Arthur H. Bleich

Contributing Editor
Digital Camera Magazine

Since the New Year is always associated with resolutions of one kind or another, I thought this would be the perfect time to discuss this often-misunderstood subject.

First, let's define some terms. The word "pixel" (from "picture cell" or "picture element") can be used in two ways. It can refer to the individual sensors on the CCD in your digicam that record images, or to the basic components of a digital image itself.

So a digicam with 640 x 480 sensors (or pixels) on its CCD, will produce an image that also has 640 x 480 pixels. The camera would be described as having a 640 x 480 pixel resolution or a resolution of 307,200 pixels (arrived at by multiplying the two numbers). When you get past a million, you're in the MegaPixel range. A digicam with a 1600 x 1200 pixel resolution (which equals 1,920,000 pixels) can also be said to have a 1.9 MegaPixel resolution.

Now remember, these figures are in pixels, NOT pixels-per-inch (ppi). That's because CCDs can be different sizes. One can be a 2/3-inch CCD, another a 1/2-inch CCD and both can have a 640 x 480 sensor array on them. In one case the sensors -or pixels- would be larger and in the other case, smaller. So digicam resolutions are NEVER measured in pixels-per-inch- just pixels, period.

Where things get a bit tricky is when you get to the stage of printing your images on paper. You can print those 640 x 480 pixels as a small image or a large one. But whichever you choose, you still have only 640 x 480 pixels to work with. Remember that.

So if you print a small picture, those pixels will be fairly close together. As you enlarge your image, though, there will be more space between them in order to cover the bigger area. Think of individual pixels as soldiers on a drill field. Let's say you have 12 of them standing in four columns, three to a column. That would give you a "resolution" of 4 x 3 (or 12) and it would be a tight little formation. If you viewed them from above, they'd look like a very compact group.

Now let's give them the command to distance themselves six feet from each other. They'd not only cover more ground, but you'd immediately see more space between them and start to notice individuals, not just the group. If they spread out even more, the large spaces between them would become very noticeable and they wouldn't be in a bunched-together formation anymore.

Take a few seconds to think about this because that's the theory of resolution. The more pixels in your final printed image, the more the image will hold together and not break into individual pixels that you can begin to see. It's really a simple concept that's been made over-complicated.

Now here's where "pixels-per-inch" comes in. When you print pictures, their sizes are defined in measurements such as 4 x 6 inches or 8 x 10 inches, and you can tell those pixels (through your imaging program) to arrange themselves closer or further apart, depending on the size of the photo they have to fill. And like good soldiers, they'll form up to fit into that space.

If you choose to print a small picture, your imaging program will order those pixels to scrunch together to fit within the defined borders. Tell it you want a large photo and the pixels will be given the command to spread out in order to fill a larger area on the paper. How close or far apart they are from each other is measured in pixels-per-inch (or ppi).

So you can have a digicam with a 640 x 480 pixel resolution and yet produce an image on paper that has, say, a resolution of 320 pixels-per-inch. It'd be a mighty small picture (2 x 1.5 inches) but full of detail because the pixels would be very close together.

If you wanted a bigger image, though, like 10 x 7.5 inches, you'd have to tell those 640 x 480 pixels to spread further apart and you'd end up with only 64 pixels-per-inch. That's a bit thin, and would probably result in an image that began to show individual pixels.

But let's say your digicam had higher resolution to begin with. You could then print a bigger picture with as much detail as your miniature because, to use our military analogy, you'd have more soldiers to start with that could keep the formation looking tight even when they had to cover a bigger piece of ground.

If you want to print large pictures, you must have the resolution "manpower" to do it. You must have a digicam whose CCD is packed with a high number of pixels so your photos will look as realistic as possible without the viewer being able to see individual pixels.

Now let's not forget the printer; it's part of the total resolution equation. Put simply, printers lay down a series of ink dots on paper that form the pixels that make up your picture. Here's something important to remember: printers use dots to form representations of the image's pixels and it takes multiple dots to print each pixel.

So even if you have a printer that can lay down 1440 dots-per-inch (dpi), it will usually just be using those dots to form the pixels that are in your image; it can't add more original pixels if there aren't any. What a high resolution printer can do is make the pixels that are there more color-accurate and sharper than a printer with lower resolution. And most high resolution printers can also make an educated guess (called "interpolation") to add some more pixels if you select that option. (You can also do this prior to printing in most imaging programs.)

To smooth jagged edges, for example, the printer will add pixels- like adding mannequins to fill in spaces in the troop formation. But like mannequins, those "manufactured" pixels are not the real thing because they weren't part of the originally recorded image. So adding those "dummy" pixels won't produce as sharp a print as you'd get by starting off with a higher resolution digicam in the first place.

That's why you should always choose a digicam with the highest resolution commensurate with the largest image size you're most likely to print.

As a rough rule of thumb, figure on a digicam with 1024 x 768 pixels for good quality prints up to 5 x 7 inches, 1280 x 960 pixels for 8 x 10's, and 1600 x 1200 pixels for 11 x 14's- provided you're going to print from the full frame and not crop (remove) any pixels. When you print selected parts of an image, you reduce its resolution by the number of pixels you remove- kind of like sending some of the troops off on a furlough. If you regularly intend to crop your images, then go to the next highest digicam resolution- or even higher.

Remember, now, we've been talking about printing images on paper. If you're going to use your pictures for web auctions or just email them to friends, you don't need a high resolution digicam. Your photos will be viewed on computer monitors, most of which are limited to displaying at 72 - 96 pixels-per-inch. Buy a cheap 640 x 480 resolution digicam or use the low resolution setting if you have a high resolution camera, and your pictures will look just fine.

Finally, on the topic of New Years' resolutions again, mine was to write this column at the beginning of the year, hoping it would cut down on the many emails I get asking me to clarify the subject. But by all means, feel free to email me if you're still confused.

© Arthur H. Bleich 2000, All rights reserved.

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