Docter Lens Spacer

[r.e.]

I have just acquired a Copal 3, 210mm Docter lens. There is a round spacer, 6mm deep, that fits on the outside of the retainer ring below the lip. When the spacer is in place, the effect is to reduce the amount of the retaining ring's thread that can be screwed into the front element. Without the spacer, the retaining ring will not screw far enough into the front element to get a tight fit with my metal lens board.

What is this spacer for? Is it supposed to make it easier to fit both metal and wood lens boards? Does it act like a washer? Does it matter whether it goes on the front or rear of the lens board?

Thanks.

[Peter K]

Spacers between shutter and lensboard are often used with cameras that are equipped with view-finders to align the infinity-stop used with this certain lens-camera combination. If your camera has no infinity stops and no view-finder the spacer acts only as a washer and can be removed.

It's a different thing with spacers between the shutter and front- or rear-element of the lens. This spacers are used to align the correction of the lens for a certain taking-distance and must be used at the same place.

[darr]

"Around 1994/1995, Docter Optic introduced an improvement of the Apo-Germinar, a version with an additional removable brass ring spacer/washer ("Trimmring" in German) between the front cell and the shutter. It is intended to optimize the performance at the edges of the field at magnifications smaller than 1:5 (up to infinity) by changing the cell spacing. For magnifications between 5:1 and 1:5 the ring is omitted, for all other magnifications it is used. There was a test report (with the 450mm version) about this in the No. 5 (Sept./Oct.) issue of the German magazine Photo Technik International from 1995, p. 32-33. "

see this page ... (http://www.largeformatphotography.info/docter-optik.html)

[r.e.]

Interesting.

The lens is a Tessar. The spacer fits outside the wall of the retaining ring, under the lip, and goes between the shutter and the rear element. Its use, at least with my metal Linhof lens board, is not optional - the spacer is essential to get a tight fit with the board. Without it, there is a gap, with the retaining ring fully screwed into the front element, of two or three millimeters - basically, the thickness of the metal board - between the back of the board and the rear element. If what I'm describing is not so easy to visualise, I can take a couple of digital photos showing the relevant pieces.

The spacer does affect the cell spacing, but the amount that it affects it is determined by the thickness of the metal board. As an aside, what do people do about cell spacing with wood boards, which are presumably thicker? Do they recess the hole for the lens?

[KLee]

I have this lens and this is the way I set it up: Front element screwed into shutter, then the spacer, then the lens board, then the retaining ring. Screw the retaining ring in tightly such that the lip of the retaining ring is flush with the back of the lens board. This part may require a little jiggling of the spacer so that the retaining ring nestles into it.

Once this is accomplished, then screw the rear element in. The spacer does NOT affect cell spacing, at least on the lens I have. The net effect of using the spacer places the shutter so it projects out about 6mm from the the front of the lens board. I think this might be to prevent binding/rubbing when you adjust the f/stop.

If my explanation isn't clear, I'd be glad to send you some pics.

[r.e.]

Thanks KLee,

I just mounted the lens with the spacer on the front of the lens board. As you say, this results in 6mm of clearance betwen the f stop lever and the front of the board, which in turn makes advancing the lever less finicky. I guess that's the point - doesn't seem to have any other obvious function. This also means that the back of the shutter is exposed/not flush with the board, but I suppose that there's no harm in that.