Hi,

I’ve got my hands on a 400mm telephoto lens to try, as i don’t own any telephoto lens, to see which focal length would suit my needs.

I took all the shots i wanted to take, but for most i was too far away.

I would like the be able to deduce from the shots i’ve made which focal length would have been appropried for the results i wanted to do, that is, by cropping the images i’ve got.

Do you know any calculations that would allow me to translate the crop i will do to relate which focal length would have matched the same field of view ?

Thanks !

  • Richard_Butler@alien.topBEnglish
    1·
    10 months ago

    Calculate the diagonal of the uncropped image:

    Diag = SQRT(Power([image X dimension],2)+Power([image Y dimension],2))

    Then calculate the diagonal of the cropped image, using the X and Y pixel dimensions of the crop. Divide the original number by the smaller number to get the crop factor.

    The effective focal length you used was 400 * crop factor.

    • Smirkisher@alien.topOPBEnglish
      1·
      10 months ago

      Thanks for the detail, perfect. I’ll crop as I wish and see what focal length would have been ideal.