>>3668415>>3668417To add to the “1000mm” not actually being 1000mm thing, getting to the focal lengths that they do (and often at impressive-sounding apertures like f/2-3.5) is much easier because the small sensors only need a small image projecting onto them. A 1000mm f/3.5 lens that can cover full frame (or even M4/3) would be huge and very expensive, and probably poor quality unless the optics are corrected to the hilt which will make it even bigger and even more expensive. See
>>3668147 