural tendency to curve inwards. The curl of film has a curvature

of approximately 3 m.

>>

<p>

Martin, you're obviously a very technically minded person, but I find

this statement pretty hard to swallow. The pressure plate pushing the

film against the film rails is machined flat and the film curves in

such a way that its centre line pushes out against this plate, so it

seems unlikely to me - given also the small size of the negative -

that it can have any residual curvature when held in position during

exposure. As I recall, Contax made a vacuum pressure plate to hold

the film absolutely flat - does this mean they had to recompute their

lenses for use with this camera?

<p>

I'm not saying you're wrong, just that it seems pretty counter-

intuitive to my limited intuition.

Rob, film plane architecture is quite standard in most cameras, it

consts of three parts

<ul>

<li> A pair of inner film rails, each about 1.5mm wide, the rails

are

24 mm apart, this film rail pair sits UNDER the film sprokect

<li> A pari of outer guard rails, 36 mm apart, they prevent the film

from

wobbling sideways. The height of the guard rails is 50 micron more

than

the height of the film rail

<li> A pressure plate

</ul>

<p> The presure plate is in contact with the outer guard rails

but never touches the inner film rails, because they are 50 micron

lower.

<p> The pressure plate, the outer guard rails and the inner film

rails form a 35mm x 50 micron tunnel, in which the film slides

through. The film has a 50 micron leeway of moving in the optical

axis direction inside the

film channel

<p>

<pre>

pressure plate

======================================

|| ------------------------------ ||

|| || film || ||

The pressure plate does not presses on the film, but on the outer

guard rails.

Film channel architecture provides much smoother film advance,

otherwise, if the pressure plate presses the film on to the inner

film rails, then the film would be very stiff to move.

</pre>

<p> The purpose of Contax RTS III vacumm sucker is to reduce this

50 micron film channel leeway, thus reduce focusing error. However

I do not own RTS III, so I don't know the details.

AFAIK, there is one camera which does not adopt 50 micron film

channel architecture, that is Minox GT-E 35mm caemra <p>

Minox GTE has four focal plane rails of equal height its pressure

plate presses directly on the film to four focal plane rails.

<pre>

pressure plate

============================

---------------------------

|| film ||

GT-E: four equal height focal plane rails

</pre>

<p> Minox GT-E's direct pressure plate film plane architecture

elimiates this built in 50 micron focusing error in most cameras,

thus enhance the overall sharpness the the lens-camera combination.

My Leica R5 has such a film channel architecture.

<p> I don't know Leica M or LTM has similar film channel.

<p> Come to think of the difficulty in loading my IIIf from the

bottom, I guess, it would be hard to have such kind of fours rails

it would make film loading even more difficult, and in case there is

need to remove film in the middle (without rewind ), the "guard rails"

would make such removal imposssible. I remembe when loading film, I

used to push in then pull out the film several times, if there was

gaurd rails, I would not be able to pull the film out and retry.

<p> So my guess is, LTM may not have film channel, instead it may have

GT-E style film plane architecture. If so, it may explain why my

Elmar 50/3.5 was so sharp

In the previous posts, 50 micron film leeway has already taking in

account of film thickness. Without the film, the the raw height

difference of "guard rails" vs "film rails" is 200 micro; average

color print film or chrome is about 150 micron thick, leaving net

50 microm for the film to move.

<p> For thin base film such as Kodak Technical Pan film, which

has a thickness of 113 micron, leaving a film channel leeway of

as much as 87 micron !

Another factor which affects the film plane flatness is more common

with auto wind camera or camera equiped with motor drive.

<p> This phenomenon is known as film undulation. When motor drive

pull film quickly then stop suddenly, the center of the film may bulge

toward the lens( some time even two bulges ).

<p> Contax RTS III has an elaborate vacuum system for keeping film

flat. However, IMO, it is rather complicated.

<p> Only the other hand, the Minox spy camera uses a simple and very

effective method to keep the film plane flat: It has a flat pressure

plate, which presses directly on the film against a oblong opening (8

mm x 11 mm), and since the film has such a small dimension, there is

practically no film curling. During film advance cycle, the film

pressure plate automatically retracts, thus the film can move freely

with little friction, when the film moved to next frame, the pressue

plate then press again on the film, keeping it flat. Since there is

no such kind of 50 micron to 90 micron deviation of film from "film

plane" there is no focusing error introduced by film plane deviation,

that explain why Minox camera can produce sharp image out

of such tiny negative. It is indeed a high precision camera.

But I thought the Minox used a curved gate to correct field

curvature. (?)

Bob, in Minox A and B, the COMPLAN lens indeed has a curved film

plane, and these camera has a slightly curved pressure plate. In

later models, from Minox C to LX and the current model TLX (titanium)

CLX (chrome ), the lens(name MINOX ) was recomputed to yield a flat

film plane.

<p> My own lens test with Chasseur d'image lens test chart and

observation with Olympus microscope, I found that the 15mm/f3.5

COMPLAN

lens of my Minox B is sharper ( 177 line pair/mm ) vs 163 lpmm of

the later MINOX lens.

In large format photography, a plane object forms a plane image, and

the plane does not necessily perpendicular to the optica axis of lens.

The rules of lens tilt and back tilt is governed by the Scheimpflug

rule and Scheimpflug Hinge rule.

<p> In large format camera lens, Petzval curvature is less a problem

then 35mm, because Petzval curvature is proportional to the focal

lens.