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Digital cameras offer ever-increasing numbers of megapixels (millions of pixels), promising greater detail in photos. In practice, more pixels doesn't always mean a better picture - but it's undeniable that not enough pixels is a bad thing.

The amount of detail in an image is vaguely referred to as its resolution. Unfortunately, even professional designers often talk complete rubbish about resolution. They'll mention a '300dpi image', for example, when they mean a photo with sufficient quality for printing. Well, 'dpi' stands for 'dots per inch', so it's fairly obvious that any image can be 300dpi. Even if it only contained nine pixels in a 3x3 grid, you could print it one hundredth of an inch wide and hey presto, full 300dpi quality!

As you can see, it only makes sense to talk about the resolution of a photo if you have a physical size in mind. When you use a photo on screen, for example in a website, you need to make it the exact size you want to see, measured in pixels. When you're preparing to print it, you don't have to be so exact - whatever size the printer is told to output it, the image will scale to fit - but you need to ensure there are enough dots for good quality.

Below, we debunk image resolution and show how to check or change it by resizing an image.

A different way to alter the size of a picture is to crop it: that is, cut out a rectangle from within the frame and throw the rest away. You might do this because you need the picture to fill a space that's not the same shape, or for creative reasons to get rid of unnecessary parts of a scene or to make a more eye-catching and artistic composition. We've provided some examples opposite. ab

Resolution in my head

There are lots of misunderstandings about resolution, partly because the same term is used to mean different things. Every digital image consists of a grid of pixels, and the number of pixels determines how big you can display or print the picture without losing the illusion that the pixels form a continuous image. Today's PC monitors pack around 10,000 pixels into each square inch, 100 across and 100 down. The screen emits light, and this helps to blend the pixels together. On paper, you need more (ie smaller) pixels to avoid seeing them. There's no hard and fast rule, but the accepted practice is to allow 300 dots per inch.

Remember we're talking here about the number of pixels in the image file, not the number of dots of ink that the printer can print per inch, which may be far more. That's because most printers only use four or six inks, so to create all the colours the eye can see - or at least a fair proportion of the 16.7 million that are stored in a standard digital image file - they have to combine large numbers of dots within each pixel.

You can tell your image editor, or any software that handles images, to print any picture at any size. So how do you know if it'll have enough pixels to come out well? Just divide the number of pixels across the image by the width in inches that you want. If you get 300 or more, that's fine (though if it's a lot more, see 'Sharpening down', previous page). Around 240 is enough to get away with. Less than this and you're going to see some compromise on quality; below 150, you'll start to notice some real deterioration and blockiness.

As a rough guide, a 5 megapixel photo will print perfectly at A5 size, or up to A4 at a pinch; 10 megapixels will be fine at A4 and acceptable up to A3. Of course, if you crop an image, that area will need enough pixels.

Image editing programs all have a Resize or Image Size dialog box similar to this (seen in Adobe Photoshop Elements). You can enter settings to change the number of pixels in the image, by ticking Resample, or just to check what its resolution will be at a certain size. (Always use Constrain Proportions, sometimes called Preserve Aspect Ratio, to maintain the photo's shape.) With Resample off, you can enter the desired resolution, such as 300 pixels per inch, and the physical size will update to show how big the photo will be at this resolution; or you can enter the physical size you want, and see how many pixels per inch you'll get. Here we're finding out that if we print this photo at A4 size, we'll only get 179dpi. Resampling to a larger size doesn't increase quality, so we'll just have to print it smaller.

	1 To crop an image rather than resizing it, you can use the Canvas Size command in your image editor. However, it's much easier to get the result you want using the Crop tool, which lets you draw and adjust a box within your photo, then delete the remaining area. You should be able to enter a specific size if required so that the crop area stays the right shape.

	2 Crop: Wrong! Pro photographers will try to compose a shot when taking it, but may well re-crop it later. Newspaper and magazine designers also often crop photos for maximum impact. Get into the habit of using the Crop tool and you'll start to see your photos in a new light.

	3 Crop: Correct!

	4 Crop: Wrong! By convention, when a person in a photo is looking to one side, or an object is moving in a certain direction, the picture is cropped to leave extra space there. You can break this rule, of course, but it's well worth bearing in mind.

	5 Crop: Correct!