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Thread: Confused by lens math

  1. #1

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    Confused by lens math

    I just got a new to me Wollensak Verito 8-3/4" f4.

    It has no aperture markings...or rather it's got some marking made in whiteout. So I thought I'd use some math to figure out what they were. The first (biggest) needing to be f/4 by default.

    So 8-3/4 inches is 222mm. When I measure the size of the aperture I get 45mm and 222/45 = 4.94...call that f/5.

    What am i doing wrong?

    Thanks for the help!
    Laurent

  2. #2

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    Re: Confused by lens math

    The aperture measurement to use is the diameter of the entrance pupil. Open the diaphragm wide, hold the lens at arm's length, front facing you, and measure the diameter of the aperture's image.

  3. #3

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    Re: Confused by lens math

    Thanks Dan,

    But I'm still measuring 45mm for the opening. If I measure the front lens element, I get 55 which gets me to f/4.

    The lens is mounted in a copal 3 shutter. Can I assume the shutter is the limiting factor and I now have a f/5 lens because of the shutter?
    Laurent

  4. #4

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    Re: Confused by lens math

    Quote Originally Posted by ljb0904 View Post
    Thanks Dan,

    But I'm still measuring 45mm for the opening. If I measure the front lens element, I get 55 which gets me to f/4.

    The lens is mounted in a copal 3 shutter. Can I assume the shutter is the limiting factor and I now have a f/5 lens because of the shutter?
    If you have an exposure meter you can put the lens on your camera and meter your gg. This works best with a microscope adapter or a short tube to eliminate stray light and to position the meter directly on the gg.

    You would have to factor in the light loss of the gg and fresnel screen.

  5. #5

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    Re: Confused by lens math

    Quote Originally Posted by Dan Fromm View Post
    ...Open the diaphragm wide, hold the lens at arm's length, front facing you, and measure the diameter of the aperture's image.
    When I first read that I thought, what the??? Must have slept through that lecture when I was in college. Then again I went to an art school so I didn't spend much time doing math.

    After doing a quick search, sure enough, it is the entrance pupil that determines the f/stop, not the physical diameter of the aperture. I tried it on one of my lenses and though it was tricky measuring the virtual image of the entrance pupil, it is somewhat larger than the physical aperture. Makes sense because the front element by itself can be used as a magnifying glass. Oh yeah and it looks like my old lens is marked in US stops so the minimum marked stop is f/256 which is actually f/45 and by my measurements and calculations it comes out to about f/43.5 -- close enough.

    @ljb0904 - Are you sure you're measuring the virtual image? On your lens it should be slightly larger than if you remove the front element and measure the aperture opening.

    BTW--measuring from the back of the lens will give you the exit pupil. On my lens the exit pupil is slightly different than the entrance pupil.

  6. #6
    Mark Sawyer's Avatar
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    Re: Confused by lens math

    Quote Originally Posted by ljb0904 View Post
    The lens is mounted in a copal 3 shutter. Can I assume the shutter is the limiting factor and I now have a f/5 lens because of the shutter?
    That's the culprit. The Copal 3 has a smaller maximum aperture than the lens' original shutter, so your f/5 measurement is right. the good news is after using the Verito a while, most people seem to like it between f/5.6 and f/8.

    BTW, it's a convertible, so if you remove the front element you'll have a 14-inch lens. Figure the f/stops the same way.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  7. #7

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    Re: Confused by lens math

    Also, be aware that SF lenses might have an increased flare/fog level in under actual shooting conditions the exposure might be (usually) even more light, so you might have to calibrate a new scale for your shooting style and materials used... And your printing process might require different approaches to the different tonality...

    Steve K

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