Megapixels and digital cameras

Let's take it backwards, starting with the printer. The assumption is to print pictures that will look like good photographic prints. A photo quality printer has a resolution of 2400 dots per inch (dpi). The rule of thumb is that you divide this by 4 to get the maximum picture quality in pixels per inch (ppi). This would be 600 ppi, but considering camera cost, memory requirement, and whether you can really tell the difference, 400 ppi is satisfactory.

Cameras take pictures that are usually processed as 4" x 5", 4" x 5.3", or 4" x 6". Using the 4-inch side, 4 inches at 400 ppi would be 1600 pixels. If the other dimension is 5 inches, 5 inches at 400 ppi would be 2000 pixels. Therefore, you would want a digital camera that has an image size of 1600 x 2000 pixels. 1600 x 2000 = 3.2 megapixels. That means a 3.x megapixel camera would be optimal for 4" x 5" images. Other things are stored, so the megapixel size is a little more than the actual calculation. An 8" x 10" would have a resolution of 200 ppi.

The 2000 pixel width could be a little different depending on the "wide" dimension of the picture. From the image size in pixels, divide the smaller number by 4 to get the ppi for a 4" high picture. Divide the larger number by the ppi to get the width in inches.

File size with minimal JPEG compression seems to be about half the megapixel number, or about 1.5 Mbytes per picture for a 3.x megapixel camera, and about 1 Mbytes per picture for a 2.x megapixel camera. The larger the file, the longer it takes to store the image in the camera, and the more memory you need for the number of pictures you'd like to be able to store.

For me, a 2.x megapixel camera is a good compromise in quality vs. file size (HP Photosmart 912). It gives a 1280 x 1600 pixel image, for a 4" x 5" image at 320 ppi. With a 64MB flash card, I can take over 60 pictures at "Best" quality. The image is stored at 72 ppi, 22.222 inches wide, and 17.778 inches high. File size runs about 700KB to 1MB at "Best" image quality, depending on picture contents. Using image editor Adobe PhotoDeluxe, I constrain file size (keep all the pixels), and change the height to 4 inches. This gives me a 4" x 5" image at 320 ppi, and still the same file size. Before emailing a picture, or for web page display, I mercifully change image density to 72 or 96 ppi so it downloads faster.

A note on digital zoom: it just does cropping like you could do with an image editor, and image resolution is reduced. Optical zoom is the true zoom.

Through-the-lens viewing: I can't say enough how important this is. In bright light you can't see the image on a display screen. Also, it is better for steadiness and framing to have the camera against your head instead of holding it out in front of you.

For more information, see my other web page on Scanning and Resolution at www.jdhartsell.com/scanning.html