Physics-wise, S/N ratios are complicated. Photon shot noise, addition in quadrature, Poisson statistics--those sort of things. Complications on top of complications.
A more reasonable question would be "Is my camera as good as the camera that was used to take the photos for the review I read on my favorite photo site? The one that convinced me to buy the camera?"
If you are lucky, yes. More likely, no. Any marketing manager worth his corner office will make sure the cameras sent to review sites were hand picked for performance. But your camera should be close or you do have the right to demand your pixel peepin' refund.
This came up in the forums recently. A new user had bought a 'bridge' camera or, as I used to call them before marketing folks invented the name, a super zoom. Those types of cameras have little sensors since, among other things, their light weight and inexpensive super zoom lenses only make little circles of focused light. Since they also pack about 12 megapixels into their little sensor--lets say the trade offs are not favoring S/N ratios and low light performance.
Our new user picked the right place to fix his noise problems even though his expectations were originally too high. RawTherapee has great noise reduction tools but there are limitations. So a question came up. Was his camera's S/N ratio within 'spec'?
Here is the workflow I used to work out the answer by using a comparison image, RawTherapee and ImageJ
I've already posted a tutorial on how to create a noise profile in ImageJ so I won't repeat the steps here. http://scribble-jpc.blogspot.com/2013/03/crreating-noise-profiles.html
Imaging Resource is the place to go for your comparison images. http://www.imaging-resource.com/ They have created a massive data base of reference images going back to the days when a 2 megapixel camera equaled a $1000 investment. And they did things right from the start, controlling details, such as consistent lighting, needed to create reference images that highlight real differences between individual camera brands.
To download your comparison image pick your camera in the review pages, go to the sample tab and then the sample image page.
Near bottom and after a multitude of jpg images you will find Raw downloads of their multi-image test shot taken with your camera's various ISO settings. It has the Macbeth color chart we will use. I downloaded the ISO6400 version to compare it with the bowling alley picture I blogged about earlier.
Using the neutral profile I converted both NEFs into jpgs before loading them into ImageJ. That conversion insures I was doing a real apples to apples experiment. If your camera doesn't take RAW files, obviously you must use jpgs but then your test image must have been taken with the same setting as the reference image for any meaningful results.
Pick the closest match you can find on your image, the wall behind Rhianna, and on the Imaging Resouces' multi image, the #4 gray scale box. Run your profiles. Compare the graphs.
That's it. Twenty minutes time max and you know how well your camera is working. Take a few more measurement, let ImageJ do the signal averaging and you have an accurate S/N number. Then you can brag about your new camera's performance in the forums. Or. more important, toss down a hard copy on the service desk if your camera needs fixing.
As for the new user who just posted his "Thanks" -- "You're more than welcome since your questions inspired this tutorial."
You can find RawTherapee 188.8.131.52 here. The package includes an updated manual