Coverage area of a lens

Discussion in 'Camera Building, Repairs & Modification' started by tkamiya, Aug 22, 2010.

  1. tkamiya

    tkamiya Member

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    This is probably a very silly question....

    What determines the maximum coverage of a particular lens? Is it design of the lens? Is it focal length of the lens? Is it the distance between the lens and the film?

    The reason I ask is that I am starting to collect a bit of junk camera body and lens... from 35mm to MF to enlarger lens. If I wanted to play with these and say, make a panorama shooter, projecting to 120 film, how should I logically approach the lens selection?
     
  2. tkamiya

    tkamiya Member

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    Yes, I am very bored today.... have been sick at home for few days.
     
  3. Ian C

    Ian C Member

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    That’s mainly determined by the design. For example, a 150/5.6 Nikkor W covers the 4” x 5” format, while the 150/8 Nikkor SW covers the 8” x 10” format.

    You have to choose a lens based upon the coverage it has. It can’t be changed.
     
  4. Ian Grant

    Ian Grant Subscriber

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    Purely design & physical constraints. The head (lens cells) of an older Leitz 35mm telephoto lens actually covers 5x4.

    Ian
     
  5. tkamiya

    tkamiya Member

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    Thank you Ian 1 and Ian 2.... DARN!
     
  6. sepiareverb

    sepiareverb Subscriber

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    In a related question- does the diameter of elements necessarily have any effect on coverage? I've heard that the 40/2.8 Sonnar that came with the Rollei 35RF (recent Voightlander clone) is optically the same as the Sonnar in the old Rollei 35 S/SE- yet the diameter of these elements is vastly different.
     
  7. Steve Smith

    Steve Smith Member

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    Have a look at my home made 6x12 camera link (below). Doesn't answer your question but you did say you were bored!


    Steve.
     
  8. ntenny

    ntenny Member

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    More specifically, I guess the optical formula and physical constraints determine the coverage *angle*, and the coverage *area* then grows in proportion to the focal length, right? Which is why it's hard to build a wide-angle lens with large coverage.

    -NT
     
  9. ooze

    ooze Member

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    I know one excellent camera repair guy. He once told me that he had made a device to compare the image circle of Leica and Nikkor lenses and found that whereas the Nikkors barely covered 35mm film, the Leica's covered almost 60x60mm. By using the best portion of their image circle, the Leica's were much sharper than the Nikkors, he claimed.
     
  10. tkamiya

    tkamiya Member

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    Thanks but that's not fair! You have access to a nice CNC machine!! Very nicely done!!!

     
  11. Steve Smith

    Steve Smith Member

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    It's amazing how many 'practice' or 'training' pieces I do on this machine!


    Steve.
     
  12. Q.G.

    Q.G. Inactive

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    Then they are not the same lens in a different mount.
    The size of the lens elements does matter, yes. But how (or how much) depends on the optical design.
     
  13. MattKing

    MattKing Subscriber

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    The design of the body of the lens affects this as well. I find it interesting that at least one of my Mamiya 645 lenses has a rectangular "gate" in it. I would guess this is to minimize image degrading flair.
     
  14. Q.G.

    Q.G. Inactive

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    Lenses (being round) 'throw' a round, circular image. Rectangular formats don't use parts of it, but the light in those parts still leaves the rear of the lens.
    It will bounce around, eventually landing on the film too, where it causes a lowering of contrast (at best) or nasty highlights (at worst).
    So rectangular baffles are put both on (and in) lenses and in cameras. The lens still has to produce a circle large enough to cover the full diagonal of the rectangular format.

    (There is at least one example of something similar, but somehow opposite: the first Kodak camera produced circular images, not - as we may think - because the lens didn't cover the format, but because the corners were spoiled by excessive flare. Instead of trying to get rid of the internal reflections, they just put in a circular mask.)

    The design of a body thus mostly is aimed at losing the extra, but unused light.
    Mostly, because the depth of the camera body does increase mechanical vignetting.
     
  15. sepiareverb

    sepiareverb Subscriber

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    Thank you- I couldn't quite wrap my head around that idea.
     
  16. ic-racer

    ic-racer Member

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    Measure the diagonal of your proposed film gate size. Select a lens with an image circle that is bigger than your measurement. So, for example if you film gate is nominal 6x12cm (actual 56mmx112mm) you should look for a lens with an image circle 125mm or greater.

    The size of the glass is not always related to coverage. A tiny Hypergon covers 8x10, whereas a huge Aero Ektar won't cover 8x10.
     
  17. Rom

    Rom Member

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    Hi !

    A very intersting post

    My related question is how to calculate the image circle ?

    Thanks for helping

    ++
     
  18. Q.G.

    Q.G. Inactive

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    You can't.
    It depends on optical design and mechanical vignetting.
    You have to either ask the manufacturer or try/measure it yourself.
     
  19. bvy

    bvy Subscriber

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    Interesting. Is there a name for this phenomenon? I'd like to learn/read more about it, and how to control (or temper) it.
     
  20. Q.G.

    Q.G. Inactive

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    There is. It's called "flare".

    There are three ways to control it:
    - antireflection coating of lens elements and matt black paint on the inside of the lens barrel and the inside of the camera,
    - baffles in and behind the lens that block as much non-image forming light as possible,
    - the use of a good lens hood, that prevents non-image forming light from entering the lens in the first place.

    The last thing is the only one you can do. And it is very important to indeed do this.
     
  21. bvy

    bvy Subscriber

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    Flare? I've heard of that (!). It sounds like something very esoteric when described like this. A common phenomenon anyway.

    I ask because I'm experimenting with homemade optics and having trouble controlling the incidental light. On one end, I'd like to tighten it up a little, but on the other, the flare gives the images a dreamy sort of quality. So I'm trying to find that balance.

    Contrast is a problem too. I'm using a DSLR to work out the kinks, but I eventually want to adapt it to a Pentax SLR I have here.

    Sorry to go off topic with my own concerns...