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

submitted 11 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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[–] Slugnan@alien.top 1 points 11 months ago

You can do this with a formula based on the megapixels. I don't know what camera you have but let's say it's 24MP and I am assuming you are using a full frame camera, otherwise that 400mm lens will have a different FOV depending on exactly the camera body you are using - for example 600mm or 640mm if it's an APS-C body. Also note all lens focal lengths are rated at infinity focus, and magnification changes near minimum focus distance are common and can result in a different effective focal length depending on lens design.

This how the math works:

(Desired focal length / actual focal length) squared

Then take your megapixel count and divide it by that value.

EXAMPLE Now let's say you want to see what 600mm would be like, using your 400mm lens and your 24MP full frame camera (feel free to correct my assumptions). Find something far in the distance to take a picture of to eliminate any magnification change caused by the specific lens design.

600/400 = 1.5. Square that (resolution is a square function) and you get 2.25. Now divide your megapixels by 2.25 (24 / 2.25) and you get 10.67MP.

Go into your editor of choice and make a 3:2 crop (the aspect ratio of a full frame camera sensor) that gives you a ~10MP final image. Crop wherever you want and that is what the image would look like if taken with a 600mm lens instead of a 400mm lens. Hope I explained that clearly.