Aperture mechanisms on large format lenses.

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Is the aperture mechanism of a large format lens located within the shutter? If so, does that mean that when you buy a shutter (copal), it comes with its own aperture mechanism? I assume that it does, but wouldn't the aperture have to be calibrated for the specific lens it was being used with it? Does this mean that the aperture is fully adjustable from completely open to completely closed when you first buy the shutter, but is just not calibrated for specific apertures?

What about the apertures of enlarging lenses? Are the apertures limited to specifically calibrated stops, or are they fully adjustable from open to closed?

-- (vector3@prodigy.net), February 21, 2000


Yes, the Iris diaphragm is integral with the shutter mechanism, and no, you can't just buy a lens and bung it on any old shutter.

Most large format lenses are designed in two parts, which screw onto the front and rear of the shutter, but the spacing between the two halves has to be maintained very accurately. This is usually done by shimming the front half of the lens to the shutter with very thin washers, carefully chosen to give the right spacing. Only lens manufacturers and some specialist optical repairers have the equipment and skill to do this. So, when a lens is bought with a shutter the makers supply the iris calibration plate already screwed in place and ready to go. The rear of the lens is user-detachable, if necessary, for fitting to a lens board.

Nearly all enlarging lenses are supplied in click-stop mounts to make it easy to count down to the desired aperture in the dark. A few of them allow for the click stops to be dis-engaged for a continuously variable aperture.

-- Pete Andrews (p.l.andrews@bham.ac.uk), February 21, 2000.

'Yes' to almost all your questions.

>> Does this mean that the aperture is fully adjustable from completely open to completely closed when you first buy the shutter...

Most apertures don't actually fully close, the smallest aperture is usually a few millimetres in diameter.

-- Alan Gibson (Alan@snibgo.com), February 21, 2000.

Yes, if you buy a new shutter assy. for your lens, you will need to calibrate your aperture scale. To do this, you need to have another (calibrated) lens/shutter assy. This is what you do: Mount your working lens on your camera. Using any light source you wish (brighter the better) illuminate a wall in your home, set your camera up on a tripod, positioned so that the entire ground glass is evenly illuminated, set the aperture to the largest aperture both lenses are capable of. (for instance, say you have a F4.5 lens as your reference, but you are calibrating an F7.7, set your reference to F8 or so). Now, using a reflective meter, measure the ground glass. Remove your reference lens, and mount the newly shuttered lens, and while reading the light on the ground glass with your meter (like before) adjust the uncalibrated aperture until you get the same reading on your meter as you got with the reference lens mounted. This is now the F8 position. Now all you need to do is stop down and mark your scale as you go to find F11, F16, F22 and so on. If you have clear skies, the sun may be a good choice as a light source.

-- Ron Shaw (shaw9@llnl.gov), February 22, 2000.

Or simply remove the ground glass and stick your 35mm TTL SLR camera body in the back of the camera to take accurate meter readings.

-- The masked informer (green.lantern@dc.net), February 23, 2000.

Some enlarger lenses have click f stops (e.g. the Schneider Componon S 50 mm and 100 mm lenses) and some have no click stops (e.g. the Schneider Componon S 150 mm lens). Some lenses with click stops allow you to disengage them, others don't. All of this just depends on the particular lens.

-- Brian Ellis (bellis@tampabay.rr.com), February 23, 2000.

I guess I should elaborate a bit on the calibration sequence. First, I would not remove the ground glass, as this isnt necessary. Place your meter against the center of the ground glass for all readings. With a decent hand held meter, this should be quite accurate. If you prefer to use an SLR for your metering, I guess you could give it a try, but I doubt it would be any more accurate, and quite possibly less so than a hand held meter.

-- Ron Shaw (shaw9@llnl.gov), February 23, 2000.

Just for the record, there were a number of varieties of the Schneider Componon S 150 mm lens. With mine, I can switch the clicks on or off.

-- Alan Gibson (Alan@snibgo.com), February 23, 2000.

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