View Full Version : Circle Image per 1 Stop

Peter Hruby
16-Dec-2004, 10:55

I am considering to look after few lenses to my portfolio and after some studying and wondering it came to my consideration the question like:

By how much the image ciecle will increase with increasing by 1 Stop? Consider lenses for 8X10 Dorf.
LF gurus and the rest of us, thread is open for answers...

Many thanks for responses.

Ole Tjugen
16-Dec-2004, 12:55
Some lenses will give a slight increase with stopping down, some will give an increase in the usable part of the image circle, and some will give no change at all. I even have a lens which I sometimes suspect of DECREASING when stopping down.

There is no mathematical formula: All lens types are different, and there are differences between lenses of the same type as well. So I'm afraid your question has no general answer.

Alan Davenport
16-Dec-2004, 14:06
Varies with each lens.

Brian Ellis
17-Dec-2004, 04:50
Ole said: "Some lenses will give a slight increase . . . "

Is that true? I always thought the idea was not that the image circle actually increased but rather that the usable portion increased because stopping down minimizes or eliminates some aberrations. I haven't before heard that the physical size of the circle can increase by stopping down. Not that my failure to hear of it means anything, I'm not a lens expert, but I'm curious. Why would stopping down actually increase the size of the circle?

Ole Tjugen
17-Dec-2004, 05:03
Brian, it's "true" but fairly inaccurate.

In most cases it's a "firming up" of the foggy useless bit, but as the aperture decreases the fraction of light vignetted by the mount decreases. So the illumination evens out too. So when it goes from dark and blurry to less dark and sharp, the coverage increases even if the total area of illumination has actually decreased.

A case in point is the much-maligned 90/6.8 Angulon, which barely covers 4x5" at f:32. At f:90, it should be capable of covering 5x7"!

Dan Fromm
17-Dec-2004, 05:11
To expand a little on Ole's reply to Brian, Schneider says of the 6/2 G-Claron: "Der ausnutzbare Bildwinkel der G-CLARONE beträgt bei voller Öffnung bereits 57°. Er kann durch Abblenden (1 : 22) beim G-CLARON 1 : 9/150 bis auf 64° und bei den anderen G-CLARONEN bis auf 68° gesteigert werden. Der kleinere Bildwinkel bei voller Öffnung ist keineswegs eine Folge zu großer Aberrationen, die erst durch Abblendung verschwinden, sondern vielmehr eine Beschränkung aus Rücksicht auf den unvermeidlichen Helligkeitsabfall." See http://www.schneiderkreuznach.com/archiv/pdf/g_claron.pdf (http://www.schneiderkreuznach.com/archiv/pdf/g_claron.pdf)

Its not a matter of reduced vignetting, but of better control of aberrations. And what grows isn't the circle illuminated, but the circle that contains photographically useful image.



17-Dec-2004, 07:23
It has been my observation that the increase in coverage gained by stopping down is a result of less vignetting in the len assembly. A wide open lens will be significantly vigneted at the edges and thus less illumination. When the lens is stopped down vignetting will disappear and there is now "equal" illumination across the film plane. You can see this effect by looking at a lens from an angle, not all of the aperture at the diaphram can be seen, however when stopped down and viewed from the same position, you will now see the whole aperture at the diaphram. It is very obvious the illumination at the edges will be effected. Of course if you are useing a lens with huge coverage such as a 12" dagor on 4x5 film there is little or no vignetting to begin with so the effects of illumination at the edges of the film will not be nearly as siginifciantly. As mentioned in previous posts there are other gains as well.