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  • #16
    Re: More Interest in M4/3 Recently?

    Originally posted by DavidB View Post
    Does anybody know how to calculate the equivalent telephoto lens focal length/aperture of a spotting scope? (Wonder if I could justify the cost of a Swarovski scope on the grounds it would be cheaper than an Oly 150 - 450mm).
    No, I don't know but had a go with digiscoping some years back. Not easy to do with careful manual focussing required plus BIF is a complete none starter. Went back to a camera with a proper telephoto lens and focussing for photography and leave my scope at home.

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    • #17
      Re: More Interest in M4/3 Recently?

      Originally posted by DavidB View Post
      Does anybody know how to calculate the equivalent telephoto lens focal length/aperture of a spotting scope? (Wonder if I could justify the cost of a Swarovski scope on the grounds it would be cheaper than an Oly 150 - 450mm).
      To answer that question... first we need to agree on what you refer to with "spotting scope".

      The focal length of the scope is the distance between the front lens and the point where the light rays from come sharply into view. The focal length as such is not of main interest for the user, it is rather the magnification which in turn will be impacted by the ocular used.

      For observation purposes, a spotting scope is fitted with a (usually interchangeable) ocular which can either have fixed or variable focal length. Together with the spotting scope this gives the setup a certain degree of magnification. The formula is: magnification = (Focal length of scope)/(Focal length of ocular). Typical magnifications for practical field use span from about 15 to 100 times.

      What I did was to test using my Nikon ED82A scope fitted with a 25x to 75x ocular, fully zoomed out at 25x. I measured the distance to the target using Google maps, got 82 meters and by counting the bricks on the targeted wall I estimated the target size to be 2.4 meters. Resulting angle of view is tan-1 (2.4/82) which is 1.67 degrees. Double checked with the specs for the scope/ocular which according to Nikon is an angle of view of 1.6 degrees which confirms my estimate.

      For long focal lengths the relationship between the angle of view and focal length is linear (this is not 100% true, however the approximation is good enough for the purpose of estimating the focal length)

      For reference the m.Zuiko 300mm gives an angle of view of 4.1 degrees.

      We could estimate the effective focal length of the setup to be about 770mm when zoomed out. The front lens diameter is 82mm, meaning the setup aperture is about F/9.4. Zoom in the ocular fully and the values would multiply with 3.

      Now, I don't know what focal length and aperture you would get by using a Swarovski camera adapter https://www.swarovskioptik.com/birdi...tx-30-p5006346, I looked for specs but could not find any. I guess it would be something between 700mm and 1500mm. Have been there myself using a similar optical device for my Nikon scope providing around 1000mm focal length. Impressive reach, but ergonomy, ease of use and image quality are inferior to using a photo lens.

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