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  1. T

    Birding Scope plus Barlow.

    Unforunately not an option on a birding scope. However, with the barlow lens screwed directly ino the 7,5 mm Eudiascopic that gets me under 6 mm for a theoretical 87x on the Kowa. In reality that's probably somewhere between 80x and 100x and a 1 mm exit pupil, which is the bottom limit for...
  2. T

    Birding Scope plus Barlow.

    The above paragraph requires some correction. I had falsely assumed that an eyepiece was an independent entity with its own characteristics, but the objective focal ratio has a considerable influence, the most obvious being the exit pupil (exit pupil = eyepiece focal length/objective focal...
  3. T

    Birding Scope plus Barlow.

    A Barlow would have no effect on the AFoV but I think designs like the Pentax XW (20 mm ER) or Nagler type 6 (12 mm ER) are intended to fulfil a simultaneous requirement for high magnification, large or acceptable ER, and a wide AFoV. Some time ago, Henry posted some cutaway and field curvature...
  4. T

    Birding Scope plus Barlow.

    A barlow is a negative lens placed between the telescope objective and the eyepiece and effectively increases the focal length of the objective to provide a magnification increase, usually around 2x. It is often asserted that a barlow will increase the eye relief, but this is not true. For the...
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