I've been shooting a lot with my new Nikon D3100 over the last few days to get a feel for it, how it performs, and whether or not it's going to meet my needs as a light-weight prime camera.
So far, I'm fairly impressed with the venerable little Nikon D3100. Image quality is second to none for a crop sensor DSLR. ISO 6400 is usable. For a $500 DSLR, the D3100 is incredible.
That said, my first impressions on the auto focus aren't as good as I was hoping.
Let me qualify that. From the specs, it would seem the Nikon D3100 inherits the excellent Nikon D90 11 point Auto Focus system, hence I expected the D3100 to be on par with the D90's excellent auto focus performance.
Unfortunately, that doesn't seem to be the case.
The D3100 hunts more than my D90 when focusing, especially indoors and in lower light situations. Obviously this type of observation is very hard to quantify. I can't stand there and shoot the same subject with both cameras at the same time, so I'm the first to admit my observations are subjective.
That said, I've shot thousands of images with my Nikon D90, and I'm intimately familiar with it's auto focus performance in all lighting situations. I've now shot over 500 images with my Nikon D3100, and I'm getting a good feel for how the auto focus performs. My initial impressions are that the D3100 is slower to focus than the D90 in all but the best of lighting conditions. Indoors in lower light situations, this is even more pronounced.
I've been shooting the Nikon D3100 exclusively with my Nikon 50mm f1.4G AF-S lens to this point. Prior to getting the D3100, in the last few weeks I've shot my D90 exclusively with the same Nikon 50mm f1.4G AF-S lens, so I have a very good feel how the lens performs on both cameras.
I've been thinking about why the D3100 would be slower to focus when on paper it should be the equal of the D90, and I think it might be battery related. The bigger 1500 mah battery in the D90 may be deciding factor. The D3100 battery is only 1030 mah so it may not be able to drive the AF-S motor in the lens as fast and sure as the D90's bigger battery can. Just a guess, but all else be equal, it's the only explanation that comes to mind.
Don't get me wrong though folks... the D3100 auto focus is VERY good. It's better than my Canon T2i in every lighting situation, and almost as good as my D90 in most lighting situations.
The Nikon D31oo auto focus is likely best in class. I'm making comparisons to a whole different class of camera, and keep in mind these are my first impressions.
Stay tuned for more on the Nikon D3100 as I continue to put it through it's paces!
Nikon D3100 Comparison of ISO 1600, 3200, 6400, & 12,800
Nikon D3100 High ISO Wedding Images
Nikon D3100 Unboxed
Nikon D3100 vs Nikon D90