Pixel density, noise & megapixels
Megapixels are not the only factor in image quality; the size of the pixels (or ‘photosites’) also matters. Bigger photosites mean less noise and better dynamic range; smaller photosites mean more noise and less dynamic range. So because the DX-format sensor in the D500 is smaller, there’s a limit to how many megapixels Nikon can cram in before these other aspects of image quality start to suffer. Nikon has used 24-megapixel DX sensors, but it clearly feels 20 megapixels gives the ideal combination of resolution and overall image quality in the D500. In principle, the smaller photosites in the D500 should put it at a disadvantage at high ISOs. But it’s a newer-generation sensor than the D750’s and its maximum standard ISO is actually higher (51,200 versus 12,800), giving greater flexibility if you’re shooting in the dark or if you need action-stopping shutter speeds. Even so, it’s not necessarily better quality at the same ISOs as the D750.
The larger photosites on the D750’s sensor should give it a theoretical noise and dynamic range advantage, though the newer technology in the D500’s sensor will offset this – broadly, though, the old rule applies to sensors just as it did to film formats: “a good big ’un will always beat a good little ’un”. The larger sensor area of the FX-format D750 means that although it has only slightly more resolution than the D500 (24 megapixels versus 20.9 megapixels), there’s space on the sensor for much larger photosites and hence the potential for lower noise and better dynamic range. Alternatively, the larger sensor area can be used to offer much higher resolution with the same photosite sizes – so you get sharper photos with no penalty in noise or dynamic range. The D810 is a good example of an FXformat Nikon that exploits the
full-frame resolution advantage.
There’s an old saying about film: “A good big ’un will always beat a good little ’un”, and it’s just as true when comparing different-sized sensors