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Wide Angle film illumination
Jan Brittenson wrote:
Rafe B. wrote: Right. So why is it necessary on LF lenses and not on "equivalent" 35 mm or MF lenses? I use the same center filter on both my 43mm Mamiya7 lens (6x7) as I do with my 75mm lens on 4x5. But I don't see any serious falloff with a 45mm lens on a 6X6 SLR because of it's retrofocus design. -- Stacey |
#2
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Wide Angle film illumination
"Richard Knoppow" wrote in message nk.net...
"Rafe B." wrote in message ... On Sat, 10 Jan 2004 23:27:46 GMT, "Richard Knoppow" wrote: The theoretical fall off of a lens of standard design is cos^4 theta where theta is the angle of the image point from the axis. Some lenses have a little more than this. Some designs use what is called a "tilting entrance pupil" to eliminate one factor giving approximately cos^3 theta. The only way to further reduce the fall off is to use barral distortion as in a fish-eye lens. The usual way of correcting the fall off is by the use of a kind of mask called a center filter. This is a neutral filter which tapers in density from center to edge at the right rate for a particular lens. They are not quite generic although a center filter for one lens may work for another of the same focal length and front diameter. Fall off is not affected by the stop once you get past the point where there is mechanical vignetting, usually about two stops down from maximum opening. Whether the effect is noticable or not depends on the scene and to some extent on the film. Fall off can be compensated for in printing, provided its not too great and provided the overall exposure has been calculated to give sufficient exposure to the margins of the image to prevent its being underexposed. Fall off can be seen even in only moderately wide angle images, but, since the eye expects the edges of a picture to be slightly darker than the center, its often not objectionable. For a 90degree coverage lens (theta = 45 degrees) the edge illumination is 0.25 of the center so the margins of the picture are down a full two stops from the center. Its easy to figure the fall off using a hand calculator or the calculator built into most computer operating systems. Call me confused. I have 35 mm primes and zooms that go down to 24 and 28 mm respectively. I've not noticed the light falloff in these, even on chromes. Now as I understand it, 24 mm would be 90 mm or so for 4x5,so it seems to me the view angle isn't quite enough to explain the light falloff all by itself. I'm still confused. Seems bizarre that in LF one first needs to put up with a seriously expensive f/8 wide angle (equiv. to 24 mm on a Nikon,) and then have to add an expensive center filter that's going to turn it into an f/16. rafe b. http://www.terrapinphoto.com I can't explain your results. I have a 24mm Nikor for my Nikon F and find it has quite noticable fall off even in B&W. This is a good lens with a tilting entrance pupil so the fall off is less than the cos^4 theta, but its still noticable. I don't know what the fall-off of this lens is compared to a Super-Angulon or Rodenstock Grandagon. Both of those also have tilting entrance pupils but are symmetrical (or nearly so) lenses. The Nikon lens is a retrofocus so it _might_ have somewhat less fall off. I suspect the difference, if there is one, is not significant, but can't say for certain. Because the inverse square law applying to the image from the lens to the film is less for a retrofocus lens than for a standard lens they have somewhat less fall off. This is because the image appears to be coming from further away then for a "normal" lens of the same FL so the difference in distance between the center and corner of the image, hense the square law loss, is less. -- --- Richard Knoppow Los Angeles, CA, USA Just an added note: in certain extreme reversed telephoto designs its actually possible to have a brighter image in the corners than in the center. I've seen this happen both with fisheyes and with completely distortionless systems. Not your average lenses perhaps, but nevertheless real systems that get designed and built. With enough chief ray bending and pupil aberrations just about anything is possible! Brian www.caldwellphotographic.com |
#3
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Wide Angle film illumination
"brian" wrote in message m... Just an added note: in certain extreme reversed telephoto designs its actually possible to have a brighter image in the corners than in the center. I've seen this happen both with fisheyes and with completely distortionless systems. Not your average lenses perhaps, but nevertheless real systems that get designed and built. With enough chief ray bending and pupil aberrations just about anything is possible! I thought that was one case. Thanks. I wish you were closer, Brian so that you could try one of my new wide lenses. I think you might be pleased, even though it's not a reversed-tele. |
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