Resolution and Compression

"How many images will fit on this memory card?". That is the most common question we get from digital photographers. The answer we always give is, "It depends". There are four main factors that determine the answer to this question, and we will look at them in order of importance.

The capacity of your memory card is the most important factor in how many images you will be able to squeeze onto that card. More memory equals more photos. It?s that simple. All types of memory cards (Compact Flash, xD, SD, etc.) are measured in megabytes (MB). A 512 MB card has twice the capacity of a 256 MB card and will hold twice as much data. We recommend always having two cards, so that you can download one while shooting with the other, and you have a backup in case something goes wrong.

Resolution Matters, But Less Than Size

Resolution is the number of pixels (individual dots) that make up an image. On a digital camera, it is measured in megapixels (MP). Most cameras allow you to adjust the resolution you are shooting at. A higher resolution means more detail and sharpness, and a larger file size. So you will get fewer 5 MP images on your card than 3 MP images. You have to decide on the trade-off: get more images on your card by lowering the resolution, get more detailed images by leaving the resolution high, or buy another memory card.

Compression, also labeled as Quality on many cameras, can also affect the amount of memory your images take up. Higher compression means smaller files. It also means less detail and sharpness. Most digital cameras describe compression with words like "Fine, Normal" or "SHQ, HQ, SQ" or some other code. However, most people will never have to change this mode unless they are trying to squeeze a few more pictures out of their memory card, or are professional photographers.

What you are taking pictures of will affect the size of your image files, because subjects with more detail will not compress as much. That is why you can only ever estimate how many images will fit on your card. Each image file is a different size because of varying levels of detail.

Here is a chart showing the relationship between resolution, compression, and file size:

Once you have purchased a card, there is an easy way to find out how many images your memory card will hold: put it in your camera and turn the camera on. You will see an estimate somewhere on your display of how many more images will fit on your card (remember that is an estimate because your subject matter will affect file size). When you change a setting, the estimate will change. Whereas film cameras count upward until the roll is done, digital cameras count down until you run out of space.

The Photo Express recommendation for most people is to leave their camera at the factory setting, which is most often HIGH resolution, and NORMAL compression. Only change to a finer compression if you are very concerned with detail; lower the resolution if you are only planning to use your images for e-mail or the web or very small prints.

Note: click here for a chart showing how many images typically fit onto certain size memory cards.

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