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 !