Petzval lens elements

CeeBee · Apr 3, 2018

Hi, I was wondering if anyone could tell me the individual focal lengths of the 3 lens elements in a Petzval lens of a certain focal length. So they would be the positive combination at the front and the two single at the rear made up of a negative meniscus and a positive biconvex I have tried working them out from lens diagrams I have found that give the radius curves of each lens but not sure if I have got it right. Thank you all.

Sirius Glass · Apr 3, 2018

It would depend on which Petzval lens, as it would with any lens design.

E. von Hoegh · Apr 3, 2018

CeeBee said:
Hi, I was wondering if anyone could tell me the individual focal lengths of the 3 lens elements in a Petzval lens of a certain focal length. So they would be the positive combination at the front and the two single at the rear made up of a negative meniscus and a positive biconvex I have tried working them out from lens diagrams I have found that give the radius curves of each lens but not sure if I have got it right. Thank you all.

If your diagrams give the focal length of the complete objective, and radii for each element, the design can be scaled for other focal lengths - but only for glasses identical in refraction and dispersion to those originally used.

CeeBee · Apr 4, 2018

I’ll put up the measurements I have later today when i’ve found them.

CeeBee · Apr 20, 2018

So I have found the lens radius measurements ( taken from a 19 C German optics book I think) for a Petzval made by Voigtländer 100 mm f3.4 12 degrees angle of view.
All radius measurements in mm. from front of lens.
Cemented Achromat. R1 52.9 R2 41.4 R3 41.4. R4 436.2
Distance to rear pair 23.3
Negative Meniscus R1 104.8 R2 36.8
Bi Convex R1 45.5 R2 149.5
Distance between above 0.7

Using lens makers formulae I get
Negative meniscus F = -57
Rear Bi convex F= 67
Not sure how to calculate F of the front achromat from the above Info.
Can anyone work that out?
Many thanks.

Nodda Duma · Apr 20, 2018

As Sirius says, it depends on the lens design. What he meant by that was the prescription for your specific design, which won’t tie to the Voigtlander prescription unless you have direct evidence of it (measurements of your own lens or if you know for sure that it’s the exact same make/model as described in the book)

Petzval is considered a design family, and Petzval lens prescriptions will vary not only from company to company but also vary within the same company as time progressed....without any apparent difference in image quality or performance.

You also need the glass index and dispersion values to model your lens. You can’t use lensmaker’s formula without knowing the index. For example, your calculations are wrong because you essentially assumed a glass index of 2.0. That type of glass is very uncommon and wasn’t available until the 1980s or so.

Dan Fromm · Apr 20, 2018

Look here: http://www.dioptrique.info/base/obj_1914/avant-1900.HTM for some Petzval prescriptions. The glasses used may no longer be available.

Nodda Duma · Apr 20, 2018

Dan Fromm said:
Look here: http://www.dioptrique.info/base/obj_1914/avant-1900.HTM for some Petzval prescriptions. The glasses used may no longer be available.

That's a good reference. I started converting those prescriptions into Zemax format and so far in all cases have been able to use currently available glass types in the prescriptions.

CeeBee · Apr 21, 2018

I think I may be out of my depth here ! The given figures for crown and flint glass index is 1.517 for the crown and 1.575 for the flint. Thank you for the link above but I am not sure what all the given information relates to, I need to do some more reading....

Petzval lens elements

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