Giz Explains: Why ISO Is the New Megapixel

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In 1975, the first digital camera took 23 seconds to record a 100-line black-and-white photo onto cassette tape. Today, a Nikon D3s takes photos with 12 million pixels at 1/8000 of a second. And it can see in the dark.

The conventional wisdom is that the romp-stomp-stomp of progress in digital imaging has proceeded on the mostly one-way track of ballooning pixel counts. Which wasn't always a pointless enterprise. I mean, 1.3-megapixel images, like you could take in 1991, aren't very big. The Nikon D1, introduced in 1999, was the digital camera that "replaced film at forward-looking newspapers." It was $5,000 and shot 2.7 megapixel images using a CCD sensor, large enough for many print applications. But still, there was room to grow, and so it did. Now pretty much every (non-phone) camera shoots at least 10-megapixel pictures, with 14 megapixels common even in baseline point-and-shoots. Cheap DSLRs from Canon are now scratching 18MP as standard. Megapixels were an easy-to-swallow specification to pitch in marketing, and became the way normal people assessed camera quality.

The now-common geek contrarianism is that more megapixels ain't more better. The new go-to standard for folks who consider themselves savvy is low-light performance. Arguably, this revamped arms race was kickstarted by the D3, Nikon's flagship DSLR that forsook megapixels for ISO. (Rumor had it that the D3 and D300 led Canon to shitcan their original, middling update to the 5D, pushing full-steam-ahead for a year to bring us the incredible 5D Mark II.) However it began, "amazing low-light performance" is now a standard bullet point for any camera that costs more than $300 (even if it's not true). Nikon and Canon's latest DSLRs have ISO speeds of over 100,000. Welcome to the new image war.How a Camera SeesThe name of the game, as you've probably gathered by now, is collecting light. And in fact, the way a digital camera "sees" actually isn't all that different from the way our eyeballs do, at one level. Light, which is made up of photons, enters through a lens, and hits the image sensor (that boring looking rectangle above) which converts it into an electrical signal, sorta like it enters through an eye's lens and strikes the retina, where it's also converted into an electrical signal. If nothing else after this makes sense, keep this in mind: The more light an image sensor can collect, the better.

When a camera is spec'd at 10 megapixels, it's not just telling you that its biggest photos will contain about 10 million pixels. Generally, it's also telling you the number of photosites, or photodiodes on the image sensor; confusingly, these are also often referred to as pixels. Photodiodes are the part of the sensor that's actually sensitive to light, and if you remember your science, a photodiode converts light (photons) into electricity (electrons). The standard trope for explaining photosites is that they're tiny buckets left out in a downpour of photons, collecting the light particles as they rain down. As you might expect, the bigger the photosite, the more photons it can collect at the moment when it's exposed (i.e., when you press the shutter button).

Image sensors come in a range of sizes, as you can see in this helpful diagram from Wikipedia. A bigger sensor, like the full-frame slab used in the Canon 5D or Nikon D3, has more space for photosites than the thumbnail-sized sensor that fits in little point-and-shoots. So, if they're both 12-megapixels, that is, they both have 12 million photosites, the bigger sensor can obviously collect a lot more light per pixel, since the pixels are bigger.

If you're grasping for a specification to look for, the distance between photosites is referred to as pixel pitch, which roughly tells you how big the photosite, or pixel, is. For instance, a Nikon D3 with a 36mm x 23.9mm sensor has a pixel pitch of 8.45 microns, while a Canon S90 point-and-shoot with a 7.60 mm x 5.70 mm sensor has a pitch of 2 microns. To put that in less math-y terms, if you got the same amount of light to hit the image sensors the D3 and the S90—you know, you took the exact same exposure—the bigger pixels in the D3 would be able to collect and hold on to more of the light. When you're looking for low-light performance, it's immediately obvious why that's a good thing.

Catch More Light, Faster, FasterOkay, so that's easy enough: As an axiom, larger photodiodes result in more light sensitivity. (So with the 1D Mark IV, Canon kept the same photodiode size, but the shrunk the rest of the pixel to fit more of them on the same-size chip as its predecessor). There's more to an image sensor than simply photosites, though, which is why I called up Dr. Peter B. Catrysse from the Department of Electrical Engineering at Stanford University. The "ideal pixel," he says, is flat—just an area that collects light—nearly bare silicon. But even at a basic level, a photodiode sits below layers of other stuff: a micro lens (which directs light onto the photodiode), a color filter (necessary, 'cause image sensors are in fact color blind) and then a layer of gunk, like wiring. So one way manufacturers are improving sensors is by trying to make all of that as thin as possible—we're talking hundreds of nanometers—so more light gets through.

One major way that's happening, he says, is with back-illuminated sensors, which move the wiring to the back-side of the silicon substrate, as illustrated in this diagram by Sony. It's currently still more expensive to make sensors this way, but since more light's getting through, you can use smaller pixels (and have more of them).

In your basic image sensor construction, there's an array of microlenses sitting above the photosites to direct light into them. Previously, you had gaps between the microlenses, which meant you had light falling through that wasn't being directed onto the actually light-sensitive parts of the sensor. Canon and Nikon have created gapless microlenses, so more of the light falling onto the sensor is directed into the diode, and not wasted. If you must persist with the bucket metaphor, think of it as putting a larger funnel over the bucket, one that can grab more because it has a wider mouth. Here's a shot of gapless microlens architecture:

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