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Thread: Question for an optical engineer

  1. #1

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    Question for an optical engineer

    Assuming all else to be equal (focal length, grinding and polishing accuracy, glass formulation, collimation/manufacturing tolerances), would a lens that is optimized for a larger image circle than 4x5 inherently have lower resolution than one optimized for the format?

  2. #2
    ic-racer's Avatar
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    Re: Question for an optical engineer

    Light passing through the center is not bent as much by the glass, therefore, can retain resolution. Thats my thought based on empiric data.

  3. #3
    the Docter is in Arne Croell's Avatar
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    Re: Question for an optical engineer

    Does "all else" include the price/manufacturing cost?

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    Re: Question for an optical engineer

    I'm not an optical engineer, but this is what I have discovered. A lens with huge coverage ability is not less sharp when used with a smaller format.

    In fact, it seems to be sharper when used with the smaller format, since it is only using the center section of the image circle.

    For example, I have a Nikkor f8 120SW and a Nikkor f9 300M. both lenses have image circles and will cover 8X10. When used for the 4X5 format, they are extremely sharp.

  5. #5

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    Re: Question for an optical engineer

    would a lens that is optimized for a larger image circle than 4x5 inherently have lower resolution than one optimized for the format?

    It is very difficult to give a clear answer to the question because it depends how you compare the final results.

    The first answer that comes to mind would be to say : yes if you compare the same lens design, same glass, same manufacturer, larger lenses for larger formats have lesser resolution than shorter lenses for smaller formats, with the same angular coverage.
    The reason is that the residual geometrical aberrations scale exactly like the size of the lens elements.
    However, diffraction effects do not scale like this since the wavelength of light is the same for all lenses. So for short focal lengths used on small image detectors in order to avoid diffraction you cannot stop down as much as you can do in large format.
    So this makes the comparison difficult between lenses of similar design but optimised for different detector sizes since the best f-stop is not the same for different focal lengths.

    For top-class lenses operating close to the diffraction limits, the best f-stop number N, for a given family of designs like the 6-element standard view camera lenses, actually follows an empirical law that you can derive by compiling the best f-stop recommended by manufacturers for the same design offered on catalogue in different focal lengths.
    For older 6-element standard lenses we have N_best approx equal to the focal length in mm divided by 8 mm : N_best is betwwen 16 and 22 for a classical 150 mm standrda LF lens.
    For top-class recent lenses like the apo-sironar-S, the apo-symmar L and "digital" lenses covering approximately the same angle (70-75░) N_best would be something like the focal length in mm divided by 11 mm, yielding something between 11 and 16 for the best 150 mm available today, something between 5.6 and 8 for the best 75mm lenses, a range of best f-numbers actually corresponding to old planar/xenotar designs in medium format for decades, and not far from what modern digital view camera lenses demand in order to keep the image with as little diffraction effects as possible.

    If we use the various lenses for various formats at the best f-stop and consider that in this situation the actual resolution limit combining aberration and diffraction is something like 1.4 times the size of the diffraction effect alone, we get a simple scaling law, the minimum detectable spot size scales like the focal length also.

    As you see the question of using with the same image format various lenses of same focal lengths but various angles of coverage is not an easy one. Arne is right to mention price and manufacturing costs. If we do take into account only a reasonable cost for the lenses in use, I would expect a standard 150mm lens covering 70-75░ on the 4x5" format to perform better in this angle that the central portion of a 150mm/110░ ultra-wide angle lens covering the 8x10" format.

    But in practice there are some good surprises and I agree totally with Gem Singer.
    I use regularly a 75 mm Grandagon-N lens, 6 elements, covering 102░. The image quality delivered by this lens stopped down to f/11 is superb, on the central portion of a 6x9 image or rollfilm is by no way poorer than what I am used to see with a Rollei or a Hasselblad MF camera fitted with a standard 75 or a 80 stopped down to f/11...
    But the full aperture of the wide-angle grandagon is only 6.8, while planars or xenotars for 6x6 (5 or 6 elements) offer 3.5 or 2.8. The optimisation was different.

  6. #6

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    Re: Question for an optical engineer

    Quote Originally Posted by Gem Singer View Post
    I'm not an optical engineer, but this is what I have discovered. A lens with huge coverage ability is not less sharp when used with a smaller format.

    In fact, it seems to be sharper when used with the smaller format, since it is only using the center section of the image circle.

