The new Nikon D7000 is out, and a lot of previews has touted the "native iso" of D7000 to be 100.
What does this actually mean? I'm assuming it means it performs at its best at iso 100, which means if you're ok to sacrifice light sensitivity, you'll get really great images...?
From what I gather, it appears to be yet another silly measurement for gear-heads to obsess over.
Here is a pretty good overview that I found regarding both Native Iso and Base Iso.
Obviously from the tone of my answer, I'm not really keen on such qualitative measurements. I suppose if you need a way to stack-rank compare bodies it might be valuable, but in my opinion it needlessly complicates things with criteria that aren't really that important.
As I understand it the "native" or "base" ISO is the sensitivity you get without amplifying the analogue signal you get from the sensor. It becomes important when the native ISO is higher than the lowest available on a camera (e.g. the base ISO is 140 and the lowest setting is 100). In this case the camera is likely to overexpose the image (as you can't unamplify the signal to recover the highlights) and the non-amplified signal is more likely to be affected by the read noise of the electronics (then read noise of the electronics is roughly constant so if you have a small signal the read noise is higher by comparison).
As already stated it's unlikely to actually be noticeable in images however if you always strive to use the lowest ISO the camera offers whenever possible, you may be wasting your efforts as the image quality may be just as high/slightly better one setting up.
For further reading:
ISO is changed by applying gain at the analog stage of the signal (which, incidentally, is why you can't change the ISO in raw), and the base ISO is the amount of gain at which the signal-to-noise ratio is the maximum.
In practice, the one at which the image is the cleanest. But that part is already obvious to everyone.
There is a noticeable difference if you are shooting in a wide light range. An example would be a bright sunny day with high contrast. I have a sony a7r. The native iso is 100 but I can shoot the iso at 50. I started shooting most photos at 50 to have as little grain in my images as possible and have that crystal clear image. On most photos I didn't notice a problem until I shot in an extreme condition. I found that my extreme lights and darks were clipped when I shot under 100 iso. A good way to test this is to go out on a bright sunny day and shoot the sky and land together. You'll notice the brighteset parts of the clouds are clipped when using an iso of 50 or 80. When I went to an iso of 100 I had no clipping in my images
Steve is right. His experience pretty much summarizes what a non-native ISO can do to our images. Native ISO is 100, 200, 400, 800, 1600, ETC the little numbers in between are not native and what they do is to digitally process our files to mimic the results of those ISO numbers (ISO 125, 160, 250, 320, 500, 640 etc) but the results generally aren't good and clipping and other artifacts may occur.