Josh Miller/CNET

Editors' note: This article originally published May 6, 2012, and was updated on February 13, 2013, and again on May 4, 2013.

In a matter of months, the high-end smartphone camera spec rocketed from a respectable 8 megapixels to an altitudinous 13.

The Samsung Galaxy S4 and LG Optimus G Pro are the freshest examples of this megapixel push, but even last January's Pantech Discover (12.6 megapixels), last October's LG Optimus G for Sprint (13 megapixels), and especially mid-2012's 41-megapixel Nokia 808 PureView piled on the megapixels.

Yet even though the technology exists, quality can be just as uneven from phone to phone as it was when an 8-megapixel shooter was the "best" that money could buy.

Championing that perception head-on is HTC, the same company that not too long ago boasted about the 16-megapixel camera in its Titan II. Now, in its HTC One flagship, the smartphone maker dials down the megapixel count to 4 megapixels, which HTC fancifully terms "Ultrapixels," arguing that the lager pixel size throws back the blinds to let in much more light.

In this lies the reminder (something photography nuts will tell you) that it's quite possible for an excellent 5-megapixel camera to produce photos you prefer over a shoddy 12-megapixel camera. The number of megapixels alone is no guarantee of heightened photographic performance.

Instead, the formula for fantastic photos comes down to the entire camera module, which includes the size and material of the main camera lens, the light sensor, the image processing hardware, and the software that ties it all together. So let's dive in.

Note: As always with this column, if you already consider yourself an expert, then this article is probably not for you.

Key ingredient No. 1: The sensor

Most budding and professional photographers will tell you that the most important ingredient in the optical system is the sensor, because that's the part that captures the light. The sensor is essentially the "film" material of a digital camera. No light, no photo.

Light enters through the camera lens, then passes to the camera sensor, which receives the information and translates it into an electronic signal. From there, the image processor creates the image and fine-tunes it to correct for a typical set of photographic flaws, like noise.

The size of the image sensor is extremely important. In general, the larger the sensor, the larger your pixels, and the larger the pixels, the more light you can collect. The more light you can catch, the better your image can be.

Josh Miller/CNET

The experts I spoke with for this story had colorful ways of describing the relationship between pixels and sensors, but "buckets of water" or "wells" were a favorite (intentionally oversimplified) analogy.

Imagine you have buckets (pixels) laid out on a blacktop (sensor). You want to collect the most water (light) in those buckets as possible. To extend the water-and-bucket analogy, the larger the sensor you have (blacktop), the larger the pixels (buckets) you can put onto it, and the more light (water) you can collect.

Larger sensors are the reason that 8 megapixels from a digital SLR camera (or 5 or 13) best those 8 megapixels from a smartphone camera. You get roughly the same number of pixels, but the pixels on the dSLR get to be larger, and therefore let in more light. More light (generally) equals less-noisy images and greater dynamic range.

The fallacy of megapixels

You can start to see that cramming more pixels onto a sensor may not be the best way to increase pixel resolution. That hasn't stopped the cell phone industry from doing just that.

Jon Erensen, a Gartner analyst who has covered camera sensors, remembers when we collectively made the leap from 1-megapixel to 2-megapixel shooters.

"They would make the pixel sizes smaller [to fit in more pixels]," Erensen told me over the phone, "but keep the image sensor the same."

"What ended up happening is that the light would go into the well [the 'bucket'] and hit the photo-sensitive part of the image sensor, capturing the light. So if you make the wells smaller, the light has a harder time getting to the photo-sensitive part of the sensor. In the end, increased resolution wasn't worth very much. Noise increased," he said.

The relationship between the number of pixels and the physical size of the sensor is why some 8-megapixel cameras can outperform some 12-, 13-, or even 16-megapixel smartphone cameras.

There's more involved, too. A slim smartphone limits the sensor size for one, and moving up the megapixel ladder without increasing the sensor size can degrade the photo quality by letting in less light than you could get with slightly fewer megapixels.

Then again, drastically shrunken pixel sizes aren't always the result when you increase your megapixels. HTC's Bjorn Kilburn, vice president of portfolio strategy, shared that the pixel size on the 16-megapixel Titan II measures 1.12 microns, whereas each of the HTC One X's 8 pixels measures a slightly larger 1.4 microns.

As a result, the photo quality on both these HTC smartphones should be comparable at a pixel-by-pixel level.

Sarah Tew/CNET

Unfortunately, most smartphone makers don't share granular detail about their camera components and sensor size, so until we test them, the quality is largely up in the air. Even if smartphone makers did release the details, I'm not sure how scrutable those specs would be to the majority of smartphone shoppers.

For more information on the interplay between megapixels and sensors, check out the excellent description in CNET's digital camera buying guide.

What about Nokia's 41-megapixel PureView?

The story behind the 808 PureView smartphone as Nokia tells it is really interesting. CNET Senior Editor Josh Goldman has written one of the best explanations of the Nokia 808 Pureview's 41-megapixel camera that I've seen. I strongly suggest you read it.

In the meantime, here's a short summary of what's going on.

Juha (pronounce it YOO-hah) Alakarhu is head of camera technologies at Nokia, where he works within the Smart Devices team. Alakarhu explained to me that although Nokia has engineered the 808 to capture up to 41 megapixels, most users will view photos as the 5-megapixel default.