    For example, I have a Nikkor f8 120SW and a Nikkor f9 300M. both lenses have image circles and will cover 8X10. When used for the 4X5 format, they are extremely sharp.
    Apples to oranges because they're different designs.

    That said, I shoot a number of lenses made for larger formats on 2x3 and in practice all of the better ones shoot about equally well.

    Its fun to theorize about how wonderful a lens can be, but the emulsions I use aren't better than my lenses and my technique is often less than perfect. I doubt that many of us routinely get the best our lenses can give. So discussions like this one have an air of, um, unreality, bring the mythical debate about the number of angels that can dance on the head of pin to mind.

    FWIW, I just shot a 4"/2.0 TTH Anastigmat, ex-F95, against a 100/2.5 Uran-27, ex-RA39, and a 105/5.6 Zircon (6/4 plasmat type). At apertures larger than f/5.6 the TTH won, from f/5.6 down the Zircon was no worse than the better of the other two. The Uran-27 isn't horrible, but the other two are better.

    So what? I mean, if I'm going to be shooting in the range f/11 - f/22, the Zircon is the clear winner; its no worse that the others at those apertures, is much lighter than either, and stops to f/22 while the aerial camera lenses go only to f/16.

    I can imagine situations in which the TTH would be the lens of choice, and it is certainly better pour epater les bourgeois than the Zircon. The Uran-27 is even more outrageous and is usable. But as a practical matter I'd hardly ever use either in place of the Zircon.

    Cheers,

    Dan

    Emmanuel, please don't remind me that I'm an ignorant barbarian. I'm a birthright barbarian and every day the whole world shows me how little I know.

  7. #7
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    Re: Question for an optical engineer

    Dan,

    I stated openly that i am not an optical engineer.

    I was attempting to answer the OP's question based on my sixty-three years of experience with many different lenses.

    BTW, those lenses that you mentioned must be before my time. I have never heard of them.

  8. #8

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    Re: Question for an optical engineer

    Gem, they're all post-WWII. The aerial camera lenses were designed in the 1950s, the Zircon some time after 1972. You've been around longer than they have. Of the three, only the Zircon is a conventional LF camera lens.

    The TTH is discussed in the VM; these days they bring silly money on eBay. The Uran-27 is very little known; a cross-section is shown in the 1963 GOI catalog. Boyer lenses are also poorly known even in their native France. Boyer had a US distributor, Rolyn Optics, when Boyer closed in 1982; several years ago Rolyn had new old stock Zircons.

    You're ok, Gem, but I should have been clearer that I think the OP's original question is, um, misguided.

    Cheers,

    Dan

  9. #9

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    Re: Question for an optical engineer

    I sincerely don't appreciated your "misguided" remarks, nor the flaming, not one bit, sir. If you don't like the question, Dan Fromm, move along.

    I'm using LF versus MF primarily for greater resolving power in order to make larger prints for the galleries I'm represented in. Already have MF gear capable of 80 lp/mm resolution. I've been using smaller formats for over 30 years (20 professionally) and have been achieving film-limited resolution using excellent technique. So naturally I approach LF expecting superior results, not just looking for enough overhead in a 16x20 that I can afford to be sloppy.

    But back to the reason for asking the original question. I have a Meridian 45B technical field camera with back moves (swing and tilt only) Back moves allow using a lens of smaller coverage, unlike front standard moves for tilt and swing, at least. Frankly, I'm non-plussed about the resolution I'm achieving with a 210mm APO Plasmat with huge coverage (Caltar IIN f/5.6, AKA Rodenstock APO Sironar N with 301mm image circle or thereabouts). Reasonably happy with my Nikkor 90mm SW, however.
    If it's a matter of simple physics, I soon may be looking for a modern 150-210mm lens with similar apochromatic design but less coverage.

  10. #10
    Wayne venchka's Avatar
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    Re: Question for an optical engineer

    I know just enough about all of this to get in trouble.

    If the only criteria you are using is lp/mm, large format lenses will rarely equal the better medium format lenses.

    Your 80 lp/mm figure for your medium format lenses is at the beginning of the upper range for the best lenses. The best large format lenses will top out at or below 80 lp/mm.

    Folks don't go to the trouble of using large format equipment just for lp/mm.

    You are correct in using the "sweet spot" in the center of a huge image circle to obtain higher lp/mm over the area you are using.

    Large Format Lens Tests

    Medium Format Lens Test

    Enjoy!
    Wayne
    Deep in the darkest heart of the North Carolina rainforest.

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