lens coverage vs. f-stop

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Hi all:

How/why/is the image circle of a lens affected by the stop (aperture size)? Is the stated IC given by the manufactuer always at a specific aperture? Is the difference in IC between the largest aperture and smallest very significant and is light fall-off more dramatic with a bigger/smaller aperture?

Thanks everyone, have a great day!


-- Matthew Hoag (hoagm@bostonpizza.com), February 03, 2002


Hi Matthew,

This is actually one of those experiments anyone can do at home and quickly disprove the conventional wisdom, which is that image circle gets larger with a smaller aperture.

What happens when you stop down is the illumination becomes more even across the image circle, not that light gets thrown farther out to the corners than it does with a larger stop.

If you put your lens on your camera and open all the way up, then look through the back of the camera with the ground glass off and watch the shape of the aperture as you move your head away from the center, you will notice the shape of the aperture (relative to your eye) goes from a circle to an oblong shape with sharp corners. It quickly goes from a large circle to a tiny sliver. Do the same thing with the lens stopped down and the same thing happens, but the change is less dramatic across the film plane. It goes from a tiny circle to a tiny sliver, instead of a huge circle to a tiny sliver. Thus, relative to the center of the film, the edges are still getting a fair amount of light with the lens stopped down. Relative to the center of the film with the lens wide open, the edges are hardly getting any light.

So the image circle doesn't get any larger with a smaller aperture, it's just that light falloff doesn't cause such a huge problem.

Also, of course, large format lenses generally produce better images on the edges of the image circle when stopped down. But the image circle itself does not grow with stopping down. Only the "useable" image circle does, and what is useable depends on your own tastes and skills in the darkroom.

-- Erik Ryberg (ryberg@seanet.com), February 03, 2002.

Manufacturers' image circle specs are usually, if not always, at f/22.

-- Chad Jarvis (cjarvis@nas.edu), February 03, 2002.

Matthew: I concur with the prior posts. An example of this illumination fall off is something I have had a lot of experience with. In the late eighties and early nineties, I spent forty days in a helicopter over colorado, shooting aerial scenics from the air for Eastman Kodak for there Kodak Colorama displays in the old Stapleton International Airport, to promote Colorado tourism. I used a Fuji 617 panoram camera. Wide open, the lens had dramatic illumination fall off from the center to the edge. I tested the lens...and found that at f-16 to f-22 the illumination fall off was almost gone. I used Kodak VPH 400 negative film, which was incredibly sharp and most of my exposures were in the 1/250 at f-22 and the illumination fall of was almost non-existant. Kodak blew them up to SIX feet high, by EIGHTEEN feet wide on translite film, with lights behind them. They were incredible and one of the most popular features of the airport terminal. In some cases, Kodak converted the negative film inmage to digital, and then printed the massive transparencies. But even with out the digital conversion, the images were massive and very impressive...and SHARP. Richard Boulware - Denver.

-- Richard Boulware (boulware-den@att.net), February 04, 2002.

If you look at the B&H Photo web site under large format lenses, you will see detailed specfications available for most of the lenses. The image circle is usually quoted at a specific f stop, or in some cases there are image circles quoted for different f stops (for some of the Nikon lenses, among others). I assume these specs come from the manufacturer.

-- Michael Feldman (mfeldman@qwest.net), February 04, 2002.

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