I will go to my grave with that question echoing in my head. I've lived through "How many megahertz?" and "How many inches?" (monitors--get your mind out of the gutter), but the megapixel question tops the charts, partly because it's so difficult to answer--especially when you have to generalize for groups of consumers--and in part because it's the wrong question. Color quality, noise, and tonal range have far more importance for determining image quality, but manufacturers encourage resolution-shopping because simple specs, especially numbers that get higher every year, are an easy sell. And let's face it: we product reviewers can also be pretty sloppy with our terminology. My theory is that much of the problem can be reduced to the misuse of the term resolution, a failing for which I'm as guilty as the next pixel pundit.

Density vs. dimension
It all started when VGA monitors hit the market, and we began referring to them as having "640x480 resolution," when what we really meant was that they could display an image with the dimensions of 640 vertical pixels by 480 horizontal pixels. In the absence of a scale, however, image dimensions are meaningless. We intuitively grasp this concept when we see examples but seem to lose our grasp when it's divorced from the visual. For example, if you look at the examples below, you immediately know that the one on the left has the same image dimensions as the right, but the right image represents a higher resolution--it has a greater dot density.

In order to achieve the equivalent dot density as the image on the right using the number of dots in the example on the left, you have to reduce the dimensions of the area they cover, like this (the change in dot shape is a compression artifact):

The number of megapixels delivered by a digital camera refers only to the dimensions of the image: 5-megapixel cameras, for instance, typically output images of about 2,592x1,944 pixels (it differs across models). If you render that image at 2.5 by 2.0 inches, the pixels will be very densely packed--possibly too densely for your output device to handle, which means it would drop pixels and potentially lose image detail. Conversely, if you try to render the image at 2,500x1,900 pixels, there aren't enough pixels to render any meaningful detail. Your software would have to guess (interpolate) what the intermediate pixels might look like, and that can result in a big mess.

So when you ask, "How many megapixels?" you need to frame it relative to the resolution of your printer or display. The output device adds the scale necessary to make the pixel dimensions meaningful and leads to my contention that you determine how many by calculating backward through the workflow.

Less is more, more or less
Unfortunately, once you've jumped that hurdle, you'll find you've landed in quicksand. Software and hardware these days have gotten good enough that output devices have a fair amount of resolution latitude when it comes to rendering images. (Note: Remember that we're dealing with the image resolution, not the resolution of the output device.) For instance, if you print a 4x6 photo from a 240dpi image file on an Epson photo printer, it usually looks about the same as if you'd used a 360dpi image. But you can achieve the former with a 3-megapixel camera, while the latter requires a 7-megapixel camera.

And that's why we get e-mail from some readers gloating, "I get great 8x10s from my 2-megapixel camera," and others griping, "To get a good 8x10, I'll need a 16-megapixel model."

Where do the numbers come from? You can calculate them yourself (or let Photoshop do it for you automatically in the Image Size dialog), but here's a quick reference for determining the largest prints you can create from an image of a given dimensions at various resolutions, plus a set that takes into account an arbitrary amount of cropping.

I bet you're wondering why I split the HP and Canon printers from the Epsons, into two different sets of resolutions. I did it because it's always a good practice to keep your file resolution as an integral multiple--2X, 3X, 4X, and so on--of the nozzle density of the printhead. Unlike all its competitors, Epson printers are based on sets of 160dpi nozzles rather than 100dpi or 150dpi. This is less of an issue for photos than for graphics, but watching out for these types of details will make it easier to get the results you like. Don't believe me? See for yourself. Download and print these PDF files, without any page scaling, and see how the printer driver has handled the various line frequencies. The one that looks most correct is the best input resolution. I didn't create these files, but the Web site where I originally found them seems to have disappeared. I'd love to attribute them to the real author; if you can help, please drop me a line.

Next up: Asset management for every style.