I use the R.D. Gray Periscope No. 6 on my 8x10. I'm curious if the lens design being described in this thread is of the same family as the R. D. Gray by the same name? I'm curious as I've traveled these forums for a number of years and have never heard mention of the Periscope lens. Now it's the talk of the town. ;-)
Basic sales info for the R.D. Grays No. 1 through No. 10 (ef 2-3/8" thru 36") can be found here . . .
Here are prescriptions for two Steinheil Aplanat Periskop lenses:
wide angle (90 degrees): http://www.dioptrique.info/OBJECTIFS1/00022/00022.HTM
narrow angle (45 degrees): http://www.dioptrique.info/OBJECTIFS4/00181/00181.HTM
Also see Eric's comments on Periskops at http://www.dioptrique.info/base/comp...s/periskop.HTM
Both lenses are symmetrical, both are, by modern standards, dreadful. If you want to calculate the performance of lenses for which you have prescriptions, you can download his program 030 from here: http://www.dioptrique.info/base/comp...s/a_propos.HTM
If you want to get a reasonably flat field, the curvatures of the lenses and the spacing between them is important. If the curvatures are fairly steep, you can get a flat field with the elements close together. If the curvatures are less steep, the elements need to be farther apart. If the curvatures are too steep or too shallow, a flat field cannot be obtained.
Dan, I am modeling ( just as a learning ex) the 90 degree wide angle prescription on the OpticalRayTracer3.2.jar
The model is predicting an on axis focal length of about 98 mm and the optimum increases to about 110 mm at about 35 degrees off axis ( ie 70 degrees coverage).
But I am not sure If i read the prescription correctly, in the 12.6 mm distance between the lenses.
Do you know if that distance is between glass to glass on the axis , or were some other reference datum used in the lenses?
What frame size would this lens have been intended for?
What does "no dioptre diaphragme :2 " mean?
The distances are measured on-axis.
means that the diaphragm goes after the second surface. The lenses are 1.3 mm thick, the distance between them is 12.6 mm. You got it right.
no dioptre diaphragme :2
If you click on "courbes" you'll see some calculated performance data. The news isn't good, horrible curvature of field. But since the OP wants to use a long lens on a small format, i.e., need the lens to cover only a small angle, he may be able to get away with it.
The calculated spot diagrams are dreadful too. The world gave up on periskop lenses for good reason.
I will keep chewing away at the model as it is a simple one to understand. I will try it on 6X7 diagonal with different apertures too.
I suppose it will be better bang for the buck to get some surplussheds and model them, then try in real life !
Some screen shots of the rays through the 90mm periscop.
The rays are from a point at 5 metre.
At f/40 and 40 degrees ( near edge of 4 x 5) the COC ( spherical + chromatic) was about 200 micrometre. Not too bad?
The biggest problem was that the focal plane moves as the aperture changes.
F/40 > F/11 it moved from 98mm to 88mm.
The second shot is at f/11 and 26 degrees to simulate the lens on 6 x7 with a practical aperture.
With best focus on the axis, the COC at 26 degrees is about 500 micrometer as shown. No good at all, I think