Focal Length Computation with Satz (Meyer) Plasmat

[Kevin Crisp]

I have picked up a Hugo Meyer Convertible Plasmat 5X7 lens. It comes with 3 elements, which are 9 1/8th", 12" and 13 1/4". This means using it, combined, it would be 9+12, 12+13, 9+13. What is the formula for coming up with the focal length of those combinations, should I choose to do this mathematically instead of putting them together and measure from the lens board to the ground glass? 12+13 appears to be around 190mm just playing with it here in the office. The set came with a nice old box but, unfortunately, no paper work so I need to figure out the f stop table for the mm scale. Many thanks for your help.

[Ole Tjugen]

The basic formula is: 1/F = 1/F' +1/F'' where F is the focal length of the whole, and F' and F'' are the focal lengths of the elements. There's also a 1/d bit in the equation, which is more or less a "fudge factor" to correct for element spacing and such. That bit can usually be ignored.

So 9+12 is about 5.2", 9+13 about 5.4", and 12+13 is 6.3".

I had a look in my old books, but the set you have must have been made for the US market, as the focal lengths were all different in my 1910 book (in German).

[Kevin Crisp]

I finally got this beast mounted on a lens board. The formula's numbers didn't seem right to me. This is a 5X7 set and with the longest combined focal length being 160mm, per Ole's post, it just seemed like something had to be wrong. The shorter combined focal lengths in a plasmat weren't going to cover the 5x7 even dead straight on. So making measurements, the focal lengths combined come out to 200, 170, and 165 at infinity. I assume the lens board is the nodal point for measuring.

[Ole Tjugen]

The nodal point should be close to the aperture, not the lens board. With single elements the nodal point can be a long way out!

The Vade Mecum mentions a set of 9.6, 12 and 13.5 inches for 5x7", with combined focal lengths of 6.5, 7.0 and 7.5 inches., very close to what you have.

IIRC a plasmat (combined) will have something like 70 degrees coverage at smallish aperture.