> Full-frame lenses tend to be markedly bigger for equivalent quality compared to, say, APS-C lenses. Only if you define quality as field of view. For light-g

> Full-frame lenses tend to be markedly bigger for equivalent quality compared to, say, APS-C lenses. Only if you define quality as field of view. For light-gathering ability and background separation/bokeh, you need a lower f/number on APS-C than on full-frame to be equivalent: A 35mm f/1.2 lens on a 24MP APS-C sensor will take pictures that look nearly identical to a 52.5mm f/1.8 lens on a 24MP full-frame sensor. (Assuming crop factor of 1.5.) Both will have an aperture size of 29.17mm (= 35mm/1.2 = 52.5mm/1.8), will capture a 37.9° x 25.8° FoV. Almost all important properties of lenses are determined by field of view and the aperture diameter: Amount of light gathered, background blur, diffraction, and weight. The illumination-per-area on the full-frame sensor will be 2.25x lower, but the area of the sensor is 2.25x larger so it cancels out such that both sensors will receive the same number of photons. Background blur is determined by aperture diameter, field of view, and the distances to the subject and background. Since the two lenses have the same aperture size and field of view, you'd get the same amount of background blur for a given scene. For many lenses (particularly telephoto lenses), the size and weight are primarily determined by the size of the front element, which needs to be at least as big as the aperture. For wide-angle lenses, you start needing a front element that's significantly wider than your aperture for geometry reasons -- the subject has to be able to see the aperture through the front element, so that relationship breaks down. (Also with lenses where focal length << flange distance, you start to need extra optics to project the image back far enough. This can mean that a wide-angle lens can be more complicated to build for APS-C than for full-frame on the same mount. Take for example the Rokinon 16mm f/2 at 710g / 87mm long versus the Nikon AF-D 24mm f/2.8 at 268g and 46mm long. This isn't relevant to phone cameras, since those don't need to fit a moving mirror between the sensor and the lens like SLRs do. Phone camera makers can put the lens exactly as far from the sensor as makes sense for their design.) Slow telephoto lenses for DSLRs are pretty much the only place where crop sensors have an advantage. DSLR autofocus sensors generally need f/5.6 or better. Thus, for a given field of view, you need a bigger aperture + front element for the full-frame lens than the "equivalent" crop-sensor lens -- e.g. a 300 f/5.6 with its 53.6mm front element is going to be heavier than a 200 f/5.6 with its 35.7mm front element. However, as mentioned above, the 300 f/5.6 on a full-frame camera will gather 2.25x as much light as the 200/5.6 on the APS-C sensor. Mirrorless cameras can typically autofocus with smaller relative apertures. This is why you see Sony selling an f/8 zoom and Canon selling f/11 primes for their mirrorless mounts -- this sort of lens just wasn't possible on DSLRs. On mirrorless, you could have a 300 f/8.4 full-frame lens that would be truly equivalent to the 200mm f/5.6 APS-C lens.