How to measure for waterhose stops? .

Discussion in 'Large Format' started by bill_zelinski, Nov 22, 2000.

  1. I have found 1891 B&J lens for 8x10 in a Unicum shutter that I want to use for portraiture but the iris blades have been destroyed (were made out of some sort of paper) I was wondering if I could just insert a piece of paper of a fixed apeture ala "waterhouse" type stop so that I can use this lens on my beat up old 8x10 , the Unicum shutter actually still works on T & B..so my shutter is working But how do I calculate the size of the hole to make my stop? It was set up for stops from F6 to F128 but I was thinking a stop around F11 or F16 would be about right or would it? Any suggestions? Thanks.
     
  2. Divide the focal length by 4, 5.6, 8, 11, 16, etc.

    <p>

    F'rnstance 300/5.6 = 53.571428571428 ad infinitum. So drill a hole as
    close to 53.5714 mm as possible and that is your f 5.6. THe same
    would apply for f/64 etc. Bear in mind though that those values are
    for focus at infinity only.
     
  3. Bill, there is actually another step to be added to Sean's procedure.
    If you were sticking the aperture on the front of the lens you could
    follow his instructions. However, since the aperture is (I presume)
    physically within the lens, more than likely it will seem larger
    viewed through the front of the lens. The view through the front of
    the lens is where the aperture needs to be the size Sean described.

    <p>

    I don't think there is any reasonable way you can accurately predict
    the effective change in size of the aperture, so I would suggest trial
    and error. Simply make an aperture of known size, install it, then
    measure the apparent size from the front. To measure from the front,
    a crude method would be to lay a scale across the front of the lens
    and read it from as far away as you reasonably can; ie, from arm's
    length, etc. Once you find the size relationship between actual
    drilled hole and apparent size through front of lens, this should stay
    constant. Good luck!
     
  4. I did this once by metering off the ground glass with a spot meter.
    You can compare the light readings to a known lens. You cannot have
    ANY stray light on the glass. I put the meter right against the glass
    and used it sort of like a general meter on a large area. It probably
    gives the most realistic idea of how the apertures compare, rather
    than trying to measure little holes. Besides, you probably don't
    really need to have perfect normal numbers like a modern lens. You
    just need to know what it is for the exposure. So what if a certain
    hole works out as f11 and a half? I would try to make the stops out
    of thin metal, however, for a cleaner edge. You could determine hole
    sizes in paper first, though. The lens I have actually is a
    Waterhouse stop lens, but is missing the metal slides. At least it
    has the slot in the barrel.
     
  5. thanks, its trial and error time.:)
     
  6. I just read about a really neat way to measure the effective size of
    the stop. You take the lens cap and (in the darkroom) put in a piece
    of enlarging paper. With the camera set up for infinity focus, you
    replace the ground glass with an opaque card having a small hole in
    the middle. Illuminate the hole briefly then develop the enlarging
    paper in the cap. There should be a dark circle that represents the
    effective aperture size. I haven't tried this and it's probably a
    pain in the rear, but thought it was kind of a clever idea.
     
  7. I've used a slightly more sophisticated version of Conrad's 'exit
    pupil' method to measure the actual aperture diameter of a range of
    35mm lenses. This involved using a diffused laser spot in the film
    plane, and measuring the diameter of the exit beam using engineering
    calipers.<br>It's a very accurate method, but I found the measured and
    stated apertures to be extremely close, usually within the limits of
    measurement (~0.1mm).<br>In other words, you can be pretty confident
    that if you simply divide the focal length by the desired aperture
    number, then the resultant diameter will be close enough for all
    practical purposes.
     

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