What's A Megapixel, And Why Should I Care?

Some of us are technogeeks. We like reading up about the latest gewgaw, gadget and gizmo, and if possible, possess it for ourselves. Our use of jargon is often so convoluted and esoteric it evolves into it's own dialect, or at least a pidgin. We wax rhapsodic about the pros and cons of various models, knowledgable about all the differences between different manufacturers and even the conditions in the factories where our favorite toys are made. Occasionally, we become so enthralled with the bundled features of a given device that we forget to actually use the device for its intended purpose.


Some of us are, instead, rather more luddite when it comes to technology. We don't really care about the latest innovations if what we're using now suits our needs. We prefer to master the technology we have on hand than to constantly have to update ourselves on current developments. We openly scoff and sneer at the early adopters of new technology, especially when the shortcomings of that technology becomes more apparent over time. Although we sometimes wake up to discover that the world has moved on without us and we have a steep learning curve ahead of us in order to catch up, we believe it's worth the occasional paradigm shift in order to avoid wasting time shopping for new gear every week when we could be using what we've already got.


I think most of us fall somewhere between the two extremes, and when it comes to photography, the technology advances in such fits and starts that sometimes you do discover that there is technology out there that really is measurably better than what you've got. So you decide it's time to buy yourself a new camera, but you haven't paid much attention to what's been happening in the market the past decade or so and don't really know what to look for. That's okay; if you haven't immersed yourself in the lore and jargon of the photographic world, it can get pretty confusing. Even for those of us who are in a deeper section of the pool can sometimes get a little turned around when it comes to the technical aspects of the field, and some photographers find experts to advise them in the kinds of gear they need so they can concentrate more on just shooting photos, rather than studying journals to find out the latest and greatest toys on the market.


So today, we're going to focus on that one word that shows up in the description of every digital camera, but that not everyone really understands: megapixels. Before we get to the real nitty gritty of the topic, however, it might be instructive to back up a bit and discuss how a camera works in general, before getting into the digital stuff.


At it's most basic, a camera is just a light-sensitive media (celluloid film, digital sensor, silver-coated tin or copper plate, etc.) encased in a housing that protects it from light until the moment you want to expose it to the scene you'd like to photograph. That's really pretty much it; everything else on the camera is a bonus feature that either makes it easier to take the photo or improves the quality of the image. There is a mechanical shutter on most modern cameras that serves as the means to let light in (you've probably seen old 19th-century cameras in the movies with the photographer under a cloth hood and sliding a piece of wood out from in front of the camera -- that piece of wood was essentially the shutter, and they could get away with it because their daguerrotype plates weren't very sensitive and required as much as eight minutes to expose properly, so a few seconds on either end while the photographer fiddled with the shutter really didn't matter). The shutter works in conjuction with an iris, which limits the amount of light that can enter when the shutter opens, and these two work together to determine the exposure you get in the final image (much, much more on that in another article). The lens brings the image into sharper or softer focus, and is a means of collecting the light coming into the camera; with telephoto zoom capability, the lens can also bring subjects closer. Flashes illuminate the subject when lighting is not optimal at the time you take the picture. And so on.


All technologies prior to the digital age relied on chemical mixtures that underwent a reaction when exposed to light. Films were coated with layers of photosensitive compounds such as silver oxide, treated to react to different colors of light. Once exposed, the film was processed with solutions that developed the film and fixed the final image to the film. The image could then be transferred to photosensitive paper and finally developed as a positive print.


A digital camera replaces film with a sensor called a charge-coupled device (CCD). It works like this: the surface of the CCD is an array of photosensitive capacitors, often made of silicon doped with boron, that develop a charge relative to the intensity of light which hits each capacitor. The capacitor is then allowed to pass its charge on to its neighbor, and at the end of each row is a connection to an image processor which measures each of those charges, calculates which charges came from which capacitors, and then converts and saves this information as digital data.


