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Thread: How are the aperture scales on shutters determined?

  1. #1

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    How are the aperture scales on shutters determined?

    I'm relatively new to LF and have just embarked on the adventure of buying an “in barrel” lens for cheap with the intention of fitting it into a shutter. And some questions have arisen that I should have thought of a while ago.

    It has just occurred to me that I don't know how (or even if) the aperture scales on LF shutters are calibrated. Since many different lenses might fit on a Copal #1 (for example) with many different focal lengths, how does one get the scale right for a given length? Or, in the case of second-hand lenses how can one be confident that the scale on the shutter currently on the lens is correct for that lens?

    Any wisdom gratefully received.

    Keith

  2. #2

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    Re: How are the aperture scales on shutters determined?

    approximately speaking, the diameter of the pupil of the iris at any given position is measured as a fraction of the focal length. Doing it this way allows one to have consistent exposures on different lenses. So there's a few ways you could verify or calibrate your own. The a priori method would be to get a micrometer or other suitable measuring device and

    1. look for a given diameter, like f/8. So if your lens is a 300mm lens for example - look up the ACTUAL focal length for that model - let's say it's 282.5mm (variances like this occur - the marked focal length is rarely the ACTUAL one).

    2. in this case we're going to set the iris so that the pupil closes to 282.5/8 mm or around 35.3 mm. Exactitude isn't super important - the degree of error implicit in being out by half a mm is around 1/12th of a stop or less.

    3. Once you have your f/8 marked on the linear scale (where the aperture actuator marker is sitting) - then figure out your f/16 or 17.6 mm in this case.

    3A. an ALTERNATE and faster method is to use the scale of a lens with the same shutter model (in this case copal 3 if that's what you have) and just use the same increments as the mechanism is identical

    go from there. you're practically done.

    An ALTERNATE method for the super picky photographer would be to measure T-stops - you could do that with a really accurate exposure meter in several trials. It would account for the trasmissivity of the glass and any errors in manufacture of the iris itself although would take a lot of trials and labor to calibrate... well that's just an aside for the über geek...

    good luck

  3. #3

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    Re: How are the aperture scales on shutters determined?

    The main problem is, the entrance pupil of a lens is not equal to the iris opening diameter. It actually is the diameter of the iris enlarged (or diminished - in case of retrofocus lenses) by the front part of the lens itself as it is seen looking from infinity. One can put a ruler above the front glass of the lens and watch it from a distance as far as the ruler is visible enough to read it to get a reasonable approximation, but the perfect way to measure the pupil involves using a collimator to emulate the infinity view - still looking from a closer distance.

    There is really too much trouble (or too little accuracy, if the instruments are primitive) to go this way when you have the original barrel/shutter at hand (or can borrow one). In this case, just make your new shutter scale to yield the openings exactly the same as the manufacturer's old barrel does.

    T-stops are not bad an alternative really - especially if your main concern is the exposure accuracy, as is with slide films. But then you'd probably want to recalibrate all your other lenses to T-stops to be sure you don't have to remember which ones are which not to make occasional mistakes in a hurry. And between the lens shutters, being less then 100% efficient and wasting some time to open and close, do not produce exactly the same exposures at wider and smaller lens apertures at faster speed settings. That makes the T-stops far less valuable in LF then in cinematography (where T-stops are the standard). And DoF and image circles and pictorially important spherical (and other) aberration changes are dependent on the f-stops, not the T-stops, and things like that would be far harder to manage with no real f-stop scale on the lens.

  4. #4

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    Re: How are the aperture scales on shutters determined?

    And btw I would not recommend using a small format (on an MF) SLR TTL meter to calibrate the T-stops. Those meters are made to be accurate with fast lenses wide open, and the vast majority of those already fail dealing with the f-stops as small as f/11 or even f/8 - to say nothing of the apertures routinely used in LF - due to the cameras' special viewfinder optics usually constructed to make the screen brighter at all costs. Some of the SLR TTLs may work, but one has to verify that very carefully - watching the meter readings with a well-known lens that is guaranteed to have its aperture scale really accurate.

  5. #5

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    Re: How are the aperture scales on shutters determined?

    Quote Originally Posted by ridax View Post

    There is really too much trouble (or too little accuracy, if the instruments are primitive) to go this way when you have the original barrel/shutter at hand (or can borrow one). In this case, just make your new shutter scale to yield the openings exactly the same as the manufacturer's old barrel does.
    I think this is the way forward. The lens have on the way is a G-Claron out of some repo machine or other and it has a diaphragm in it, so I'll attempt to "clone" the indicated apertures from that. I usually shoot B+W negative film with substantial latitude so I don't need complete accuracy.

