Jpeg-schmapeg, what’s all this about?

For some years now, digital has been the default method for capturing photographic images. In fact I haven’t had a client ask me to shoot film in the last eight years, but despite this there still appears to be a lot of mystery surrounding digital.

This handy guide is part of a series I’m writing to help explain some of the processes of shooting and supplying digital images, and bust some of the jargon that might otherwise sound like the teeth-sucking obfuscation of a car mechanic.

I’ll start with an explanation of the JPEG format, which is something you should at least be familiar with even if you’re not sure what it means. You’ll probably know of jpeg as the standard image format you get when you use a compact digital camera.

So what is jpeg? Jpeg (normally seen as “.jpg” on the end of an image file name) is a data compression algorithm. Or in simple terms, it’s a way of squeezing lots of data into a smaller space on your storage device, be that a camera storage card, hard drive, CD etc. You should notice that the file size of the image on your hard drive is much smaller than the stated file size when you open the photo in Photoshop or similar other software. This is because the algorithm is designed to take data away (mostly neighbouring pixels) when the file is being closed, and put data back in when it is opened.

One way Jpeg works is by throwing away information which is repeated in the picture. A good example is a blue sky. The jpeg algorithm might say that since there is a lot of blue in one part of the picture, it can safely discard a percentage of blue pixels, but leave a little note to Photoshop saying “when you open this picture, stick some blue in here mate and it’ll look fine”. So when you re-open the file, Photoshop will see the note and know to fill in the spaces where colour information has been discarded. This system works well, but can lead to strange patterns, sometimes called artifacts, when too much compression is applied.

This image detail shows normal compression.

This version shows over-compression.

The amount of jpeg compression can be set according to different needs and preferences. You can compress a file a lot so it loads quickly within a web browser, or compress it not very much to achieve higher quality for printing.

However, there are pitfalls (such as the artifacts I already mentioned) associated with over compression. Whenever you compress a jpeg, some of the image data is lost so that the next time you open the file it won’t be as high quality as it was before you compressed it. Imagine folding an oil painting to fit it through a letter box. It will never look as good as it did before you folded it.

So used carefully, jpeg is a great way of moving images around and displaying them on web pages, but needs to be handled with care – especially when you want to reproduce images in print.

Personally I shoot all my assignments in the RAW format, which I’ll explain in my next article. Future articles will look at post production, colour reproduction and whatever else pops into my pretty little head along the way.

As with all my blogs, if you have any questions or comments you would  like to make, please feel free to contact me or post a comment here.

If you would like a copy of this article for reference, I’ll be hosting it in the Free Resource section of my web site in due course.

Article and photos © Tim Gander 2009. All rights reserved.