Usually, when you use the digital zoom on your phone, you're blowing up and cropping an image to see each pixel up close. You all know what that can look like: grainy, blocky, and not always as sharply focused or as colorful as you'd like.

Nokia

In the 808 PureView, Nokia uses a process called "oversampling," which -- for the 808's 5-megapixel default resolution -- condenses the information captured in 7 pixels into 1 (they call it a "superpixel"). If you zoom in on an object, you're simply seeing part of the image that's already there, rather than scaling up. This method should translate to higher-resolution digital printouts and zoom-ins than you'd normally see.

It's taken over five years to create the technology within the 808 PureView, Alakarhu said. Not only does the 808 lean on the physical size of the sensor (specifically 1/1.2-inch), there are also custom algorithms on top of the sensor to adjust the image to reduce imperfections like noise. It's this set of instructions that Nokia terms PureView, not the sensor size alone.

As CNET's Goldman has pointed out, this is an unusually large sensor for a smartphone, and it's also larger than sensors found on the vast majority of point-and-shoot cameras.

Key ingredient No. 2: Image processing

In addition to the size and quality of the lens and sensor, there's also the image processor. Most modern high-end smartphone CPUs have dedicated graphics processors built into the chip, which, being hardware-accelerated and not just software-dependent, can quickly render images like photos, videos, and games without overtaxing the main application processor.

HTC and Samsung have been pushing continuous-burst mode hard, averaging one shot in a tenth of a second or less, thanks to separate hardware-accelerated image processors that can capture shots like nobody's business. However, since burst mode doesn't give you time to focus, expect to see some blur.

I promised that there was software bridging the hardware and the final image, and there is. Algorithms and other logic are what create the final image output on the phone's screen. This is where the most subjective element of photography comes in: how your eye interprets the quality of color, the photo's sharpness, and so on.

The image processor is also what helps achieve zero shutter lag, when the camera captures the photo when you press the capture button, not a beat or two after.

Wait, there's more

There's much more to know about the competing technology that goes into sensors, but backside-illuminated sensors are starting to be used much more in smartphones.

This type of sensor is often synonymous with better low-light performance because it increases photosensitivity. However, if you shoot in bright light, it can also blow out your image. Here are more details on how backside illumination works.

CNET

Low-light performance, by the way, is turning into a serious battleground for bragging rights. Nokia's Lumia 920 and Apple's iPhone 5 currently rule this shadowy realm, with HTC's One vastly outperforming the Samsung Galaxy S4, which has no automatic low-light adjustments to speak of (though there are several shooting modes).

Backing out of the low-light rabbit hole, it's important to note that the camera's sensor size and image processor may be the most crucial elements for creating quality smartphone photos, but other considerations come into play. Higher-quality components, for example, can help tease out better photos, but they could also cost more, which could lead to a marginally pricier camera.

While the cost of a camera module is only one part of the total cost, Gartner analyst Erensen said that high-end parts could double the price of a basic camera set, and thought that parts could cost $15 per phone. The smartphone makers I contacted for this article, like Samsung and Nokia, wouldn't share sourcing or pricing information.

Usability is king

It's quickly becoming a well-worn adage that the best camera is the one you have on you.

Despite the intense engineering focus that goes into the camera's physical elements, it's hard to overstress the importance of both convenience and the total customer experience. How easy it is to open the camera app from a locked position, how quickly photos capture, and how desirable the special effects and shooting modes are all add up to either a camera you want to use or one you don't.

Increasingly, some phone makers, like HTC, LG, and Samsung, include extra logic in their big-ticket phones, like detecting smiles and selecting the best group photo of a bunch. New modes have emerged among the top-tier Android handsets, including taking photos and videos from both front and rear cameras, as well as grouping action shots into a single frame.

Fancy filters and tricks are fun party games, but for most phone owners, said Drew Blackard, Samsung's senior manager of product planning, being able to quickly and easily share photos on the fly is far more important than pixel count. Just look at Twitter's and Instagram's (now Facebook-owned) runaway success in sharing simple, small photos.

Josh Miller/CNET

Gartner's Erensen agrees. "What do you actually gain from going higher than you need, in a practical sense?" he asked, adding that most people upload smartphone photos to online albums, or e-mail them to family and friends, formats that require many fewer than 8 megapixels, or even 5.

A trip to Indonesia illustrates what Nokia's Alakarhu and the others mean by the whole experience taking precedence over the specs. While trekking with 22 pounds of gear on his back -- including a high-quality dSLR -- Alakarhu repeatedly reached for the Nokia 808 PureView he kept in his pocket.

Although he considers himself an amateur photographer who will put in the time to frame a great shot, Alakarhu said he found himself using the PureView more because of its easy availability and quick start time when he didn't want to take the time to set up a more involved shot on his digital camera.

I have my share of similar stories, and I suspect that you do, too.

We definitely shouldn't scrap pixel count when weighing smartphone camera specs against others, but when it comes to all the hardware and software that create a great photo, the megapixel count alone just isn't enough. It's time we shift the focus somewhere else -- like maybe to that undersung sensor.

Thanks to CNET Senior Editor Josh Goldman, who contributed to this story.

To see which phones have the latest, best camera technology, check out our current list of the best camera phones.

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Smartphones Unlocked is a monthly column that dives deep into the inner workings of your trusty smartphone.