Light fall off and aperturegreenspun.com : LUSENET : Large format photography : One Thread
I have a Rodenstock Grandagon-N 90mm F6.8 lens and do not have a center filter. I have noticed some light fall off with this lens and 4x5 film. I have not experiemented in a controlled manner at various aperatures, but, was wondering. Are there aperatures in which the light fall off will be the least? Are there aperatures where it is the worst? I generally use this lens at F22-45 and the fall off is minimal. Just wondering if I can optimise the perfomance without a centering filter.
-- Dave Schneider (firstname.lastname@example.org), March 12, 1999
Probably f/22. If it isn't a problem for you, then you can skip the CWF, if you feel it is becoming a problem, try the Heliopan 0.45 (3x) version. Mine works fine on my 90mm f/4.5 Grandagon, especially when I use it for a shifted 6x17
-- Ellis (email@example.com), March 12, 1999.
What would answer your questions best is the 2 page Rodenstock sheet on wide angles and cosine failure and center filters. We can mail you a copy if you like. All wide angles currently made have cosine failure and the use of a various stops won't eliminate it. The more of the full coverage of the lens or the more movements you make the more the effect is notic
-- bob salomon (firstname.lastname@example.org), March 12, 1999.
I've always thought I've understood why wide angle lenses produce uneven field illumination....the light entering at an angle creates an aperature with an ellipse rather than a circle, causing light falloff. Therefore, the need for Center Filters. Why doesn't the same thing occur with medium format w/a lenses? I've also wondered whether the leaf shutter may contribute to uneven illumination particularly at high speeds. The shutter opens at the center (similar to the aperature)and closes at the center. Unlike a focal plane shutter that works like a curtain. Wouldn't the center portion of the film consequently receive more exposure with a leaf shutter? This may be more of a question for someone rather than an answer. Wouldn't longer shutter speeds (thus smaller aperatures) help alleviate this problem? I've never had problems with falloff from both my 75mm and 90mm lenses. But generally my speeds are 1/4 second or longer. Maybe Bob can correct my theory.
-- John Wiemer (Wiemerjo@slcc.edu), March 16, 1999.
You would also like to see Rodenstock's explanation of the fall off.
-- bob salomon (email@example.com), March 16, 1999.
There are 4 normal reasons for light falloff in a lens.
1) one is vignetting: the lens elements are simply not big enough to delivery full illumination over the negative. Put another way, the metal body of the lens partially shadows the corners of the negative. This cause of light falloff can be eliminated by stopping down. Most lens vignet wide open but not when stoped down by about 2 or more stops.
The remaining reasons come for optical laws. Define theta as the angle between the center of the lens and a spot on the negative, measured with theta = 0 on the optical axis of the lens. On most lenses (when stopped down) illumination goes as cos theta to the power of four. 2) two powers come from the light traveling a greater distance and the inverse square law, 3) one power comes from the exit pupil of the lens being tilted and looking like an ellipse rather than the full circe, 4) another power comes from the rays striking the film at an angle [the same reason for winter: the sun's rays strike the earth at an increased angle].
Wide angle lenses use an optical trick (the Slussarev effect) to improve the falloff to something like cos theta to the power of three.
The short answer is that the best aperture re light falloff is any aperture more than about 2 stops (maybe 3) down from wide open. Further stopping down should not change the relative illumination, center to corners.
This is well explained in the book Applied Photographic Optics by Sidney F. Ray, published by the Focal Press. Excellent but rather expensive. It is a big book, more than 500 pages.
-- Michael Briggs (firstname.lastname@example.org), March 17, 1999.