edge to edge sharpness on very short lenses.

Discussion in 'Large Format Cameras and Accessories' started by fhovie, Jul 20, 2003.

  1. fhovie

    fhovie Subscriber

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    I just did a 20 x 24 from a color 4x5 of some flowers and I noticed in the corners, the image got soft. I used a polarizing filter. It is sharp every where but the corners. it is not dim in the corners. It is from a 75mm F5.6 Caltar II N (80s Grandagon by the Serial Number and the info on View Camera Magazine.) I am under the impression that this is as good a short lens as there is. I used a slight forward tilt, was focused at hyperfocal and was at F32 (pushing difraction here?) I suppose it is possible that the filter could cause it. It is a good double coated filter. I was just wondering if anyone else had this kind of experience with their 75 mm lens and at what aperature do you consider the limit (either way)

    Frank
     
  2. Donald Miller

    Donald Miller Member

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    Frank,
    The view camera magazine article indicates two 75 mm Caltar lenses. The earlier a Komura manufacture and the latter Rodenstock. The earlier lens had an image circle of 170 mm and the latter an image circle of 195 mm. Both were mounted in Copal O shutters.

    The fact that you indicate softness in the corners leads me to believe that you were exceeding the coverage of the lens. The diagonal of the 4X5 format is approximately 6.4 inches (sq root of "a" squared plus "b" squared) that converts to 162.56 mm. Thus a lens would need to have an image at least that large to cover the format. When you indicate that you used slight tilt (wouldn't make any difference if front or rear) you are increasing the effects on the size of the diagonal of the format. Tilt is the greatest user of a lenses covering ability.

    Most usually the coverage ability of a lens is not a sharp demarcation point. Rather it has an increasing degradation effect until the image circle is bypassed. The reason for this is the penumbra effect that you have noticed when you dodge a print. As the dodging tool is raised from the paper the shadow edges become less distinct. This softness that you noticed would become more noticeable the larger the degree of enlargement of your print.
     
  3. Loose Gravel

    Loose Gravel Member

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    We don't have enough info to really figure this out. I've had several soft-in-the-corners lenses over the years. Komura was one. A fast xenar was another. Wide angle lenses are tough to design and maintain critical focus across the field. To solve this, you will need to reproduce the setup, but photograph something flat, maybe a sheet of newspaper. Try it with and without the polarizer and let us know.

    Since you said it didn't become dim or have light fall off, I don't think you were running out of image circle. Also, with flowers you may have had a little bellows extension, giving you a larger image circle.
     
  4. Robert

    Robert Member

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    Depends on how you define image circle. If he had 15 lp/mm in those corners wouldn't it look okay on the ground glass? But enlarge it 5x and those corners are going to get soft. Don't some lenses have relatively large circles of ilumination but relatively small circles of good definition?
     
  5. Thilo Schmid

    Thilo Schmid Member

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    fhovie, did you check with a good loupe whether the negative is the source of your lack of sharpness? It could as well be a misaligned enlarger or some lack of flatness in the carrier.

    It is not unusual that a short lens has a curved (not flat) field, esp. when using it close-up. But you should be able to check this on the groundglass. Use a small Maglite at the subject to check this at f32.
     
  6. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    Unless these were enormous flowers, I suspect you were relatively close to them, and therefore nowhere near the edge of the image circle.

    Ultrawide lenses just aren't the best choice for macro work, where DOF is short anyway. A better choice would have been a macro lens or an enlarging lens (unless you have much more bellows than I would have guessed from your initial choice of a 75mm lens). It can even be a lens that doesn't cover the format at infinity, like a macro lens for 35mm or medium format or an 80mm enlarging lens. At macro distances, lenses for smaller formats will often cover 4x5".
     
  7. Donald Miller

    Donald Miller Member

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    David raises a valid point. I wonder if Frank would not have gotten into bellows extension considerations if he were really that near the flowers?

    Considering his choice of materials (color) one would think that if he were that near the object his exposure complaint would have been one of underexposure. I guess we will wait for Frank to weigh in and tell us "the rest of the story".
     
  8. fhovie

    fhovie Subscriber

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    I don't know if this is the "rest of the story" but I was under the impression that if you ran out of image circle, you would get light fall off. The softness is in the negative. It is a Rodenstock Lens. The enlarging lens was a 105mm Nikkor. The scene is a field of flowers with the closest at 3 feet - The camera was 24" off the ground and I focused using a good loupe. The Image seemed sharp end to end at F5.6 and I stopped down to F32 because of the slight tilt. The image went from 3 feet at the foot to infinity and the mountains in the background in sharp focus. The flowers in the forground center are sharp, The flowers at the left and right and the mountains are sharp. The lower corners are soft where there is the greates distortion caused by the short lens. The exposure was 1/2 sec at F32 with Fuji Color sheet film at the rated ASA 160 and processed dip and dunk commercally at a local lab. I did the enlarging myself. I posted the image - poppy field
    Frank
    [/img]
     
  9. Loose Gravel

    Loose Gravel Member

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    That's a very short enlarging lens for a 4x5.
     
