Calculating aperture of Petzval lenses?

[EASmithV]

I have a magic lantern Petzval I want to use with slide film, and to do so, i'm going to need a more accurate aperture measurement than what i've been working with.

With a multi-element lens with the elements having plenty of airspace between them, where do I measure from?

Here is what I did, I racked the lens back so it was sitting about half and half in the lensboard. Then, I measured from the ground glass for the lensboard (middle of the lens-ish) and took the measurement (5") then I took a measurment of the front diameter of the lens (1.6").

5" = 127mm
1.6" = 40.64mm

127 / 40.64 = f 3.12

Did I do this correctly? Are there better methods to calculate the aperature of multi-element lenses such as this? Can anyone offer improvments to what I did?

[steven_e007]

Rather than calculate, why not measure?

How about using the same camera that you are going to mount the lens on - but mount a 'normal' lens of similar focal length. Shoot a test scene or test card at 1/2 stop increments (or less if you can) - mount the Petzval lens, shoot the same scene - compare the negs and see which is closest.

This way you take into account lens transmission as well as optical geometry. More information could be gained by comparing a colour chart - as an uncoated petsval may have a very different colour rendition to a modern coated optic.

[EASmithV]

That it does. On a very saturated film it renders a very nice desaturate color palate.

[jnanian]

you are pretty much doing it right ...
this video might help
http://www.youtube.com/watch?v=CIFNdfjem18&feature=user

thanks jason !

have fun
john