For consumer-oriented cameras, deciding how large a memory card to buy can be tricky. Image size varies widely, depending on the camera's design and which settings a user chooses. For working professionals or amateur enthusiasts, digital SLR is more predictable. Image sizes still vary, depending on the camera's number of pixels, but they're usually taken at high-quality settings. Major manufacturers such as Sandisk provide charts for photographers, estimating how many photos can be stored on inexpensive 8GB cards, as well as smaller and larger sizes.
File Size and Image Quality
The two main factors that affect a camera's image size are its number of pixels, and the image format chosen for storage. Each individual picture element that makes up the image is one pixel, and higher numbers indicate better images. For comparison, think of the difference between a grainy newspaper photo and a similar image in a glossy magazine. Resolutions between 14 and 22 megapixels are common in 2012, but those numbers will continue to increase. The images are stored on your camera as compressed JPEG or RAW files. JPEG files lose some quality, but are useful for websites and low-resolution printing. RAW files are larger, but higher quality.
JPEG Image Storage
Cameras vary in the size of their images, because some use higher compression ratios or have compression algorithms that are more efficient. Overall, JPEG images are much smaller than RAW images. According to Sandisk's estimates, a 10 megapixel (MP) camera averages 3MB per JPEG image. At that size, an 8GB memory card holds approximately 2,288 images. In comparison, higher-quality 14MP cameras create 4.2MB JPEG images. An 8GB memory card holds 1,634 images at that size. At 22MP, digital SLRs create JPEG files averaging 6.6MB in size. An 8GB memory card holds approximately 1,040 of those.
RAW Image Storage
RAW images have 24-bit color depth per pixel, which makes the images larger but also crisp and vivid. Sandisk estimates the size of an average uncompressed RAW image as 30 MB in a digital SLR with 10 MP resolution. An 8GB memory card holds 222 of those. The same files would be 42MB, in a 14 MP camera. An 8GB card holds approximately 163 photos at that resolution. Uncompressed images from a 22 MP camera will average approximately 66MB, and an 8GB memory card holds 104 images at that size.
The actual capacity of your memory cards is slightly lower than their stated size, because formatting the card to hold data takes up a small percentage of its capacity. If your computer has a memory-card reader, you can learn its true capacity, and the real size of your photos, by inserting the card into your computer and looking at it in Windows Explorer. Take four or five photos and save them in both JPEG and RAW formats. Divide the size of your memory card by the average size of your own photos, to arrive at an accurate estimate for your own camera.
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