The capacitors are grouped into trios that are able to sense red, green and blue light (the primary colors of the  visible light spectrum), and these trios are what are referred to as pixels (if you look very close to an older CRT television screen, you'll probably see tiny squares or rectangles that are either red, green or blue -- those are pixels, too; the modern 1080P HD plasma screen you just bought means that it can disply 1080 pixels across the width of your screen.) So when you see a camera that says it has 8 megapixels (MP), they're saying that the CCD inside the camera has a total of eight million pixels on the surface of that sensor. Very generally speaking, the more pixels there are on the sensor, the better the camera's resolution. If you take the exact same picture with two different cameras, one with 3 MP and one with 18 MP, you'll end up with the same basic image but the 18MP camera takes a picture that you can zoom in on many more times without loss of image quality because it can capture more data in that image.


Okay, you say, so a digital camera with more megapixels is better. So I should go out and buy one with the most megapixels I can afford, right?


Well, no, not necessarily. I think photographer and teacher John Greengo explained it best with a little visual aid.

Here is the world's simplest digital camera CCD: a four pixel camera. Let's say you bought this four pixel camera a few years back and have gotten a lot of use out of it, but you're considering getting a new one. So what about doubling the number of pixels? Well, it's hard to arrange eight pixels in anything other than a rectangle, like this:

So you've doubled the resolution in only one direction while leaving the other dimension the same (two pixels high). This is fine if you're buying a panoramic camera; coupled with a wide-angle lens, you could take really wide landscape photos at the same basic image quality of your old camera, but if you want to improve overall resolution, you're going to need to increase it in both the vertical and horizontal directions.

This nine-pixel camera accomplishes that: you go from two to three pixels in both dimensions, for a 50% increase in resolution overall. But to get that 1.5X increase in resolution, you actually had to have 2.25X the number of actual pixels on the sensor. This leads us to our rule of thumb: a given increase in digital resolution requires the square of that increase in pixels.

So going back to our four pixel camera, in order to double the resolution, we actually need four times as many pixels, so you'd need to buy a sixteen pixel camera.

So let's apply this to the real world. The Canon EOS 50D digital SLR camera features a 15.1 MP sensor, and you're thinking of upgrading to a Canon EOS 5D with its 21.1 MP sensor. Well, 21.1/15.1 is 1.397x the number of pixels. Take the square root of that, and you see that it's 1.182X the resolution, i.e. you get an 18.2% increase in resolution. Will you notice the improved image quality? Maybe you will, maybe you won't; it kind of depends on what subjects you like to photograph. If you do a lot of close-up work of really detailed subjects, then you might notice the difference. If you do a lot of really broad landscape photography, you might not.

Here's where the difference lies: the sensor is smaller in the 50D, less than 3/4 the area of the sensor in the 5D. There are physical issues involved with trying to cram ever more pixels into a limited amount of sensor real estate, principal among which is noise. At high ISO or long exposures, all cameras are subject to a certain amount of noise (in film cameras, this was known as grain or graininess), but with a sensor that is smaller relative to the aperture you're shooting through, there is a higher probability that the light that is sensed by each individual capacitor or pixel will either be a slightly different shade from the actual absorbed photon due to variations in the charge developed for a given photon energy level, because the individual pixels are crammed in closer to one another and may have a tendency to bleed charges into one another. So although you might not notice the 18.2% increase in resolution, if you add in the reduction in noise from having a sensor that is 25% larger, it might be worth it, if you plan on doing a lot of long exposures or photography requiring a high ISO (frequently in underlit environments, for instance)

Therefore, it might still be a tough decision if you wanted to upgrade from a 50D to a 5D, but at least you have one bit of information: There will be an improvement in image quality based on a larger sensor with more pixels available. The question is whether that improvement in image resolution is worth paying an extra $1400 (the difference in price between a 50D and a 5D on Amazon at the time of this writing). For the casual vacation photographer, it's almost certainly not worth it; for the $1400 you save, you can be happy with a really good consumer camera and spend that money on jetski lessons and umbrella drinks.

For the semi-professional or professional photographer, a little more research is required; the sensor offers a somewhat better image, in terms of increased resolution and decreased noise, but what may provide as good an improvement in image quality as a better CCD might be better quality lenses. So while you grapple with the decision whether or not to upgrade your camera body, you also need to ask yourself if you need better glass. That will be the topic of my next article.