Edit Learned a few things since I posted this. For the current thinking also see the next post: D7000 noise part 2
In a recent D7000 club post a low light handhold photographer wished he could afford the $2700 for a D700 body so he could take even lower light handhold photos. Since taking low light handheld photos is one of my things I became curious about how much better the full frame D700 is vs the cropped D7000. So I did some Internet research.
Everybody seems to go to dpreview nowadays but when it comes to comparing two camera's low light performance no review site beats imaging-resource. Why? Their Dave Box. They have been posting full size jpg images of that well designed target under pretty much the same lighting since 1995. Combine it with ImageJ-free NIH image analysis software- and you have everything you need to separate the good low noise cameras from the bad. And with hardly any work.
An A/B comparison of the d7000 vs the D700 Click on the image to see the full size screen clip.
What does the clip show? The smaller graph is the D7000. The larger is the D700 which I stretched out a little because it's actual ISO6400 is about 12% less than the D7000's 6400 ISO. The noise profiles were taken on the five gray patches and show the RGB signal and noise. (See my Nov 2009 post 'Fun with ImageJ' for a procedure on how I did the measurement.)
Bottom line--save your money. When it comes to ISO 6400 jpgs straight out of the camera with the default ISO noise reduction set to normal there isn't a nits bit of difference in the noise.
And if you want to really waste money go for a $9000 D3x. With that older design the 6400 ISO signal to noise is worse.
How do I defend my heresy--that big sensors aren't the greatest of the great. With the jpgs that we print up or post the S/N is a combination of many factors. But with modern DSLR cameras the wavelet noise reduction in the firmware trumps them all. The myth that big sensors are always less noisy is a hold over from the days before decent noise reduction routines. Even in the old day, the noise difference wasn't all that much. Double the sensor size with a big jump in price and you only end up with 40% less photon shot noise. (See my two Nov 2009 posts on noise for a fuller explanation.)
This is assuming the noise reduction is working. When you look at the S/N in the D3x white patch you might think that's super great except for the little spike at the beginning. Ain't true. When the noise reduction when on stage to do its strip-out-the-noise dance it got carried away. Went totally nude. What you are seeing is flat line with no detail.
Which brings up the fine art of marketing a camera. The marketing folks try to send a sample camera to a review site as soon as possible so great reviews appear as the camera hits the stores. And if the camera they send isn't quite ready for prime time? Who cares. Unless someone does measurements that patch looks white and OK.
I assume the firmware was fixed before it hit the stores. Us ordinary folks might be fooled but a pro is going to notice and not put up with that sort of performance.
Here are the details. The images I downloaded are found in the sample section of the camera review. The lighting is supposed to be 1 foot candle or 11 lumens--they messed up a little in the D700 review. Eleven lumens is what you find in a well lit city street at night. They also post a full series of shots where they cut the lighting back a stop at a time to show what you will see under worse conditions.
How accurate are the Imaging-resource images? Here are the measurements on a photo of a young rock hound I took a few days after I bought my D7000. ISO 6400 and NR normal. I've circled the areas where I took the noise profiles. If anything my camera maybe be slightly less noisy than the one used in the Imaging-resource review.
Edit- I forgot to explain the result box in the top photo. If you look at the S/N in the D7000's second darkest patch you might say it's worse than the D700's S/N. But when you calculate the rms S/N (mean/sdev) the D7000's S/N is 20 and the D700's S/N is 17. A minor quirk of the measurement and not significant either way. After all we are dealing with random noise.