Lens coverage, iris, groundglas

[DraganB]

hi friends,
i am building a new camera right now, practical tests showing image on groundglas up to the edge no problems, but i have read somewhere that the full iris must be visible from the groundglas corner to form the image, i dont understand because the iris is not visible full only partial, the image is there on the groundglas. when i look on the shema how the light travels trough the lens i see that only the oposite edge is needed to form the image.
so practicaly and from my understanding of physics not the complete iris must be visible, do i miss something? in large format books they say full iris??

 

[reddesert]

You don't need the full round aperture to be visible to form an image, but as you move off axis and you see less of the aperture and/or it tilts into a cats-eye shape, you will get vignetting (lower illumination).

I don't know what the CAD rendering is of. If you have a prototype camera and lens mounted, you can remove the groundglass and look through the corners of the film gate to the rear of the lens to see if anything is vignetting (camera body, lens hood, etc). This is one reason that view cameras sometimes use a ground glass with clipped corners, so you can look through the clipped corner at the lens.

 

[Vaughn]

Light is entering the lens from many angles, perhaps that comes into play. In most cases it is not a matter of losing the image in the corner, but having less illumination in the corners and/or loss of sharpness.

I have cut corners on the GG of most my cameras. Once I get focus and the movements basically set-up, it is easy to close the lens down until I can see the entire aperture of the lens from the cut corner/s. This gives me the max aperture I can safely use...or I might make change my use of movements/focus.

For example, the Fuji W 360/6.8 on my 11x14 and focused near infinity with no movements, I have to close down to f16 to see the entire aperture from a cut corner. I have seen reduced image quality otherwise.

And I can easily see an aerial image in the corners, so I know what they'll look like on the negative.

 

[bernard_L]

1. When on-axis ("focal point" on your sketch) there is generally one internal lens aperture of the lens that limits the extent of the beam: that is the pupil, circular or near-circular.
2. Moving off-axis, there will generally be one or more other internal apertures that overlap the abovementioned, and also limit the beam; the aperture as seen from the focal plane has a crescent shape (like a moon between full and quarter). This is normal, and most lenses show corner darkening at the largest aperture settings.
   In other words it is a property of the lens, not of your camera design, unless you are exceeding the design field of view; see some of Atget's photos.
3. The lens's aperture may also be obstructed by "foreign bodies" like, e.g. a sagging bellows. Maybe that was the intent behind what you read: "but i have read somewhere that the full iris must be visible from the groundglass corner to form the image"

The full aperture was not visible from the bottom corners (top of scene) of the groundglass. This corresponds to case #2 above.

https://www.moma.org/artists/229-eugene-atget

 

[DraganB]

this is the prototype, goal is to support lenses up to 105mm and cover 6x8 the prototype is showing the full 6x9 image on the goundglas 1mm more then needed so 6x8 should be good, i just saw on the internet the thing about full and round iris from clipped edge and was unshure because i dont see the full iris.

next i am just going to mill the final version and test with film.

 

[Dan Fromm]

To some extent coverage is, um, subjective. The lens projects a circular image. The diameter of the circle covered is the diameter of the image of acceptable quality. Acceptable quality is your choice.

You want to use a pre-WW II 10 CM/2.9 Xenar. I wouldn't use it on 2x3 (6x9 is a metric approximation) but it might be good enough for your purposes. The only way to know is to try it out.

Do you know which camera the lens was originally mounted on?

It appears that your ground glass will be mounted with the ground side facing the photographer. This will put the focused image out of register with the roll holder's film plane. The GG's ground side should face the lens.

Don't forget to darken the camera's interior. If you were in the US I'd suggest Krylon Ultra Flat Black spray paint. I don't know if it is available in Switzerland.

 

[Ian C]

Expanding on post #3,

Photography—Its Materials and Practices

C. B. Neblette 1945

D. Van Nostrand Company: Princeton, New Jersey, Toronto, London, New York

Page 128

"The field of view of a lens is circular in character, and this may be seen if a lens of relatively sort focal length forms an image on a large ground glass. The larger circle, as seen in Fig. 7.13, is often called the circle of illumination. It is limited in size because, since corrected lenses are thick, light beyond a certain angle is cut off, or vignetted, by the lens mount. The circle of illumination is almost always larger in diameter than the circle of good definition, sometimes considerably larger. The size of the circle of good definition depends upon the design of the lens."

