Help with Prosch shutter f/stop sizes

[rrunnertexas]

After completing a rebuild of the Prosch shutter for my 8x10, I have no idea what aperture values might be on the rotating plastic disc behind the shutter.

Can anyone comment with some experience with the aperture sizes for this shutter? The lens elements look clean and clear, but are not labeled as to who made them or focal length, etc.

As a side note, with the shutter spring tension set to the lowest level my guess is the shutter speed is maybe 1/60th.

Thanks

[Jim Noel]

The number sequence would be a help in determining opening in f numbers.

[Mark Sawyer]

They probably used the same disks for different focal length lenses, so it's best (and easiest) just to measure aperture and focal length and divide. BTW, I have a couple of Prosch shutters, and they both run at about 1/40th, regardless of tension. The lens is probably a Darlot Hemispherique Rapide, the most common in Proshes, but there are other possibilities. If you take the front element out of its mount, there may be writing on the glass edge.

[Two23]

Using a micrometer, measure the diameter of the holes, then divide the focal length by that. Worked for me. Usually each hole is a stop, starting with wide open. I found all the speeds were the same regardless of spring tension, I guessed about 1/100s on mine.


Kent in SD

[Mark Sawyer]

Using a micrometer, measure the diameter of the holes...

Remember to measure through the front element, as it will magnify the hole, and that must be taken into account.

[rrunnertexas]

Great information and help.

There are 5 holes in the plastic disc:

1 (largest) - 1.250"
2 - .865"
3 - .620"
4 - .450"
5 - .335"

Removing both of the lens elements, I could not find any markings at all as to maker or anything. If the lens is a Darlot Hemispherique Rapide, would that mean it is of good quality?

Forgive my ignorance, but as far as focal length, is that the measurement (length) of the lens installed in the shutter? If so, it is about 2 1/4" in length.

David

[Two23]

My lens is just a generic rapid rectilinear, which is probably what yours is too. In different countries they seemed to put different lenses into the shutter. No doubt Prosch sold some upgraded lenses with the shutter too. Don't worry about it--all of them work fine. Focal length is the distance from the ground glass to the middle of the lens when lens is focused at infinity. Mount the lens on the camera, focus on something far way, take a ruler and measure the length from the lens board or aperture wheel to the ground glass--that's the focal length. Don't have to be super precise with that.

If lens is unmounted, there's another trick I use that comes close. Find where sunshine is coming in through a window and hitting a wall. Place lens against the wall with front of lens pointing away from wall. Slowly pull the lens away from the wall. A beam of light will begin to focus on the wall. When that beam is at its sharpest the lens is focused (roughly.) Measure the distance with a ruler from the middle of the lens to the wall. That is the focal length, or close enough. You use that number to divide the diameter of the holes in your aperture wheel.


Let's say you find the lens focuses the beam at 10 inches. Using your micrometer measurement for hole #1, that's 10 / 1.25 = 6.4. That would make the lens roughly an f=6.4. Divide the other numbers by 10 inches (or whatever it is) to get your other f-stops. Your smallest hole is roughly f30. I write them down and tape them on the back of the lens board for reference.


Kent in SD

[rrunnertexas]

Two23,

OK, measuring infinity focus, it is just about 15". So, using the formula you provided for the stops:

1 - 15/1.25 = 12 (f/11)
2 - 15/.865 = 17 (f/16)
3 - 15/.620 = 24 (f/22)
4 - 15/.450 = 33 (f/32)
5 - 15/.335 = 44 (/f45)

This is the information that I need to make a reasonable exposure with this shutter/lens combination. If the rain and dreary weather will clear up, it's time to make a few test shots!

This is why this is website is such a great resource! Help from others with experience and time to share it!

Thank you all!

Oh, if anyone has some images of their Prosch shutters, please add them....

David