ge Given the provided Image taken at 50mm, what focal length would I need to get the 80% fo the rabbit to fill the frame?

  • To do actual calculations we would need to know the distance to the rabbit and the sensor size of the camera. Just out of curiosity: is this a quiz or does your question have a practical point to it? – Kai Mattern May 9 '20 at 23:30
  • The math en.wikipedia.org/wiki/Angle_of_view – Kai Mattern May 9 '20 at 23:33
  • A rough calculation would indicate 1450mm. If a doubling of the focal length doubles the rabbit size then we can divide the width of the image by the width of the rabbit, so 1930 pixels for the image divided by 83 pixels for the rabbit gives a factor of 23.if you want the frame to include 80% of the rabbit then multiply 23 by 1.25, which gives ~29. 50mm x 29 = 1450mm. – BobT May 13 '20 at 14:05

Your rabbit, including ears, takes up roughly 70 vertical pixels out of a total of 1,288 vertical pixels.

80% of 70 is 56 pixels. 56 pixels divided by 1,288 pixels is 1/23 of the image height.

You'd need roughly 23 x 50mm, or 1,150mm, to fill the frame with what 56 vertical pixels is currently showing.

You might as well round it off to 1,200mm, because anyone selling an 1,150mm lens will market it as 1,200mm, anyway.

  • Using the arctan formula, it's ~1172mm based on your assumptions. Rounding up seems warranted indeed. – Alexis Olson May 10 '20 at 4:49
  • Small error there: the rabbit needs 70 pixels. In order for it to fill 80% of the frame, the frame should be 125% of 70 pixels, not 80%. So you'd need a frame of 88 pixels, and consequently you would "only"need a lens of 732 mm. – slingeraap May 10 '20 at 19:41
  • 2
    @slingeraap Except the question doesn't read that way. It asks to get 80% of the rabbit to fill the frame, not to get the rabbit to fill 80% of the frame. – Michael C May 11 '20 at 5:33
  • @MichaelC: you're right, I see now. It made more sense to me the other way around, so that must have biased me. – slingeraap May 11 '20 at 8:06

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