Calculating aperture of Petzval lenses?
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?
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.
That it does. On a very saturated film it renders a very nice desaturate color palate.
you are pretty much doing it right ...
this video might help
thanks jason !