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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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[–] tdammers@alien.top 1 points 10 months ago

Here's the simplest (and easiest to remember) formula:

Crop factor equals ratio of focal lengths.

That's it. To simulate a 600mm lens by cropping an image shot on a 400mm, calculate the ratio of the focal lengths: 400mm/600mm = 2/3. Then multiply the image size by that factor (e.g., a 4500x3000 px image would become a 3000x2000 px image), and crop accordingly.

Obviously this ignores things like lens distortion, and you do lose a fair amount of pixels, but if your subject isn't too close, and all you want is an idea of the kind of reach you'd get from a longer lens, then this is perfectly fine.

Oh, and calculating the effective focal length of a lens on a crop sensor works the same way, only in reverse: say you're using a 400mm on an APS-C sensor with a crop factor of 1.5; c = f1/f2 means that f1 = c * f2, so you just multiply: 400mm x 1.5 = 600mm; that's your effective focal length.

The only caveat is that it might not be immediately obvious which focal length goes where in the fraction, but since cropping always makes an image smaller (and the subject larger in the frame), it's easy to figure out whether you got it right.