Lenses, sensors & image circles
DX super-wide zoo ms: Smaller choice
If you want a super-wide-angle lens for a DX-format Nikon like the D500, you’ve no choice – you have to get a DX-format lens, typically a 10-24mm zoom, and almost all are variable-aperture ‘consumer’ lenses.
DX stand ard Zoo ms: More for amateurs
Again, the only way to get a useful range of focal lengths in a standard zoom for a DX-format Nikon is with a DX-only lens, typically in the range 18-55mm or 18-105mm. There are some constant-aperture lenses, but the only Nikon one is the old and expensive non-VR 17-55mm f/2.8.
Telephotos : DX has an advantage
With telephotos, though, an FX-format lens makes sense, even on a DX camera. The effective focal length increase is a benefit rather than a drawback, so Nikon’s 70-200mm f/2.8 becomes a 105-300mm f/2.8 equivalent – and you can still use it if you switch to an FX-format Nikon later. You can use FX lenses on DX cameras but the lens’s effective focal length increases by a ‘crop factor’ of 1.5. All lenses need to produce an image circle large enough to cover the sensor area. Nikon’s DX sensor is smaller than its FX sensor, so it’s possible to make smaller, cheaper and lighter lenses specifically for this format. The drawback is that these lenses are little use on FX-format Nikons (except in Nikon’s lower-resolution ‘crop’ mode). Sometimes, though, a DX-format lens is the only option for owners of DX cameras because although you can use FX-format lenses too, their longer focal lengths usually rule them out for everyday and wider-angle photography.
The larger area of the FX-format sensor in the D750 means that you need lenses with a larger image circle to cover it – in other words, full-frame or FX-format lenses. You can physically mount smaller DX lenses on these cameras, but they must switch to a ‘crop’ mode that uses a smaller area of the sensor with half as many pixels (actually, rather fewer than half), so this is an emergency measure rather than something you’d want to do every day. The larger image circle easily covers DX sensors too, of course, so you can use FX lenses on DX cameras as well.
Dx sensor 24x16mm Sma llerDXlens imag e circle
50mm lens on a ‘crop’ sensor
Actually looks like 75mm
fx sensor La rger FX lens imag e circle 36x24mm
Actually looks like 50mm
50mm lens on FX camera