See also Circle of Good Definition, Camera Craft 1918, page 239-240

https://books.google.com/books?id=EWAVAAAAYAAJ&pg=PA240&lpg=PA240&dq=circle+of+good+definition&source=bl&ots=x-InYS-Jhv&sig=CaOdMFfFaGekPU3HqlQadu6qQeo&hl=en&sa=X&ei=jUStVMylJ9HhsASJ84HoDw&ved=0CEQQ6AEwBg#v=onepage&q=circle of good definition&f=false


Lenses in Photography
The Practical Guide to Optics for Photographers

Rudolph Kingslake

Director of Optical Design
Eastman Kodak Company
Garden City Books, Garden City, New York, Copyright 1951

Page 7: “Every lens projects light into a circular field which is limited in size by the vignetting or cutting off of oblique pencils by the lens barrel. However, in very few lenses is the definition sharp to the extreme limit of this circle of illumination. Since good definition is required in any a practical application of the lens, it is customary to state the field of a lens in terms of the angle over which good definition is obtainable (Fig. 7). This angle generally increases somewhat as the lens is stopped down to a smaller aperture.

Since most photographs are taken on a square or rectangular film area, it is necessary that the film format fit into the circle of good definition of the lens. Thus, the diameter of this circle must be equal to, or greater than, the diagonal of the film.”

This is the reason for recommending that the full aperture be visible from the image plane as a minimum requirement for a suitable lens.

 

[DraganB]

To some extent coverage is, um, subjective. The lens projects a circular image. The diameter of the circle covered is the diameter of the image of acceptable quality. Acceptable quality is your choice.

You want to use a pre-WW II 10 CM/2.9 Xenar. I wouldn't use it on 2x3 (6x9 is a metric approximation) but it might be good enough for your purposes. The only way to know is to try it out.

That's right, my goal is to fill the negative with the image. It's more important to me that something is on the negative than how sharp it is—at least regarding the edges. The negative should have an evenly illuminated image; sharpness only in the central area is even good for my purposes. I don't take landscape shots with it, but photograph tree roots and trunks from very close up. The Xenar 100mm/f3.9 was on a Welta Perle, a 6x9 or 2x3 camera. I plan to use it on 6x7/6x8, where at f/11 everything will be sharp even to the edge. The Welta was unfortunately in very poor condition, but the lens is like new. That's why I'm building the camera—I have many old folder cameras with unusable bodies but top lenses, so a universal body to use all my great character lenses from 75-105mm.

Do you know which camera the lens was originally mounted on?

It appears that your ground glass will be mounted with the ground side facing the photographer. This will put the focused image out of register with the roll holder's film plane. The GG's ground side should face the lens.

The prototype body is 37mm deep. I flipped the ground glass to simulate 2mm more depth, to check the image circle size—and it helped. Believe it or not even the Xenar 75/3.5 covers an "image" to the corners of the marked 6x9 area. I plan to use this lens with a 6x8 back for street photography. You know only the key subject in the frame needs sharpness. Corner-to-corner perfection matters only in landscapes and document reproduction.

Don't forget to darken the camera's interior. If you were in the US I'd suggest Krylon Ultra Flat Black spray paint. I don't know if it is available in Switzerland.

thats what i am going to do.

next weekend i am going to mill and test the kamera i will post some high res scans of what i get.

 

[Hassasin]

Im not sure what the problem is. All lenses have their spec sheet to show what they cover. Coverage is not as much about focal length as it is about optical design.

 

[gary mulder]

The pictures should speak for themselves.

 

[Dan Daniel]

I don't take landscape shots with it, but photograph tree roots and trunks from very close up.

As you focus closer, i.e. extend the lens farther from the film plane, the coverage circle grows.

 

[Dan Fromm]

Im not sure what the problem is. All lenses have their spec sheet to show what they cover. Coverage is not as much about focal length as it is about optical design.

Spec sheet? Finding spec sheets, especially for older lenses, can be a large problem.

The angle covered depends on the lens' design. The circle covered depends on angular coverage and focal length.

 

[Hassasin]

Spec sheet? Finding spec sheets, especially for older lenses, can be a large problem.

The angle covered depends on the lens' design. The circle covered depends on angular coverage and focal length.

I never said coverage does not depend on focal length but since it also depends on optical design proper lens can be found to fit a project

 

[mmerig]

Post #9 is accurate enough. "Coverage is not as much about focal length . . ."

Who would use a 300-mm telephoto lens for a 35-mm camera on an 8 by 10 view camera?