Professor Bob Newman takes a look at the implementation of the new base ISO 64 on the Nikon D810, and how this relates to other, similar cameras
One of the less heralded specification updates of the Nikon D810 over its D800 predecessor was a reduction of the base ISO from 100 to 64. All other things being equal (and it seems that, in this case, the sensor used is the same, so all things should be equal), that reduction of roughly 2⁄3 stop in ISO will allow the camera to gather 2⁄3 stop more light, and thus provide a smoother, less noisy image. The likelihood is that Nikon has made this move because it sees the D810, at least in part, being in competition with medium-format models.
For example, the Pentax 645Z has a sensor of 33 x 44mm and a lowest ISO setting of 100. That sensor has 1.78x the area of the D810’s and can thus gather 3⁄4 stop more light. By lowering the base ISO of the D810 to ISO 64, Nikon has all but made up for the deficiency in sensor size with respect to the Pentax.
The downside is that a wider aperture setting will be needed to get the same shutter speed as the Pentax due to the lower ISO setting, but such lenses are available. The D810 has f/1.4 lenses with 24mm, 35mm, 50mm, 58mm and 85mm focal lengths, while the 645Z has f/2.8 lenses at 45mm, 55mm, 75mm, 90mm and 150mm. Thus, with only 3⁄4 stop deficit in sensor size, the D810 can more than make up for this in the availability of lenses 2 stops faster. The D810, with its new lower base ISO, should therefore be able to match the image noise and smoothness of the 645Z – although it still has fewer pixels.
The general principle is that a smaller-format camera can capture the same amount of light, and thus produce similar results in terms of image noise, to a larger-format camera if used at an ISO that is divided by the square of the crop factor. Thus, if manufacturers were to design their cameras with base ISOs that produce the same ‘image quality’, we would see smaller-format cameras having lower base ISOs than larger-format ones. If the ‘reference’ base ISO is 100 on an APS-C system, we would expect a base ISO of 225 on full frame, ISO 60 on micro four thirds and ISO 30 on 1in cameras, such as the Nikon 1.
In practice, this doesn’t seem to happen, with ISO 100 the usual base ISO, and some cameras having it at ISO 200 or higher. This means that owners of smaller-format cameras are compromised with respect to the ultimate image quality available, although they do get the advantages of lower cost and better convenience in return.