Hi: I have one dagor (or dopple anastigmat) that has two scales, one for one element, and one for two elements. I have a second dagor without such scales, and I have been told that all dagors are convertables. Is there a mathmatical factor when you take off an element? Can I just figure out from the known scale, or are these optic equations non-linear? I'm not good at math, but good enough to know I'm not good at it. Can't seem to find this one in the archives. Thanks Dean

-- Dean Lastoria (dvlastor@sfu.ca), December 18, 2001

Answers

I know this is not terribly precise but you can expect a focal length of approximately 1.9 or so. One way to determine the focal length more accurately is to set up a camera so that you have an image at 1:1 magnification. Measure the bellows draw, which should be approximately two times the focal length (again this is not precise because when you remove an element, the principle plane typically moves to behind the element but it will be close enough to be workable). Alternatively, if you can measure the distance between the principle planes, either by autocollimating or by measuring the intercell distance approximately, you can use the lesn equation

1/f = 1/f1 + 1/f2 -d/(f1*f2)

where f is the combined focal length, f1 and f2 are the individual focal lengths of the two elements. In the case of the symmetrical Dagor elements, f1=f2 and the expression reduces to

1/f = 2/f1 - d/(f1*f2)

Also, while Dagors were patented and sold as convertibles, the individual cells are not corrected for coma, a somewhat teardrop shaped spreading of the image points away from the center. A complete Dagor lens relies on symmetry to correct coma, lateral color and distortion. When using the single component, these corrections obviously disappear. Distortion may be apparent near the edges of the image circle but may not be too problematic near the center. Lateral color is corrected in convertibles by using a strong monochromatic filter. However, to reduce the coma in a Dagor, you will need to stop the lens down considerably. Think of f/45 as the minimum aperture you will need to work at. Having said that, some superb images have been made with single elements of Dagors (Ansel Adams "Cliffs and frozen lake" for example).

Incidentally, the Angulons are convertible also. Triple convertible actually, since the design is non-symmetrical. Cheers, DJ.

-- N Dhananjay (dhananjay-nayakankuppam@uiowa.edu), December 18, 2001.

Hi Dean, I think it( the conversion factor) is: multiply by 1.8 when you are using one element. Of course I'm far away from my files right now, and so I can't check that figure. Try 1.8 or 1.6, something like that. Best, David

-- david clark (doclark@yorku.ca), December 18, 2001.

Hi: Thanks for your answers. So does my math sound right? With both elements the scale says f16, so with one it is f30.04; with 5.6 it is f 10.6; and with f22 it is 41.8? That seems easy enough. Thanks Dean

-- Dean Lastoria (dvlastor@sfu.ca), December 18, 2001.

>>"Incidentally, the Angulons are convertible also. "

Does this include Super Angulons?

-- Andrew Cole (laserandy@aol.com), December 19, 2001.

So, if you want to convert your Dagor in a pinch, do you think it's better to use the front element or the rear element alone?

-- David Goldfarb (dgoldfarb@barnard.edu), December 19, 2001.

With both elements the scale says f16, so with one it is f30.04; with 5.6 it is f 10.6; and with f22 it is 41.8?

>>> That is about right.

"Incidentally, the Angulons are convertible also. " Does this include Super Angulons?

>>> The Angulon was derived from the reversed Dagor design - both the Dagor and the Angulon consist of two groups of three elements. The power of the elements is reversed in the Angulon. In addition, the Angulon is not symmetrical and shifts some power from the front to the rear cell to optimize for infinity. The two cells of an Angulon are of slightly different focal lengths (to optimise the complete lens for infinity). When the lens was first sold it was claimed to be a triple-convertible. Like other Dagor derivatives, the individual cells are not corrected for coma. Image quality can be quite grim at stops short of around f/45. The Super Angulon is of a different design incorporating air spaces and is not, to the best of my knowledge, convertible.

So, if you want to convert your Dagor in a pinch, do you think it's better to use the front element or the rear element alone?

>>> It is immaterial - the design is a symmetrical design. However, in general, using single elements behind the stop is a good idea since the stop position helps to correct some aberrations. However, when using the cell behind the stop, the bellows draw can be come quite long (since the nodal point lies behind the lens and well behind the front standard). Thus, when bellows limitations prevent using the cell behind the stop, it is sometimes used in front.

Cheers, DJ.

-- N Dhananjay (dhananjay-nayakankuppam@uiowa.edu), December 19, 2001.

DJ: Thanks again, and thanks for checking my math!

Dean

-- Dean Lastoria (dvlastor@sfu.ca), December 19, 2001.

Thanks, DJ. My Dagor is a 12" and I can just barely eke out 24" of bellows on this camera, so I suspect if I ever try it, I'll be using the front element rather than the rear.

-- David Goldfarb (dgoldfarb@barnard.edu), December 19, 2001.