Digital Photography - Megapixels and Resolution

Even though this is a special family issue of Now What?, we thought it appropriate to include this Computer Art 101 article, because it is likely that this holiday season just past saw more photos being taken (by more confused photographers) than ever before in history.

Digital cameras proliferate, and whether you got one as a Christmas gift or are still trying to figure out the one you got last Christmas, there is a lot of mystery involved. Many of these cameras have automated much of the photo-taking process, and claim that you can “point and shoot”. But then what? There’s no roll of film to run to Walmart with, so how do you get those great prints you want?

And how many of those doggone megapixels do you need anyway?

What is a megapixel? And how many do I actually need?
Remember our illustration from last issue? Here it is again, because, really, it all boils down to these little guys.

Instead of recording the light reflected from an image (whatever you are photographing) onto grains on the surface of film, a digital camera records it on an image sensor in pixels.

The megapixel number for a camera refers to how many pixels the camera’s image sensor is capable of recording. The more pixels, the more detail. Each megapixel is approximately one million pixels.

You are stuck with this number of pixels. You can’t increase that total, but you can change the size of each pixel by choosing how many of them you want to see per inch. This is called resolution.

Digital cameras generally present the pixels at 72 ppi (pixels per inch). This results, because you have millions of pixels, in an image with a lot of inches! That is why you will see that the print-out dimensions for the pictures from your camera are poster size - like 23" x 33" etc. Bigger than your printer and bigger than your paper, and worse yet, if you could print them out, they would look very bad - big pixels make bad prints.

What do I do with this big image? I don’t want posters. I want prints!
This is where your trusty computer steps in - you will use it to increase the resolution of your picture - to get better detail and cleaner prints. Your camera’s importing software may have the necessary controls, and if not, an image editing software program will. There are lots of them, but Adobe PhotoShop Elements 2.0 packs the biggest bang for the buck and sells for around $50. I highly recommend it.

Since you have a finite number of pixels to work with, what’s going to happen if you ask for more of them per inch? The image is going to get smaller, of course. So, you want to open that picture in your software, find the commands that allow you to choose Resolution (Image Size in PhotoShop) and ask for 240 pixels per inch (the minimum resolution for a good print). Make sure the Resample option is not checked when you do this. Depending on how many pixels you are starting with, this will give you a smaller print size within acceptable ranges. Here follows a chart that will tell you what size print (at 240 ppi) you will get from which camera. You can always reduce the print size, but it’s much trickier to enlarge it, so make sure to choose more megapixels if you will want to do large prints.

Here is an easy way to figure out how many megapixels you want in a digital camera - expressed in the size (approx) of the optimal print you will get at 240 ppi (optimal meaning the true image - not resampled by the computer).

Optimal Print Size
2 megapixel = 5" x 6.5"
3 megapixel = 6" x 8.5"
4 megapixel = 7" x 9.5"
5 megapixel = 8" x 11"

Of course, if you choose 300 ppi, which will give you an excellent print, your images will be a little smaller than this. So, if prints between, say, 3"x 5" and 5"x 7" (approx) are okay for your purposes, you can get a 2 megapixel camera and save yourself quite a bit of money.

Note: You can make your pictures larger using PhotoShop’s resampling feature - which means that the software adds the necessary pixels to get a bigger print at the same resolution. Because the computer guesses at the colors to make these add-on pixels there can be some loss in quality. It has been found, however, if you start with at least a 240ppi file, and size up only 10% at a time, that loss is very minimal and you can take a 3" x 5" print all the way to poster size without a discernible loss in quality.

Beware the Digital Zoom!!!
This is puzzling for most folks and I hate to think it is intentional deception on the part of camera manufacturers, but they certainly do nothing to clear up this confusing factor. Digital Zoom is not zoom at all - it is an enlargement of the image that is interpolated by the camera - just like the resampling we were talking about above. The result is a very grainy, very pixelated image that will not please you.

Look for Optical Zoom!!!
If you want a true zoom lens (telephoto lens), you must pay attention to the Optical Zoom number on the camera, not the digital. Optical zoom is actually zooming closer to get a more detailed image of your subject. There is some kind of formula to compare Optical zoom on a digital camera with focal length of 35mm lenses. I think you can find that formula on the Nikon site. I only know my own camera which has a 10x optical zoom - which equates to a 380 telephoto lens on a 35mm camera.

Most digital cameras that retail below $250 do not have optical zoom at all - so read those descriptions carefully before buying a camera. If a camera has no optical zoom, you will only obtain your optimal images without using the digital zoom at all - using the set focal length of the camera - usually equivalent to a 50mm lens and appropriate only for snapshot photography.

Now, let’s turn the page and talk about quality settings and file formats as they affect your digital photography.

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