Our thanks to DxOMark.com for allowing us to quote their test data for this story.
Fortunately, the photography community now has a tool online that helps us determine how high we can go, ISO-wise, with specific cameras before the digital noise is too much. DxOMark.com is an independent test lab that determines each camera's image quality based on data from its RAW images, and they have measured over 50 cameras for Low-Light ISO capabilities.
Low-light image quality is not the only criterion on which to base a camera purchase (color accuracy, layout and ergonomics, lag time and image control features are among other things to consider). But if you prefer using existing light, any test results that show how cameras will perform under low light conditions should be considered when deciding on your next camera purchase.
Anyone who understands DxOMark's data (such as--ahem!--veteran photography writers and editors) can now determine which of the tested cameras offer the best balance between speed and quality in low light, among other critera that we'll save for another day. I have divided them into different categories. If you want to learn how it works, read the technical stuff in the next paragraph. If you just want to know which cameras tested best, just scroll down to "And the winners are..."
Raw data: This graph charts low-light ISO performance for more than 50 cameras relative to their cost. Most cameras are bunched between an ISO rating of 421 and 842, and a price range of $500-$1,500. But the best image quality did not necessarily come from the most expensive cameras! Chart © DxOMark.com.
The technical stuff
To determine each camera's best ISO low light image quality, DxOMark tested each camera's signal-to-noise ratio (which indicates how much digital noise will be produced), expressed as dB, and charted performance at each ISO setting. The goal was to determine the highest ISO at which a signal-to-noise ratio of no more than 30dB could be attained. At the same time, they looked for a dynamic range of at least 9EVs (or 9 stops). Higher than 30dB indicates an unacceptable level of noise (decibels!), while less than 9EVs means the dynamic range is too limited and gradations between shades and colors will not be smooth. Note that the Low Light ISO number might not fit squarely within any given camera's actual ISO settings. The nearest lower ISO setting is the highest one at which acceptable signal-to-noise ratio will be achieved.
And the winners are...
With 100-plus digital cameras tested so far, I picked the top performers in the following categories: Pro, Prosumer, Hobbyists, first-time DSLR user, and compact cameras.
 PRO DSLR
Nikon D3s
The Nikon D3s is the best low-light camera on the planet. It's so good, according to lab results, that it's already getting pro shooters to rethink their approach to low-light photography. In extensive lab tests, the D3s was able to record images with acceptably low grain at up to ISO 3253 (the ISO 3200 setting), which blows away everyone else. With a full-frame 12MP sensor and a pixel pitch of 8.4 microns, it has an eye-popping ISO range of 100-102,400. That's sensitive enough to help out many a sports and concert photographer shooting at night or in a dark performance space without flash. Color depth is excellent, and the camera's dynamic range is 12 stops, which is very good. (Yes, Canon has a camera with a similar ISO range, but its smaller sensor means digital noise is more prominent when you reach into the four-digit ISOs.)
 PROSUMER DSLR
Sony A850
Among all the cameras we consider prosumer, the Sony A850 turned in the most impressive image quality, with usable images at up to ISO 1415 (or ISO 1200 if you're looking for actual camera settings). The 24MP sensor also has an outstanding color range and a 12.2-stop dynamic range. With an overall sensor score of 79 out of 100, this is one of the top cameras for overall image quality, and yet it costs under $2,000. Such a deal!
 HOBBYIST DSLR
Nikon D90
With all the changes in the camera world since it came out, I was surprised to see the Nikon D90, which was introduced two years ago, is still the best of breed, turning in outstanding image quality at ISO 997 (ISO 1000 in the camera's settings). This goes along with its high color accuracy and 12.5-stop dynamic range, all towards the top of any class of camera and way at the top of APS-sensor models.
 FIRST-TIME DSLR
Nikon D5000
First-time DSLR users may be stepping up from compact cameras, so any of these models would offer a substantial image-quality upgrade. The Nikon D5000 (low light ISO: 868) is head and shoulders above the rest of the pack, with only the new Canon T2i coming anywhere close. Basically all of the cameras in this category will deliver good overall image quality at ISO 400-800.
 MIRRORLESS INTERCHANGEABLE-LENS COMPACT
Panasonic Lumix DMC-GH1
Among 4:3 sensor cameras, there's a new subset, the Mirrorless Interchangeable Lens Compact, or MILC. The Panasonic Lumix DMC GH1, with a low-light ISO of 772, scored best of the batch. You can easily get good quality images at ISO 800. Most other cameras in this can deliver good images up to around ISO 500. The Olympus E-P2, at ISO 502, is a typical example.
 COMPACT CAMERAS
Canon PowerShot S90
Image quality drops dramatically when testing cameras with smaller sensors. The 10MP Canon PowerShot S90 delivered the best low-light image quality at only ISO 185, edging out its system compact sibling camera, the G11 (ISO 169). You can probably get away with ISO 200 or perhaps a bit higher if you turn on noise reduction, but don't expect the kind of clarity or dynamic range you'd get with a big-sensor camera.
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