View Full Version : Has anyone mounted a filter to the back of a Copal 3?

17-Mar-2017, 19:06
I've mounted a f/2.8 lens to the front of a Copal 3. Because the lens was never meant to have a shutter, I can't use the aperture in the Copal 3. To account for changing light levels, I want to be able to use ND filters to reduce the exposure to a speed the Copal 3 can handle. The rear thread for a Copal 3 is supposedly 58x0.75, which means I could thread a 58mm filter into it. However, the threads are somewhat recessed and I don't think a normal filter has enough threads to actually extend down into the threaded portion of the shutter and screw in securely. Unfortunately, I have no 58mm filters to test this idea.

Has anyone actually tried this?

Doremus Scudder
18-Mar-2017, 02:52
Mounting a glass filter behind the lens will change the focus. You will have to focus with the filter in place, not an insurmountable problem unless you're using a rather dense ND filter. Ideally, you would mount the filter in front of the lens. Maybe for such a big lens and gel would be a good solution. A gel could be mounted behind the lens as well with no appreciable change in focus (gel filters are thin enough that the focus shift is negligible).


18-Mar-2017, 03:21
I don't think a normal filter has enough threads to actually extend down into the threaded portion of the shutter and screw in securely. Unfortunately, I have no 58mm filters to test this idea.
I've a 58mm ND4 filter - the male tread is not more than 2mm long and the outside diameter of the filter is 60mm


18-Mar-2017, 11:45
Why not mount a holder for glass or gel filters on the front of the lens?
I'm thinking of the Cokin system or similar.

Then you can insert/remove/change as needed with minimal effort.
Those filters don't change the camera's focus like filters behind the lens do (see above).

- Leigh

18-Mar-2017, 12:24
It's a projection lens so there's no front threads.

I'd prefer to use a front filter but I can't figure out how to attach one to this lens.

18-Mar-2017, 12:31
There are filter holders that attach to the lens using three rubber-tipped screws, bearing on the outside.

As long as you have a cylindrical surface (not tapered), these work fine.

- Leigh

18-Mar-2017, 14:58
It's a slide projector lens so the very front is tapered and much wider (~8mm side/16mm dia larger) than the barrel. The barrel is cylindrical. Can you link to one of the filter holders you're talking about? If it can "reach around" the bezel on the lens to grab the barrel that would be a nice solution for me.

18-Mar-2017, 17:54
This should work: Metal Spring Type 75mm Filter Holder (http://www.largeformatphotography.info/forum/showthread.php?133411-FS-More-LF-and-MF-Accessories)


18-Mar-2017, 18:20
Kumar, thank you for pointing those out but I don't think they'll work for me. 75x75 (3x3") filters seem fairly uncommon in the US. The few I could find listed in the densities I need are special order only and very expensive.

18-Mar-2017, 18:43
Would 100mm filters work for you?

18-Mar-2017, 19:29
Where are you located?

Might I suggest you add that to your profile so we don't have to ask.

- Leigh

18-Mar-2017, 20:07
Licking County, Ohio; it's close to Columbus.

Kumar, yes, it looks like the 4x4" (100x100mm) filters in 2mm thickness are substantially more commonly available and are also cheaper.

18-Mar-2017, 20:16
I think I have one for that size as well. I'll check and send you a PM later.


Jim Andrada
18-Mar-2017, 22:59
I have a compendium lens shade and it accepts gel filters - very useful.

One thing you might think about is a variable density cross polarized filter. Just twist the front ring to vary the strength.

Doremus Scudder
19-Mar-2017, 03:23
A bit of tape and a gel filter and you're set to go. A gel filter just needs to be in the light path and relatively parallel to the lens' front surface. I used to tape gels with "skin tape," which is normally used for bandages. It holds well enough and peels off easily. (It's even reusable; I still have a couple of strips on a number the cardboard sleeves that the gel filters come in). Heck, you can even just hold a filter in front of the lens when you trip the shutter. I've done that often. Taping a gel to the back of the shutter would work too. When it was windy, I'd just remove the camera back, tape a filter to the rear lens element and replace the back; no need to dismount the lens that way and wrestle the filter through the opening in the front standard.