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Digital photography has brought with it a whole host of acronyms, technical terminology and undecipherable jargon, image resolution being one of the most mysterious. In this short guide we will attempt to explain and hopefully demystify the strange pixelated world of resolution.
So lets start with what comes out of your camera. Your sensor is measured in megapixels, but what does this mean? Put simply, your camera sensor is not unlike the photographs you see in a newspaper. If you look closely, in the case of a newspaper print, it is in fact nothing but a series of tiny ink dots that combine to make up an image. You camera sensor is much the same only the ink is replaced by photodetectors, the pixel. To make up an image you need millions of these pixels close together, in order to fool the eye into believing that you are looking at a continuous series of tone and colour. The more pixels you have the more realistic that image will look.
Whilst your camera specifications may state 10, 12 or 16 megapixels, the more important figure is the number or pixels width and height, in the case of my 12 megapixel camera it produces images that are 4256 pixels wide and 2832 pixels in height. However there is a third number not usually quoted but important to the whole equation of image resolution: PPI.
PPI means pixels per inch and defines the resolution of an image. When your camera saves an image it needs assign a pixels per inch value to the image. Most modern cameras will save either as 72ppi or 300ppi. So what is the difference, well in terms of the overall digital image quality, there is no difference, but when it comes to outputting the image for print or web there are important differences.
So lets explain PPI a little. If we assume your camera saved the image at 4000×3000 72ppi this means if you were to print the image directly, the size of the image would be roughly 55 inches by 41 inches, 4000 pixels divided by 72 pixels in each inch, giving us as shade over 55 inches. Sounds great doesn't it? There is or course a big caveat and that goes back to what we were talking about at start. 72 pixels per inch is quite a low resolution so looking at that magnificent 55 inch print you will probably be sorely disappointed with it’s quality. This is because those pixels have been spread wide apart.
Now lets take that image and in Photoshop change it’s ppi to 300. If with do the maths again we will now only get a print size of 13 by 10 inches. Much smaller in size but because those pixels are crammed much closer together the image will be of a much higher quality when looked at from the same distance as the 72 dpi image.
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So what ppi should I use? Well if you are sizing an image for the web, use 72ppi. The reason for this is two fold: firstly you will be down-sizing the image to fit on a computer screen. Most computer screens only display roughly 72ppi so there is no point in using 300ppi. Secondly because the resolution is lower, the file size is smaller hence the image will load faster on a web page.
Now lets assume you wish to print the image, ideally you would work from 300ppi, your 4000×3000 pixel image giving you a good quality print at 13×10 inches. But what if you wanted to print bigger. Well you have two options here, firstly you could reduce the ppi. Whilst this will lower the print resolution it probably will not become visible until you start to go below 200ppi. So getting the calculator out again, our 4000×3000 pixel image, if set to 200ppi would give us a print size of 20 by 15 inches. Obviously this is much bigger but maybe not enough.
Should you wish to go further, you can do what is known as up-rezzing. This can be done in Photoshop using the image size tool or using 3rd party software. What happens in this case is the software basically recreates new pixels in between the originals and based on what the pixels surrounding it look like. The most current algorithms used in Photoshop and others are extremely efficient and can allow images to be enlarged by up to 400% with careful manipulation.
So that is a brief explanation of PPI – many of you will also have heard of the term DPI, this relates more to printer settings and we will talk about that in a later tutorial.
Jason Row is a British born travel photographer now living in Ukraine. You can follow him on Facebook or visit his site, The Odessa Files. He also maintains a blog chronicling his exploits as an Expat in the former Soviet Union