  10. fhovie

    fhovie Subscriber

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    Yes - agree - It is my friend's lab and that was his longest lens - I do regret not brining my 150mm Omegron.
     
  11. Thilo Schmid

    Thilo Schmid Member

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    Frank, this sounds all correct (except for the EL-Lens, which makes me wonder that this works for 4x5). F32 is a lot for a short lens like that, but shouldn't cause diffraction problems for 4x5. Could it be possible that the flowers got simply blurred by wind? The center flowers could have been in your lee.
     
  12. fhovie

    fhovie Subscriber

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    On any other day in the Antelope Vally the wind would have killed this shot. This was actually one of the only windless days this year - I was very lucky.

    If it was the polarizing filter causing a distortion, that is unfortunate - This shot really benefited from the filter both in sky and color saturation. If it is the lens, that is unusual and confusing because I thought the Caltar/Grandagon was at the top of the food chain for design and quality with only the Angulon or Nikkor left to consider. That is why the thread. I have not had similar problems with this lens on other occasions. This shot is unique in that it is color and I almost never shoot color and so almost never use a polarizing lens. The most common filter for me is a Red 25 with maybe a split ND filter, Or a Yellow if I feel less dramatic. I had read that the smallest aperature should be no greater than the folcal length over 4. So the 75mm shouldn't go any smaller than F22 and a 210mm lens shouldn't go smaller than f32. When i am faced with needing DOF that cannot be obtained with tilts (tall trees and far away hills) I often challenge this idea and stop down anyway. This was one of those occasions that I didn't want to take a chance and went to F32.
     
  13. RAP

    RAP Member

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    First I think it is a lovely image.

    As for how it was done, I think I would have used a 120 mm lens for more coverage on the 4x5 and moved back so to frame the image as it is here, taking note of the groupings of flowers. You would have gotten a more realistic image without the fall off, plus the feel of the flowers being as close with the longer lens. I also might have waited for a lower angel of sun light. Still it is a nice image.

    I personally have a 65mm super angulon I use for just such a purpose. I intentionally move in close so to get that distorted effect and use it creatively. The lens just barely covers the 4x5 at neutral. But I do not get any blurring at the edges or corners even stoped down to f/45.
     
  14. Jorge

    Jorge Inactive

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    I think the softness comes from the enlarger lens, probably not closed enough or enlarger misaligment. I have seen this before, the center very sharp and the corners soft. The fact that you dont have light fall off indicates to me that it might not be the taking lens. Although Don has a good point, you might have had enough coverage but ran out of circle of "good" definition.

    Have you checked the negative with a 10x loupe and see if the corners are soft in the original?
     
  15. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    "The lower corners are soft where there is the greates distortion caused by the short lens."

    I've seen this effect with very wide lenses that will just cover when movements are used. It looks like an increase in coma at the corners of the frame. I'm not sure whether this is actually coma or just an effect of the normal wideangle distortion (or perhaps coma is this effect). Maybe the optical gurus have an idea about this.

    Scratch my earlier theory. I was thinking this was like a still life, not a landscape.

    If "the softness is in the negative" as you say, then it's not the enlarging lens, although that is a short lens for 4x5".

    Another possibility is a film flatness problem in the camera--either the film popping in the heat or sagging with the camera pointed down, if it was pointed down.
     
  16. Ed Sukach

    Ed Sukach Member

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    It is not properly "coma" ... that is usually an element centering error. The more I consider this image, the more I am convinced that is is the result of a lens stretched past its design criteria for coverage - and beyond the area of distortion correction - mainly as as result of uncorrected "flatness of field". The "pincushion" effect is rather easy to see in the ~ even pattern at the lower left and right corners - it is far more difficult in the "sky" area where there are no natural patterns/ distibutions. "fhovie" says he is going to continue with another, longer, enlarging lens. I'd be interested to see what the results look like.
     
  17. Jorge

    Jorge Inactive

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    Yeah, after seeing the picture it seems to me to be lens distortion not from the enlarging lens. It does look like pin cushion so I think fhovie tilted past the correction of the lens.
     
  18. Ole

    Ole Moderator Staff Member Moderator

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    There seems to be two effects on that image: The "stretching" of the flowers in the lower corner, due to the tilt bringing the corners of the film outside the area of good definition for the lens. There is also quite a lot of light fall off, some of which could be from the enlarging lens being pushed beyond its capability.

    An aperture of f:32 seems rather large to me for a tilted wide-angle, I think I would have stopped down to at least f:45 to improve the definition in the corners. Diffraction won't be a visible problem until f:128 at least. It's more important to avoid soft corners, in my opinion...

    Next time try tilting the back instead of the front. That way the entire film stays indide the circle of good definition. if using front tilt for this, you should also use front fall to bring the center of the film on the optical axis of the lens. It's amazing how little it can take to tilt the front too much!
     
  19. fhovie

    fhovie Subscriber

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    That is all really good feedback. Since I am going to get some distortion anyway, tilting the back seems like a good idea for the next time. I did use front fall on this one and on a 75mm lens, a little movement really does go a long way! I will also try stopping down even more. I really appreciate all the good comments - You are all a great resource of ideas. - Thanks