New Year’s Resolution!

Or, as John Lennon never sang, “You say you want a resolution.”

So to continue with my series on the basics of working with digital images, I’m addressing the issue of image size and resolution. Pay attention at the back.

Let’s start by explaining DPI, which stands for Dave’s Pizza Index. Or Dots Per Inch, I forget which but no matter. What’s important is to remember that the dpi and the image dimensions (width and height) are linked when it comes to file size (the space taken up on your hard drive by the image file), while height and width on their own will only affect the printed or displayed image size. In other words, if you have a photo which is A4 in size it doesn’t matter if you print it at 72dpi or 300dpi, it will always print out at the same physical size.

Where dpi is important is when matching the image resolution to the desired output quality.

For example, images to be displayed on a web page should be set at 72 dpi because that’s the most detail a computer screen can resolve. Newspapers vary, but 200 dpi is ample, while magazines will generally require 300 dpi. Printing on a high-end ink jet printer sometimes works best when the image resolution is set to 360 dpi.

This image is displayed at 300x200 pixels, 72 dpi. A higher dpi would make no visual difference here.

For print purposes, image dimensions (height and width) are measured in inches or centimetres, while web images are measured in pixels.

A large web image might be  around 600 or 800 pixels along its longest side, while a thumbnail image might be 75-100 pixels along its longest size.

These are approximate sizes, but you can experiment to get the size which suits the purpose of your page. Some web services, including blog services like wordpress, will automatically size the image for you after you upload it, but it’s still worth ensuring the image is large enough to display decent quality, but not so large it slows upload. Upload speed and quality will also be dictated by the image compression, which I talked about in the “Jpeg – Schmapeg” article.

One temptation is to take a photo which is very small, and try to “upsize” it (otherwise known as interpolation) to something much bigger, by increasing the dpi or the physical size. All this does is add pixels which weren’t there before, so quality starts to fall off quite badly once you get beyond a certain percentage increase. It’s always best to start with a much larger image and have to scale it down, rather than the other way around. I sort of refer to this in the RAW article, when I say RAW files start out larger and with more pixel detail than jpegs.

Of course if you’re working on an important project, for example a company brochure or annual report, it’s wise to allow the photographer and printer to liaise on the best resolution. At the same time they can also liaise on the best colour space for the images to be saved and supplied in, but colour is the subject of my next blog article, so be patient my sweet, be patient…

Those of you who tuned in for my instructions on baking the perfect meringue, sorry. I ran out of space. Ask Nigella Lawson.

All the articles in this series are being adapted to pdf documents which you can download from the Free Resources section (inside Galleries) of my web site – https://timgander.co.uk/