Size Matters (From The Westmorland Gazette)

Size Matters

8:59pm Sunday 29th March 2009 in

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In the past couple of weeks I’ve been trying to explain the concepts of compression and resolution to my students, and it’s not easy. Three things can be varied. The width-and-height of the image is the easiest to understand. Then there is also the amount of electronic memory needed to store the digital image. I call this ‘memory space’ to distinguish it from the picture dimensions.

Compressing the image allows you to pack more images into a given space. The more image information per inch packed into the space, the higher the quality of the detail in the image. The smallest unit of picture information is the pixel, but no solution is perfect. If you shrink or enlarge the width-and-height dimensions of a JPEG image, all the pixel information has to be recalculated for the new size, losing a tiny bit of picture detail in the process. It’s what’s called a lossy process. Not that you’d normally notice, unless you edit the same picture an awful lot.

Trying to keep it simple for now, unlike the fine chemical grain of traditional film, the problem with pixels is that they are square. They don’t go round corners. Try taking a piece of squared graph paper and colour in the squares to make a circle, oval or diagonal line. You’ll see immediately what I mean. In close-up detail the shape is going to look stepped or jagged. Pixilated, in fact.

Which is where anti-aliasing comes in. Look at any digital photo, magnified and you’ll find to smooth the jagged edge, pixels are faded in or out along the curve. At normal 100% viewing size your eye sees a smooth curve, as a direct result of this clever trick. OK so far. But there is still a problem. In order to digitally re-construct the image after it has been taken, information on the position, colour and size of each and every pixel needs to be stored in electronic form. This takes a lot of electronic memory and processing power. Luckily, help is at hand.

As long ago as 1986 the Joint Photographic Experts Group was formed. After six years of head scratching and some complicated maths, they came up with a solution - the JPEG (sound a fanfare).

JPEGs are a method of compressing image data, allowing images to be inserted into a variety of documents and software programmes. Because the data contained in JPEGs is space efficient, it is also the most common format for storing digital images on digital cameras. As the Wikipedia entry says, “JPEG is very popular”. They then give the detailed calculations which I admit I skipped, but take a look if you are mathematically inclined.

The thing to remember is that each image has a certain ‘memory-size’. Small, space-efficient JPEGs, created using low resolution settings on your camera, are perfect for email attachments and web images. They do not contain sufficient picture information to print out at a large, A4-or-above sizes. High resolution images contain more detailed image information, but also take up more space on the digital film card. Watch that film counter as it tells you how many images your camera can store at particular settings. You will find that the better quality, larger dimension settings allow for less images to be stored in the available space. Mind you, if you’ve replaced the manufacturer’s 16 or 32MB card with a 2GB one, you’ll not be short of storage on your camera.

A couple of years ago one of my students complained that she could only get nine (9) images on to her film card before it was full. I asked if she was happy with the quality she was getting. She replied, “Very!” Like many beginners she had set the camera to superfine quality, and largest 2272 x 1704 pixel setting. To make the problem worse, she had not yet replaced the 8MB card. A quick explanation set things right.