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  1. #11

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    Quote Originally Posted by Ara Ghajanian
    What was happening to my negs was a very very subtle version of this example. The only way I noticed it was because I photographed something on a very green lawn and the grass at the bottom corners of the frame (vertical orientation) was a bit darker.
    Ara
    Ara, I realize that there's probably no grass in the upper corners of the frame. But is whatever is there darker in the corners too?

    And what filters and lens hood did you use? If you used one of those collapsible rubber hoods, take a shot or two without it.

    Dave, the cutoff isn't due to the diaphragm, its due to a field stop (shouldn't be one in a 50/1.4 Nikkor, but there is one in the 75/4 Apo Rodagon D 1:1) or some other mechanical obstruction, usually at the front of the lens or to the lens' design (the 38 Biogon with which I took my example shot). I hang a number of lenses in front of a #1 on my little Graphics, always worry that the shutter's barrel will block the peripheral rays.

    Cheers,

    Dan

  2. #12
    Dave Parker's Avatar
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    Dan,

    Seems you and I have learned photography in a different way, which is cool as long as we make the images we want.

    Regards.

    Dave

  3. #13

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    Check this page out. It shows the darkening of the corners at 1.4 very clearly.

    http://www.physics.montana.edu/stude...g/50mmTest.htm

  4. #14
    skahde's Avatar
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    The fast Nikkors of 50mm and wider *all* vignette very obviously wide open and I have and had a handfull (24/2,8, 28/2, 35/2, 50/1,8;1,4;1,2). Use it to your advantage (concentrates the viewers attention to the center together with great swirly bokeh with eg. the 1.2/50mm) or stop down a stop or two.

    best

    Stefan

  5. #15

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    Quote Originally Posted by Satinsnow
    Dan,

    Seems you and I have learned photography in a different way, which is cool as long as we make the images we want.

    Regards.

    Dave
    Not to quarrel, but when the lens is wide open how can the diaphragm cut off the outer part of the cone of light that the lens projects?

    About field stops, take a look at Rodenstock's published MTF and illumination curves for the 75/4 Apo Rodagon D 1:1. They're flat to the edge. I had one, it gave even illumination across an 80 mm circle as claimed, and beyond that all was dark. Its a wide angle lens with a stop.

    Regards,

    Dan

  6. #16
    Ara Ghajanian's Avatar
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    Quote Originally Posted by Paul_Baker
    Check this page out. It shows the darkening of the corners at 1.4 very clearly.

    http://www.physics.montana.edu/stude...g/50mmTest.htm
    This is exactly what is happening. As long as it's not something wrong with my lens, then I'm fine with it.

    Stefan,
    I can appreciate your advice. I will use it to my advantage and at least I know now that I can stop down to lose the effect. I've taken some great shots wide open with this lens, that wouldn't have been possible had I not had the extra stop.

    Thanks to everyone,
    Ara
    Just because you're not paranoid doesn't mean they're not out to get you.

  7. #17
    Helen B's Avatar
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    Ara,

    Vignetting wide open isn’t uncommon among fast lenses. Broadly, there are two types of vignetting that are inherent in a particular lens design: “natural” and “artificial” or “optical”. Vignetting introduced by filters or lens hoods can be referred to as mechanical vignetting.

    Natural vignetting is, not surprisingly, a natural property of the lens that is related to the angle of incidence of rays (the Cos^4 law), and the designers will try to overcome it. As far as my limited understanding goes, they do this by selling their souls to the devil. The wider the lens, the more natural vignetting will occur.

    Artifical or optical vignetting is caused by things in the lens getting in the way of the oblique rays but not the axial rays, as mentioned by Dave and Dan. These could be deliberate, like field stops, or they could be mismatches between the location of the entrance pupil and the iris*, or the physical size of the front and rear elements – the effect being to obstruct the entrance pupil when viewed obliquely. As the iris is closed down, the obstruction to the axial rays matches the obstruction to the oblique rays and the vignetting disappears. Poof! It’s gone.

    The full cure may be worse than the disease. So it’s not so much a sign of cheapness, more a sign of how much the design is balanced between different goals. One opinion is that a little vignetting at f/1.4 is not such a bad thing in a lens intended for pictorial use, because of the typical circumstances in which f/1.4 is used. The Leica Noctilux (50 mm f/1) has a lot of vignetting wide open, but who would notice it in blurry snaps taken in the gloomy dungeons that typical Leica users frequent?

    Best,
    Helen

    *Which is the answer to Dan's question "...when the lens is wide open how can the diaphragm cut off the outer part of the cone of light that the lens projects?"
    Last edited by Helen B; 09-13-2005 at 03:15 PM. Click to view previous post history. Reason: Paragraph arrangement in original was misleading.

  8. #18

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    Helen, don't you mean the exit pupil?

    In some lenses vignetting is clearly intentional. I have two ~ 4 inch reversed tessar type macro lenses -- 90/6.3 CZJ "M" and 100/6.3 Reichert Neupolar -- with rather small rear elements located well behind the exit pupil. At fairly small angles off-axis the rear of the lens barrel obscures the exit pupil. As a result, both have small coverage at infinity but more than enough closeup. I really regret this property of the Neupolar, since over the field it covers -- wider near than far -- it does very well at all distances.

    About cos^4 dropoff of illumination, Brian Caldwell has asserted repeatedly on usenet that he has designed short rectilinear lenses that are brighter at the edges than in the center. His trick, if I understand it correctly, is to design the lens so that on the image side of the lens the edge rays are nearly parallel to the central ray.

    I'm not sure that Ludwig Bertele sold his soul to the devil, even though some of his designs smell a little of hot sulfur. I have in mind the 44/5.6 Super Aviogon I once got to hold. I held it out in front of me, and as I rotated it from "straight ahead" to "lens' axis 90 degrees off straight ahead" its beady little exit pupil wouldn't stop staring at me. Brrr!

    Cheer,

  9. #19
    Ara Ghajanian's Avatar
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    Quote Originally Posted by Helen B
    The full cure may be worse than the disease. So it’s not so much a sign of cheapness, more a sign of how much the design is balanced between different goals. One opinion is that a little vignetting at f/1.4 is not such a bad thing in a lens intended for pictorial use, because of the typical circumstances in which f/1.4 is used. The Leica Noctilux (50 mm f/1) has a lot of vignetting wide open, but who would notice it in blurry snaps taken in the gloomy dungeons that typical Leica users frequent?
    Helen,
    Thanks for the good answer. I'm figuring if a rediculously expensive Leica lens shows some vignetting, then a $150 Nikkor shouldn't bother me much. I actually hadn't even noticed it before the last roll of film I just shot, so I'll pretend it never happened.
    Ara
    Just because you're not paranoid doesn't mean they're not out to get you.

  10. #20

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    Just to clear up a little terminology here, what your seeing is really "light falloff" and not vignetting. As stated above vignetting is ths term usually reserved for something in the way blocking the light. Light falloff is just an optical problem where the light has trouble being uniformly distributed over the focal plane.

    Paul B.

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