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Andrew James

Andrew is a highly experienced writer and photographer – if you have a problem, he is here to help.

A large amount of the water surface detail present is uniform in tone and colour, meaning that it has been compressed efficiently to a 5.5MB jpeg file.

The darker water has more colour and tonal variations within it, so it isn’t as easy to compress efficiently, resulting in a larger jpeg file of 7.9MB.

Andrew James

Image compression lesson

Q I’m a beginner and besides PPI was wondering what factors affect the resolution of a photo. Looking at the photos I’ve taken, although the pixel dimensions are similar, the jpeg file size varies. Why is that?

Kath Smith

A As a beginner, you’ve clearly been looking hard at some of the technical aspects of image quality, Kath. The term PPI is an abbreviation of pixels per inch, and you are correct in thinking this is an indicator of image resolution. Essentially, it’s simply the number of pixels that are present within one inch of an image and the more you have, the greater detail you get within that image.

But it’s normal for two jpeg images with identical pixel dimensions to have different file sizes and this is down to a couple of factors. Firstly, a jpeg is a lossy compression file and, when compressed, the process gets rid of certain information that the algorithm decides won’t be noticed. Secondly, it’s down to the amount of information within the photo itself, so an image with a larger amount of detail present, such as greater colour variation, won’t compress as efficiently and will therefore be a larger compressed file size. If you look at the two example shots here of a razorbill on the water, both have identical pixel dimensions of 5472 x 3648. However, the high-key (lighter) image has much less information and the compression has been more efficient making its file size 5.5MB. The second lower-key image (darker) of the razorbill on the water has a greater amount of information within it, therefore the compression process has resulted in a higher jpeg file size of 7.9MB.