Counting Reflections

[Frank R]

I need help identifying the type of lens I have.

I recently bought a Kodak Slide Projector to salvage the (supposedly) petzval-style lens ("Petzval Madness" indeed, Jim Galli, you are a very bad man!).

The lens is in like-new condition. It is a Kodak Projection Ektanon.

From the research I have done, a petzval lens is supposed to make four reflections when I shine a flashlight into it. Well, this one makes six reflections (or is that two sets of three reflections?).

Can anyone tell me how to hold the light and count reflections correctly? Maybe post a picture of a real example?

Or is this lens too good for "Petzval Madness"?

 

[Ole]

The front group should give two strong and one weak reflection, the rear four strong. But that alone is not enough to determine whether it's a Petzval, it might be a reversed Tessar! So you must try to determine the approximate curvature of the surfaces, which is when it becomes difficult. It gets easier with practice, though.

Hold a light somewhere in the vicinity of your head. I just sit down in a chair under a lamp. It shouldn't be too close to your eye, a little angular separation is a good thing.

Tilt the lens slightly while observing the reflections. Tip it back and forth.

Do all the reflections move the same way, or is one or more moving in the opposite direction?

The outer surface is usually convex (there are exceptions), so that reflection will move the "normal" way. If there' an internal concave surface, that reflection will move the opposite way.

With little care and a little practice (and a loupe to make sure which reflections belong to which surfaces), you can draw a surprisingly good diagram of the lens. Good enough, at least, to place it in the right "design family".

 

[Ole]

And by the way: My first guess would be that the Ektanon is a triplet. Petzval would be way down the list of possibilities. I could be wrong, of course...