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I got into birding recently and got a pair of 10x42 binoculars and got a camera with a crop ratio of 1.6 and a 100-400mm (focal length) lens.

With binoculars and spotting scopes everything seems easy - you get a magnification number, so it easy to grasp the idea of how close the objects will appear through it. My 10x42 binoculars have 10x magnification - easy.

Now with lenses, it get's much more tricky, I read several SE questions about that and I understand that full frame camera x 400mm will produce similarly magnified image as a camera with 1.6 crop ratio and a 250mm lens (1 x 400 = 1.6 x 250). But I cannot find any formula how to convert that into magnification and compare with my binoculars. And I think I also understand what zoom means - my lens has a zoom of 4 (= 400 / 100), but it is a useless number in this conversation.

So I read in multiple places that a naked human eye is very close to 50mm (one place gives 43mm) camera lens on a full frame camera. So going by that, 400mm lens on a full frame camera would have 8x (= 400/50) magnification and 400mm lens on a 1.6 crop ratio camera, would have 12.8x (= 400/50*1.6) magnification? It this how it works? And either way, what is the fundamental reason, that photographers don't talk about magnification of lenses, but binocular users do?

eddyP23
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  • Best is to [edit] the question to make it clear it's about photography, with binoculars only as a comparison item. Then folks can vote to reopen. Or, since I don't know if that's an option for duplicate, write another question that's clearly about photography -- assuming the linked dupe doesn't answer the question you wanted to ask. – Zeiss Ikon Nov 04 '22 at 12:10
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    @ZeissIkon In general, reopening is an option for duplicates. I believe that applies even though I mod-closed it but if not I'll be happy to re-open this if it's edited to explain why it's a different question from the linked duplicate. – Philip Kendall Nov 04 '22 at 12:16
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    Your understanding of camera magnification is roughly right, which is as good as it gets. Because binoculars provide an image to your eyes one can look at the ratio of the angular size of an object with and without the binoculars. For a camera the similar thing is a ratio of focal lengths but there is no easy standard for x1. 50mm full frame is often used, as you say, and I would use that. – Ross Millikan Nov 04 '22 at 14:05
  • Thanks @PhilipKendall, the question you linked is basically the same – eddyP23 Nov 04 '22 at 14:05
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    "full frame camera x 400mm will produce similarly magnified image as a camera with 1.6 crop ratio and a 250mm lens". The terminology here is not correct. They have the same angle of view. That's not the same as magnification. Remember that a camera lens is designed to project an image of a real-world object onto a comparatively small sensor. The image is then "enlarged" for viewing. – osullic Nov 04 '22 at 18:12
  • The amount of magnification that takes place in the viewfinder must also be taken into account. The same lens on a camera with the same size sensor/viewscreen which has a larger viewfinder will magnify the subject more in the viewfinder than a camera with a smaller viewfinder. – Michael C Nov 04 '22 at 18:14
  • Back in the film SLR days, most "35mm" cameras actually gave 1.0X magnification at between 55-62mm focal length. The idea that looking through the viewfinder of a film SLR using a 45mm or 50mm and seeing 1.0X magnification is oft-repeated, but it wasn't true for any of the most popular models. Maybe that idea was a carryover from the 120 film format? With smaller viewfinders on lower end digital cameras, one needs to go up to around 70mm to get 1.0X magnification, so that the subject in the viewfinder is the same size as viewed with a naked eye. – Michael C Nov 04 '22 at 18:27
  • For your camera 30mm is "normal" magnification 1. Thus mount a 300mm and the magnification is 10X. – Alan Marcus Nov 04 '22 at 18:48

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