Shutter/aperture question

[Flynnie]

Newbie question, I've just got a Sironar 210/5.6 in a faulty Compur Electronic and just noticed when stopping down the aperture that the blades only appear once it's down to about F10, i.e. the actual hole in the shutter right in front of the aperture seems to be smaller than the aperture hole from 5.6 to 10. Does that mean this is the wrong shutter for the lens? The image on the GG is nice and bright though, so maybe I'm worrying about nothing, but the image stays just as bright till I get to F10 so something is up, isn't it?

[Leigh]

I have an Apo-Sironar-S 210/5.6 in a Copal #1 shutter, and the aperture blades close uniformly throughout the range.

Sounds like you have the wrong shutter for the lens.

- Leigh

[Flynnie]

Does sound like I have the wrong shutter Leigh, I'm sure the aperture stops down just fine but the widest settings are just hidden, still, the GG is surprisingly bright considering F10 is the effective max aperture, and I probably wouldn't use it wide open anyway.

[Leigh]

...still, the GG is surprisingly bright considering F10 is the effective max aperture, and I probably wouldn't use it wide open anyway.

It's certainly usable, although your range of apertures may be less than nominal.

You'll probably need to do exposure comparisons with it and a lens with a correct aperture scale, since this one is likely off by a couple of stops.

The other lens need not be of the same focal length, although the tests would be a bit easier and perhaps more accurate if they were similar.

- Leigh

[John Koehrer]

Are you looking through the front or rear of the lens. If it's from the front, the aperture should be very obvious as it stops down.
If you're viewing from the rear and don't see the aperture until f10, something's awry.
I just looked a four lens/shutters that I have and as you begin to stop them down they immediately begin to enter the light path.
If nothing is mechanically wrong with the shutter, I'd think it is the wrong shutter for the lens.

[Flynnie]

Leigh, I hadn't thought that the apertures might be off, perhaps it'd be best to find the proper shutter for this, I think it's a Copal 1.
John, this is looking from the front but it's just the same from the back - you can hear the aperture opening up but the blades don't start peeking out of their slot till F10, perhaps this shutter is meant for an F8 or 9 lens, but what's the point of the wide aperture when its widest settings are hidden, strange! Thanks for chiming in guys.

[Leigh]

Leigh, I hadn't thought that the apertures might be off, perhaps it'd be best to find the proper shutter for this, I think it's a Copal 1.

If the aperture blades are first visible at a scale marking of f/10, then that corresponds to a true aperture of f/5.6, so the scale is off by almost 2 full stops.

... what's the point of the wide aperture when its widest settings are hidden, strange!

Remove the rear cell and see if the aperture starts to close immediately from the full-open position.

That shutter is made for many different lenses. The aperture area actually used depends on the optical and mechanical design of each particular lens.

- Leigh

[Flynnie]

Hi Leigh, it's the same story from the back, the aperture is sandwiched between metal rings with about a 29mm hole, and 210:5.6 = 37.5mm, so it's the wrong shutter basically. Still don't understand why they would bother to put an aperture assembly in a shutter that can't take advantage of its wider openings.

[Leigh]

As I said before, shutters are made to work with a wide variety of lenses.

The full aperture for a 75mm/5.6 is only 75/5.6 = 13.4mm.

The metal rings are unusual. They're not present in standard shutters (none that I've seen, anyway, and I repair them).

I would speculate that what you have was originally set up to mount cells of a barrel lens, and the rings are spacers to hold the cells at the proper separation.

- Leigh

[Jon Shiu]

Hi, the max iris in a Copal 1 is about 30mm. So you're shutter/aperture should be okay. Measure the apparent size of the hole looking through the front lens. It should be closer to your calculation.

Jon