Lens Coverage at Apertures Smaller Than f22

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OK, I know the real answer is "try it and see", but I still have to ask. If I have a 210mm, f6.3 lens with a coverage angle of 72 deg and a 308mm image circle, is there any way to make a wild ass guess as to what the coverage would be at f32 and f45?

Thanks in advance.

-- sheldon hambrick (shambric@us.oracle.com), June 10, 1999



The Schneider Optics website (I don't have their website address handy, but it's listed on this LF site, under "commercial links") clears up this commonly-held misconception. Once you get to their site, look under "Commonly asked questions about LF lenses", " Image circle increase".

According to Schneider, the physical size of the image circle does not actually become larger as the lens' aperture is stopped down. What changes, or improves, if you will, is how even the illumination becomes across the entire area of the image circle as the the lens is stopped down.

As you reduce the size of the aperture, the illumination across the entire field of the image circle, from the center progressively out towards the image circle's perimeter, becomes more even. The "hot spot" at the center of the image circle at wide open aperture is spread out more evenly across the entire 308mm image circle. This effectively "increases" the usable area of of the lens' image circle, it does not actually physically increase the size of the image circle.

So I can only assume that once you have achieved optimally even illumination across the entire 308mm image circle, at f22 let's say, further stopping down the lens will not actually yield a greater usable area of image circle? That's how I would interpret Schneider's information.

What do you think? Hope this helps.

-- Sergio Ortega (s.ortega@worldnet.att.net), June 11, 1999.

You can see how this works if you have a ground glass without the corners, or if you take the back off and position your eye at the corner. Set the aperture at f64 or whatever. You see the whole aperture, and the light through it. It will be in a circle. Now enlarge it. Pretty soon, if you are at the extreme of your image circle, you will see the barrel start to vignette the aperture and you will be looking at an elongated oval. So the lens is still producing an image here at the edge of your negative, but it is a dim one, produced by only a fraction of the lens.

At smaller apertures than f22 you should have a larger usable image circle than f22 but the difference between f5.6 and f22 is a whole lot greater than the difference between f22 and f64, since the physical dimension of the aperture is not much greater. So I would say the f22 circle is a pretty reasonable one for the manufacturers to choose.

-- Erik Ryberg (ryberg@seanet.com), June 11, 1999.

I'm no optics mayven and I certainly wouldn't challenge Schneider, but, in "Corrective Photography",Lewis L. Kellsy of Deardorff fame has this to say about a 12 inch 6.8 Dagor.....

"The circle of illumination and area of critical definition in this type of lens are very large.....This type of lens in the 12-inch focal length, is listed to cover an 8 X 10 plate at f/6.8, a 10 X 12 plate at f/16, and a 14 X 17 plate at f/45."

Then he says this about a 12 inch Tessar IIB

"In this type of lens, the circle of illumination and the area of critical definition are samller than in the Dagor. The Tessar IIB in the 12-inch focal length, is listed to cover an 8 X 10 plate at f/6.3, and a 10 X 12 plate at f/16. It will not cover larger sizes than 10 X 12 even when stopped to f/45."

Presumably there is a formula out there somewhere which could help Sheldon calculate this sort of thing?

-- Sean yates (yatescats@yahoo.com), June 11, 1999.

Most ground-glasses have a spare-opening at every corner to look through the bellows to the lens directly. If you move your cameraback to an extreme position in the image circle, e.g. to the right and look through these holes you'll see that at every stop down you can move your camera-back more to the right to fill out the format.

-- Lot (lotw@wxs.nl), June 12, 1999.

I guess I'll add my 2-cents worth on this issue as well. As I understand it, the angle of coverage (and therefore, indirectly, the image circle) is determined by the lens design. The cone of light that leaves the lens and is projected on the film is intersected by the film plane and makes a makes a circle of varying size depending on the distance from lens to film and the angle of the cone to start with. This is further complicated by the varying aperture. At a large aperture the film is being illuminated by a rather large "circle" of light, not a point. The more you stop down, the smaller this illuminating circle becomes. If you imagine moving your eye from the center of the image circle to the edge you can see that this illuminating circle only appears as a circle when your eye is at the very center. As you move toward the edge you are seeing the aperture from a greater and greater angle and it becoms an elipse. This accounts for some of the light fall-off at the edges of the image circle. Next you must imagine doing this at a very large aperture. With the very large aperture, as you approach the edge of the image circle part of the illuminating circle will be cut off due to vignetting, i. e., one side of the illuminating circle will not be able to cast light on this part of the image circle. This penumbra accounts for the rest of the light fall-off at the edges. The size of the penumbra (the area illuminated by only part of the aperture) decreases as the lens is stopped down since the outside of the aperture edge moves towards the center as the lens is stopped down. When this "menal experiment" is repeated at a very small aperture, your eye can move farther towards the outside of the image circle before the vingnetting begins. The penumbra has become smaller and begins farther from the center of the image circle. This accounts for the practical increase of usable image area at smaller apertures. The above assumes that the lens casts an acceptably sharp image over the entire image circle which is not always the case. Resolution and sharpness decrease towards the edges and this also affects the usable image area. Sometimes the limits of acceptable resolution are reached before the light fall-off becomes a limiting factor, but in most modern lenses it is the illumination problem that determines the usable image circle size. As can be seen from the above, for any give lens design this is dependant on aperture size. Hope this helps. ;^D>

-- Doremus Scudder (ScudderLandreth@compuserve.com), June 12, 1999.

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