    Another naive question: are shutters and (reasonably modren) barrel diaphragms all the same thickeness, or should I expect to have to put some shims in (or even cut something down a bit) to get the front and rear cells the right distance apart? Once I have the thing in hand a bit of micrometer work will tell me this, of course, but "forewarned is forearmed" as they they.

    Thanks,
    Keith

  6. #6

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    Re: How are the aperture scales on shutters determined?

    Quote Originally Posted by keithb View Post
    Another naive question: are shutters and (reasonably modren) barrel diaphragms all the same thickeness,
    Often, but not generally. One thing to be aware of is that shutter size originally defined diameter and thread, not thickness. Shutters of same size (number, for 00, 0, 1 and 3) mostly are interchangeable between Compur, Prontor and Copal, for regular (Schneider, Rodenstock, Nikon, Fuji) view camera lenses, from the seventies on. But beware: There have been shutters made or modified to match (older) non-standard lens cells, without obliterating the enumeration on the body. And many of the widespread and cheap Prontor and Copal Press shutters from Polaroid and other technical recording cameras have a different thickness (and are useless until someone finds a way to ship negative strength shims from some parallel antigeometry universe). The above usually extends to barrel mount, for example, Schneider original barrel mounts of lenses primarily marketed for view cameras in shutter mount generally are the same dimensions all around as the shutters - but again, this all does not apply to lenses not made for view camera use, which came in barrel by default, or which already were a legacy product by the mid seventies.

  7. #7

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    Re: How are the aperture scales on shutters determined?

    Quote Originally Posted by Sevo View Post
    many of the widespread and cheap Prontor and Copal Press shutters from Polaroid and other technical recording cameras have a different thickness (and are useless until someone finds a way to ship negative strength shims from some parallel antigeometry universe).
    I wondered about that. You mean that they are thicker? If it's only a matter of a millimetre or two I'd expect to be able to deal with that by carefully rubbing down the outside ends of the threads, but that sounds like too much, too tricky work.

    Quote Originally Posted by Sevo View Post
    for example, Schneider original barrel mounts of lenses primarily marketed for view cameras in shutter mount generally are the same dimensions all around as the shutters
    This is what I would expect: manufacturers tend to optimize for uniformity as it reduced their cost base and so would likely design a model of lens intended for view camera and reprographic use with the same dimensions in both cases. We shall see...

    Thanks for the input,
    Keith

  8. #8

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    Re: How are the aperture scales on shutters determined?

    I just look at the size of the hole, approximate the aperture, give it an extra stop and almost never miss. He number doesn't matter if your exposure is good..

  9. #9

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    Re: How are the aperture scales on shutters determined?

    Isn't that very way to ship negative strength shims for the Polaroid's Copal Press and Prontor Press shutters called a file (the steel one, not a digital nor a paper one) in this particular universe? :) For me, that simple tool used to work just OK.

    Also, modern shutters (and those barrels that are threaded identically to the corresponding sized shutters) are usually standardized to the same thickness so one can take lens halves out of a shutter or barrel and just screw them into another one without any trouble. But the apertures are not positioned in the same places in all the shutters; to overcome this, the modern threads just start in different places so the front lens part goes deeper in one shutter, and the back part goes deeper into another - while the distance between the lens parts remains exactly the same.

    Older lenses, shutters and barrels have their threads staring at the very edges without the modern style gaps. So to be properly set into a newer shutter, those lens halves (or the shutters) may have to be filed a bit. In any case, it's important to make sure the distance between the lens elements remains the same as in the original barrel, but the aperture position is not as critical. Even for a 50mm small format lens, the iris displacement as large as ±1 mm is acceptable. And with longer focal lengths, those tolerances get correspondingly greater.

  10. #10

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    Re: How are the aperture scales on shutters determined?

    Quote Originally Posted by ridax View Post
    Isn't that very way to ship negative strength shims for the Polaroid's Copal Press and Prontor Press shutters called a file (the steel one, not a digital nor a paper one) in this particular universe? For me, that simple tool used to work just OK.
    At least on the Prontor Press #1 shutters I have around (presumably off German market Polaroid recording cameras) the shutters are not only too thick in their thread flange spacing, but there is also a inner protrusion around the aperture that gets into the way of screwing down Tessar and Symmar type lenses to nominal distance. YMMV, there may be other lenses that fit nonetheless, but for the above a file is not enough, you'd have to take apart the shutter and ream out its internals with a lathe.

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