I ran down the specs of the new Nikon D3100 against the time-tested Nikon D90 to get a feel for how they compared.
Yes, I know, the D3100 is replacing the D3000, not the D90, but take a look and see how well the D3100 stacks up against the D90.
The biggies here are the sensors, lens compatibility, monitors, HD movie capability, and full-time autofocus for video.
The D3100 sensor features a few more megapixels, 2.1 to be precise. Not really much in term of real resolution, but hey... all take it given it doesn't appear to have impacted negatively on performance.
Which brings us to the other big point on the sensor... high ISO performance. Looks like the D3100 is going to beat the D90 by at least a stop for high ISO performance, and it's still giving us two more megapixels! This is HUGE! I'm always looking for better high ISO performance, and this comes to us in a very nicely priced DSLR.
Lens compatibility is where the D90 beats the D3100 hands down. The D3100 won't auto focus with anything other than the newer G lens that have an autofocus motor. The D90 will auto focus with any AF Nikkor. This will be a big issue for some, and a non-issue for others. For me, not so much. I'll be buying the D90 replacement (D7000) as well, so I'm thinking my Nikon 50mm f1.4G prime will be permanently glued to my D3100. :-)
The monitor is another big difference between the D3100 and the D90. The D3100 monitor is a lower resolution monitor similar to the D5000's, while the D90 is the sweet 920,000 resolution monitor just like it's bigger brothers, the D300, D300s,D700, D3, D3s, and D3x. This is a biggie for me. I didn't like the low resolution on my D5000's monitor, and it was one of the main reasons I sold it in favour of my D90. Then again, the D3100 is only going to be about $500 so you can't have everything.
On to the HD Movie capability! The D3100 is the first Nikon DSLR to feature full 1080p HD! The D90 is only 720p, so the D3100 wins here. Remember though, the D90 is now about 2 years old and was the first DSLR ever to have HD movie capability.
And last, but not least... in fact, perhaps the BIGGEST difference of the D3100 and ANY OTHER DSLR currently available, is the full-time autofocus on the D3100. The Nikon D3100 is the first Nikon DSLR to feature it, and nobody else has it. Can you say Gamechanger! I can't wait to see how the D3100 performs at 1080p HD with the new full-time autofocus!
For the first time, I'm excited to buy Nikon's entry level DSLR. I've never owned anything in the low-end Nikon DSLR line-up before. The Nikon D5000 was the lowest down the chain that I have owned, and I sold it in favour of the Nikon D90. The D3100 though has SO much to offer that I think it will be worth having, especially if it turns out to be the perfect prime camera as I hope.
Nikon D3100 vs Nikon D90 Spec Comparison
Sensor - D3100 14.2 megapixel vs D90 12.3 megapixel
HD Movie Mode - D3100 1080p with full-time autofocus vs D90 720p (no full-time autofocus)
Auto Focus - 11 point AF system on both D3100 and D90
Compatible Lenses - D3100 only AF with G with AF motor vs D90 AF with all Nikkor AF lenses
Metering - 3D Color Matrix Metering II on both D3100 and D90
ISO - D3100 ISO 100 - 12,800 at 1 EV steps vs D90 ISO 100 to 6,400 at 1/3, 1/2, or 1 EV steps
Active D Lighting on both D3100 and D90
Built-in HDMI port on both D3100 and D90
Monitor - D3100 3" 230,000 dots vs D90 3" 921,000 dots
Battery - D3100 EN-EL14 Li-ion Rechargeable Battery vs D90 EN-EL3e Li-ion Rechargeable Battery
Dimensions - D3100 4.9"wx3.8"hx2.9"d vs D90 5.2"wx4.1"hx3.0"d
Weight - D3100 16oz (455g) vs D90 22oz (620g)
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