Stupid shutter question (Copal detents & speeds)

[seezee]

I think I know the answer, but can anyone confirm: a typical modern LF shutter, such as a Copal, has detents on the speed adjustment that correspond to the marked speeds. I assume the speed is controlled by a tension spring somewhere in there, so if I set the dial in between detents, e.g., halfway between 1/30th- and 1/60th-second, will I get a 1/45th-second exposure?

Anyone know how this works on mechanical SLRs or rangefinders?

[Dan Fromm]

I think I know the answer, but can anyone confirm: a typical modern LF shutter, such as a Copal, has detents on the speed adjustment that correspond to the marked speeds. I assume the speed is controlled by a tension spring somewhere in there, so if I set the dial in between detents, e.g., halfway between 1/30th- and 1/60th-second, will I get a 1/45th-second exposure?

no

[stawastawa]

nop, you get either 1/30th or 1/45. this is what the detents look like:


If the B is at 3 O'clock then the detents are at about 1-2 o'clock. there are a few more detents at 4/5 o'clock. As you can see they are stair stepped, not continuous.

[seezee]

nop, you get either 1/30th or 1/45. this is what the detents look like:


If the B is at 3 O'clock then the detents are at about 1-2 o'clock. there are a few more detents at 4/5 o'clock. As you can see they are stair stepped, not continuous.

So spring tension isn't what controls the speed? There's something less-direct, like clockwork, based on the stairstep cams?

[Luis-F-S]

I think I know the answer... I assume the speed is controlled by a tension spring somewhere in there, so if I set the dial in between detents, e.g., halfway between 1/30th- and 1/60th-second, will I get a 1/45th-second exposure?

Wrong! it's not a Leica! Sometimes you can "adjust" the speed on older Ilex and Compound shutters, but with the more modern Copal and Compur shutters, it doesn't work.

[Bob Salomon]

Wrong! it's not a Leica! Sometimes you can "adjust" the speed on older Ilex and Compound shutters, but with the more modern Copal and Compur shutters, it doesn't work.

In fact, you can cause damage to the shutter by trying to set intermediate speeds.

[Pere Casals]

I'd add that mechanical shutters often vary from their marked speeds.

Not uncommon that 1/10 is in fact something like 1/14, or 1/25 is 1/30, 1/250 is 1/160. This is specially true with old material and with not regularly serviced units.


Film has a lot of latitude, but if you need precission (with Velvia...) the first you have to do is to buy a shutter tester and check what the real speeds are, you can write down true speeds on a label and place it on the lens board.

You can buy shutter tester as cheap as $60, or even there is one for $15 new (ebay... a photocell connected to PC soundcard, then record sound with Audacity software, and measure it)


http://www.kenrockwell.com/tech/expo...rge-format.htm


Regards

[RedGreenBlue]

So spring tension isn't what controls the speed? There's something less-direct, like clockwork, based on the stairstep cams?

The spring tension is essentially constant. Some shutters do have a secondary booster spring to reach the highest speed: 1/500, typically in small and medium format shutters.

Yes, a clockwork type mechanism called an escapement retards the closing of the blades after they reach full-open. Speed cams are stair-stepped, so intermediate speeds can't be set.

[seezee]

In fact, you can cause damage to the shutter by trying to set intermediate speeds.

Good to know!

[seezee]

I'd add that mechanical shutters often vary from their marked speeds.

Ah, this I already knew.