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A place to politely discuss the tools, technique and culture of photography.

This is not a good place to simply share cool photos/videos or promote your own work and projects, but rather a place to discuss photography as an art and post things that would be of interest to other photographers.

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How to calculate how to crop an image to reflect a longer focal length specifically please ? (alien.top)

submitted 10 months ago by Smirkisher@alien.top to c/photography@viewfinder.pro

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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 !

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[–] Richard_Butler@alien.top 1 points 10 months ago (1 children)

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.top 1 points 10 months ago

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