OJBs Web Site: Image Size

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Photography Tips: Image Size

Digital cameras have settings to control the size and compression quality of the images they store. This tip discusses what image size is, what compression does, and how to set the camera for the highest quality results.

The size controls how big the images are, up to the maximum resolution of the camera (for example my 7.1 megapixel camera creates 3072 x 2304 pixel images. This means the picture is made up of a rectangle of coloured dots 3072 across and 2304 down. The more dots you store, the more detailed the photo can be. Its possible to take photos smaller than the camera's capability, which allows more images to be stored in the camera, and sometimes makes processing faster.

Compression controls how the images are processed for storage. You still get the same number of dots, but the camera uses compression to store the photo in a lot less memory. The more compression used the smaller the image is (on the memory card, the photo itself is the same size) but highly detailed areas tend to become blurry. Its also possible to store images without compression, but they can get really big, for example only one image might fit on a small memory card!

Left is a highly compressed image, right is a moderately compressed image the same size. Notice that the compression blurs detailed areas, such as the clock. The less compressed image required 5 times more memory of the small one to store though.

My policy has been to use the highest quality settings available, even if I don't need all the detail that provides. For example, I display most pictures on my 1440 x 900 screen (about 1.3 million pixels) but my camera takes 3072 x 2304 pictures (7.1 megapixels). The reason I take the photos at the highest quality is to give me some extra information to work with when rotating, cropping and doing other manipulations. You can see on the image above how I zoomed in on the church by cropping the bigger photo. Because the image was big to start with I still got good detail. Also, it gives extra detail when printing on high resolution printers.

The disadvantage is, of course, that the memory card in the camera gets full more quickly, but the price of high capacity cards isn't that much so I use a 512 megabyte card which can store 150 highest quality images. If I was going to be away from a computer for a long time I would consider buying a second card instead of reducing the picture quality.

Update

Since I wrote the information above I have a new dSLR camera which takes 8 megapixel images. I still use full resolution (highest quality, but not raw). I also have a second SD card, which is essential for storing a decent number of high quality images. I prefer two smaller cards instead of one large one. Its not likely, but if a card did fail for some reason, at least I've still got half my images on the other card!

Related Subjects

Cropping

Selecting the best parts of larger images.

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