October 4, 2004
Q: I am about to buy a digital camera and don’t know how big of a memory card to get. How do I figure out how much space each picture will take, and should I get more than one card? - Connie
A: It sounds like that you’ve already discovered that digital camera manufacturers don’t like to include a usable memory card with the camera.
Typically, a measly 32MB card or less is included, which doesn’t allow for many pictures with today’s cameras.
Before I get into the technical end of the answer, the bottom line is that there is no such thing as too much memory when it comes to digital photography. The price per MB is likely to be the driver for your decision, so simply compare prices on the various sizes and buy as much as you can afford.
If you plan on taking a lot of pictures or travel often, having a couple of cards is a very good idea.
The exact amount of space taken by each image for any given camera varies and is determined by the image sensor, the image resolution and the file format used to capture the image.
The image sensor’s capability is generally denoted by a "megapixel" rating, which determines how much information (in pixels) can be captured. The number of pixels that can be captured translates directly to a maximum image resolution.
According to PC Magazine:
1 megapixel = 1,280 x 960 image resolution
2 megapixel = 1,600 x 1200 image resolution
3 megapixel = 2,048 x 1536 image resolution
4 megapixel = 2,290 x 1710 image resolution
5 megapixel = 2,592 x 1944 image resolution
This means that the higher the megapixel rating, the more space each image will take on your memory card.
The single biggest factor that will determine how much space each image will take is the file format. Most cameras default to a compression format known as JPG or JPEG, which can reduce the image size by as much as 90 percent. More sophisticated cameras will often include the capability of capturing images with no compression at all with either the RAW or TIFF formats.
To calculate the space taken by each uncompressed image you would multiply the image resolution by the color depth. Most cameras use a 24-bit color depth, so a 4 megapixel camera would calculate as follows: 2,290 x 1710 x 24 bits = 93,981,600 There are 8 bits to a byte, so 93,981,600 / 8 = 11,747,700 bytes 1,048,576 bytes = 1 megabyte, so 11,747,700 / 1,048,576 = 11.2 MB This means that you could only take two uncompressed images on a 32MB card.
By default, digital cameras capture in a compressed format (for obvious reasons) and can be configured to decrease the captured image resolution to further reduce file sizes. For example, my 5 megapixel Sony Cyber-shot in the Normal (compressed) mode takes just over 2MB per image, while in TIFF (uncompressed) mode it takes over 14MB per image. If you really want to know all of the technical image quality details of a potential camera, be sure to visit the manufacturer’s Web site before you make your final decision.
Ken Colburn is president of Data Doctors Computer Services and host of the "Computer Corner" radio show, Saturdays at noon on KTAR (620AM). Readers may send questions to firstname.lastname@example.org