Measuring focal length and aperture?

Discussion in 'Beyond the Basics' started by Yan Lauzon, Oct 28, 2018.

  1. Yan Lauzon

    Yan Lauzon TPF Noob!

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    Is there a way to approximate focal length and aperture of an unmarked lens that does not involve making a complex test bench?


     
  2. petrochemist

    petrochemist TPF junkie!

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    For the rear focal distance, set the lens to infinity, hold the lens up so it projects an image of a bright sky onto a suitable screen & measure how far it is from the mounting flange to the screen.
    (I've used a fence for the screen, with a tape measure or ruler held alongside the lens when I wanted a more accurate answer & just my hand for rough assessments). This test also allows you to check the image circle. Both of these factors are important if you are adapting the lens. For simple lens designs the rear focal distance will be the focal length.

    Many camera lenses are either Telephoto or retrofocal designs where the rear flange distance will not even be close to the lenses focal length. The easiest way is to mount the lens on your camera & compare image size to known lenses, but if that's not practical you can also find it by stacking the lens in front of your camera lens (both focused to infinity) and see how much of a ruler scale fits in the FOV when the image is focused and compare that with the sensor size. The magnification here = focal length of cameras lens/focal length of unknown lens.

    When the focal length is known aperture is easily determined by measuring the apparent diameter of the iris (or baffles etc) from the subject side of the lens. This dimension divided into the focal length is what defines the f-stop. A 25mm (unobstructed) diameter lens with 100mm focal length is F/4
     
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  3. Yan Lauzon

    Yan Lauzon TPF Noob!

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    Thanks @petrochemist

    I have what I begin to believe may be a large soviet enlarger lens that I am in the process of adapting to my camera, and wanted to get more information. I may just compare with another lens once adapted as it doesn't have a focusing mecanism of its own.

    Free-lens tests demonstrate that I can focus at infinity when it's less than 20mm away from my camera, but measurements while holding the camera, lens and ruler while looking into the camera prove tricky at best!

    Unsure how to measure while "set to infinity" when there's no focusing mecamism so everything I read online for calculating the specs is... less than ideal

    I ordered m42 helicoids to be able to test between 12mm & 34mm to see where I stand. I guess I'll tripod the camera, and compare with a similar size lens to get focal length.
     
  4. petrochemist

    petrochemist TPF junkie!

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    Where the lens doesn't have a focusing mechanism it can be assumed to be already set to infinity. In most cases the reversed lens will not change when focused closer it just extends the mount further from the lens elements (reducing working distance for the test). If the lens on the camera is focused closer you'll get a higher magnification.
    Internal focusing lenses often change focal length when the lens is focused so get more complicated.
     
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  5. Yan Lauzon

    Yan Lauzon TPF Noob!

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    I didn't think you could just assume infinity...

    So I use the lens as is, project an image on a surface, and measure at what distance I get focus? I can do that :)
     
  6. Yan Lauzon

    Yan Lauzon TPF Noob!

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    If I did this properly...

    I have an about 60mm focal lens.
    And it turn to about 72mm if I use the "corrected to be able to focus to infinity" m42 adapter.

    Now, the iris fully open is about 20mm, so the widest aperture would be 3 and 3.6 respectively.

    ... I thought it would be wider than that!

    Still, 60mm f1:3 ain't bad



    Fully closed, it goes to 4 or 5mm, so... F 12-15?
     
  7. petrochemist

    petrochemist TPF junkie!

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    If there's no moving parts then the distance to the image screen controls whats in focus. Choosing a subject like a bright sky makes the subject distance effectively infinite, at which point a simple lens produces an image at the focal length. The lens then focuses closer as it is moved further away.
    Simple lens formula 1/focal length = 1/subject distance + 1/image distance (1/infinity is zero)
    The problem with more complex lenses is working out which bit of the lens is the right bit to measure from. The correct point can be outside the dimensions of the lens, but I wouldn't expect it to be with this lens. For the purposes of adapting the lens the actual focal length isn't too relevant, it's the distance from your mounting point to the sensor you want right.

    I use the same helicoid for several of my projector lenses, with the mounting done so they all focus to infinity with just a touch of extension on the helicod - so I get left with plenty of focusing movement available. For a few of my lenses a few sections of extension tube need to be added too - fortunately I have some that are 52mm threaded :)
     
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  8. Yan Lauzon

    Yan Lauzon TPF Noob!

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    I received two hélicoïdal focusing adapters today, but not the adapter to connect to my camera without using the correction lens.

    Using the correction adapter, by comparing to another lens, I'd say it's an 80mm lens. My calculations were close!!

    So... 80mm f/4?

    I'll see without correction where I stand.

    The reason I ordered two hélicoïdal was mainly to be sure to get correct focus, my measurements were too much eyeballing to rely only on that. 12-17mm seems to be great for focusing portraits with it.
    ... Now I have a hélicoïdal that has no lens. I guess I need to shop for more lens. Lol

    It's actually quite natural to focus that way.
     
    Last edited: Nov 15, 2018

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