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Looking at wildlife photography, the general beginner recommendation seems to be a cropped sensor with a 400mm lens. But, then I found out (I think this is right?) that you can't interchange lenses between different formats, e.g. FX and DX for Nikon. When you search for 400mm lenses they always seem to be for a full-frame camera. Have I got the wrong end of the stick or something? Or are they just naturally harder to get hold of for DX and APS-C? Thanks for any advice, apologies for my amateurism.

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You can use Nikon FX lenses on Nikon DX cameras. The smaller DX sensor will only capture the center portion of the larger FX image circle projected by the lens. The angle of view with a DX camera will be the same with a 400mm FX lens or a 400mm DX lens (if any existed), just like the AoV with an AF-S DX Nikkor 35mm f/1.8G lens and a Nikon AF-S Nikkor 35mm F/1.8G ED (FX) lens are the same.

If you use DX lenses on FX cameras the image circle won't fill the sensor. But many Nikon digital cameras will detect the DX lens and use "crop mode" to only use the center part of the full frame sensor that is the same size as a DX sensor.

For more, please see:

Why arent there supertele lenses for APS-C sensor sized cameras?
Is it beneficial to use a crop sensor camera with a full frame telezoom?
Will there be any difference in photos taken using full frame and crop lenses on a crop camera?

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  • \$\begingroup\$ Note also that FX lenses are massively larger, heavier, and more expensive than DX lenses of comparable zoom. An FX lens will often offer advantages that you can still take advantage of on your DX camera, for example faster focus or wider aperture. \$\endgroup\$
    – JakeRobb
    Commented Jan 22 at 21:53
  • \$\begingroup\$ At telephoto focal lengths that is not so much the case. The front group of the lens must be the same size regardless of the image circle size in order to allow for the entrance pupil to be large enough for a specific focal length and f-number. APS-C telephoto lenses are smaller lighter because they also tend to have much narrower maximum apertures than FF telephoto lenses, not because they are APS-C. A 300mm f/2.8 must have an entrance pupil of 107mm, no matter the size of the image circle it projects out the other end. \$\endgroup\$
    – Michael C
    Commented Jan 22 at 23:30
  • \$\begingroup\$ A 250mm f/5.6 DX lens, on the other hand, need only have an entrance pupil of 45mm. But that is also the case with an FX 250mm f/5.6 lens. \$\endgroup\$
    – Michael C
    Commented Jan 22 at 23:32
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But, then I found out (I think this is right?) that you can't interchange lenses between different formats, e.g. FX and DX for Nikon.

No, this is not right.

Provided that the mounts are compatible, you can absolutely use lenses designed for larger sensors on cameras with smaller sensors too; but not the other way round.

As the other answer explained, this means that only the center of the image circle projected by the lens will be used.

If you try to do it the other way round (lenses designed for smaller sensors on a larger sensor), you'll get a black circle (vignetting) around your image; some cameras may auto-crop the image to get rid of that, leading to loss of resolution.

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Nikon makes a Nikon 70-300mm f/4.5-6.3 DX AF-P lens. It also comes in a variant with VR (Vibration Reduction). The lens is very sharp and focusses fast - both useful features for wildlife photography. Here is a useful review: https://www.kenrockwell.com/nikon/70-300mm-vr-afp.htm

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  • \$\begingroup\$ You may also want to mention the lens has been discontinued since that 2016 "review". And Rockwell is... problematic. I would actually point to Thom Hogan's review laying out all five models of Nikkor 70-300 as being far more useful. He doesn't recommend purchasing the non-VR version. Telephotos without stabilization are far too limiting. \$\endgroup\$
    – inkista
    Commented Jan 30 at 8:08
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    \$\begingroup\$ Ah, I was not aware it had been discontinued. Thom Hogan's resource is very insightful - thanks! \$\endgroup\$ Commented Jan 31 at 9:44

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