Lens & Aperture...

Discussion in 'Large Format Cameras and Accessories' started by vickersdc, Sep 27, 2011.

  1. vickersdc

    vickersdc Member

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    Despite the fact that I've been out using my last home-made camera, and the fact that I've got to build another camera for my rapid rectilinear lens that is sitting in the cupboard, I've got hold of a single lens from a projector (one of those projectors that used to be used in the classroom).

    I reckon the focal length to be about 320mm, but in order to get the depth of field I'm going to need to build (another camera, but also...) a diaphragm. But how far away from the lens does the diaphragm need to sit? Is it just experimentation to find the best spot, or is there a formula one should follow?

    Note, I'm not looking to work how large the aperture is, just where the diaphragm as a whole (hole?!) should sit.

    Thank you,
    David.
     
  2. Monito

    Monito Member

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    You say the lens is "a single lens". Do you mean it is a single element? Or sealed barrel? Or just mean that you have only one lens for your camera?

    If it is a sealed barrel or a single element lens, put the diaphragm as close to the rear of the lens as you can. If the barrel is funny such that there is a big distance between the rear of it and the rear element, then consider putting the diaphgragm as close to the front of the lens as you can.

    Depending on the lens and the barrel and what you mean by a diaphragm (traditional iris?) you might be able to cut a slot in the barrel and put in Waterhouse stops (metal slides with different sized apertures).
     
    Last edited by a moderator: Sep 27, 2011
  3. Monito

    Monito Member

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    Take a look at http://en.wikipedia.org/wiki/Cardinal_point_(optics) which explains that there are two nodal points, which in lenses with air on both sides are the two principal points. I haven't found a definitive explanation of where to place the aperture, but putting it between the two principal points would seem best, or perhaps at one or the other if they fall in an air gap in a multi-element lens.

    The entrance and exit pupils are just images of the aperture, so they are a result of choice of placement and not possible to use in choosing where to place the aperture.
     
  4. Monito

    Monito Member

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  5. vickersdc

    vickersdc Member

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    Thanks for that Monito, I'll check those links out.

    What I meant to say was that the lens is just a single element, my 'diaphragm' will be made like a Waterhouse stop - a hole in a piece of blackened metal essentially!

    Cheers,
    David.
     
  6. numnutz

    numnutz Member

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  7. vickersdc

    vickersdc Member

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    Well some of those links, or rather the information contained within them, is more than my head can handle at the moment! Anyway, it is a positive meniscus lens that I'll be using.

    @numnutz: those focus stops are really weird, never seen anything like those before, might take a bit of messing around with in the milling machine to make some of those!
     
  8. Jim Jones

    Jim Jones Subscriber

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    David, the stops nn linked to are for a special soft focus lens. The amount of softness was controlled by using more or less of the light through the edges of the lens. For most photography, a traditional round aperture is better. The classic box camera used a positive meniscus lens with the convex side towards the film and the aperture on the concave side. Later box cameras, when convenience was preferred over image quality, had the concave side of the lens towards the film, with the aperture also on the concave side. This resulted in a more compact camera with the shutter protected from dirt and moisture by the lens. However, the lens focused on a curved surface, not a plane, so image quality suffered. Aperture was limited to maybe f/14 in the older cameras. The curvature of both surfaces of the lens and the position of the aperture affected performance.
     
  9. jnanian

    jnanian Advertiser

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  10. vickersdc

    vickersdc Member

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    Thanks Jim, now this is getting interesting... so the aperture sits open to the air (as it were) with the convex side to the film plane. Does this result in improved image quality - your post seems to suggest that it does. But how do I set about finding out where that aperture should be placed?

    I'm not too concerned about the final size of the camera, the lens itself is 90mm in diameter, with a focal length of around 320mm, so it's not going to be small anyway! I'm thinking of another sliding box camera as it is, so I can make it to suit whatever is needed. I'll be using paper negatives too, as that is what I am into at the moment. Hmmm, time to open up Google SketchUp...
     
  11. vickersdc

    vickersdc Member

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    Thanks John, that's perfect... to quote Reinhold... "The location of slot for the stops is 10~15% of the focal length in front of the lens"

    Heavens, this camera is going to be enormous, it'll need wheels! :whistling: