Resizing images can be a confusing business, someone sends you a digital image attached to an email and all you can see is a tiny part of the picture, it’s so large on screen – “What’s going on?” as they would say in Eastenders.
Someone has told you to order a quality digital image from your photographer, your boss after discussions with the designers of your brochure have said make sure it’s at least 300dpi – what are they talking about?
Hopefully the following simple explanation will help. What’s dpi – Dots per inch. All digital files and computer based images are made up of little dots called pixels. Put simply the more dots in each inch the greater the information in the image, the higher the quality the picture and the bigger you can blow it up.
Most people with little experience of digital imaging are these days having to receive and distribute images, because their computers are set up to cope with whatever their job function is they seldom have imaging programs installed so pictures will open up in their web browsers when they click on them.
As standard the web browsers will display an image file at the basic screen resolution of 72dpi so the picture will be very big on your screen unless your photographer has already altered the basic instructions of each file as that is how the camera generally captures the images.
The average digital captured picture today will have perhaps 6 million pixels at the point of capture, space those out at 72 dots were inch they are going to take up a lot more space than if you can cram them tighter together. So to reduce the image and make it so you can see the whole picture on the screen without loosing any of the information in the picture we need to resize the image.
For example our six million pixel (or 6 mega pixel) image at 72dpi is about 27×42 inches, if we could put 150 dots in each inch we could get that size down to 13×20 inches and if we managed 300dpi we could get the overall image size down to roughly 10×6.5 inches – small enough to fit on the screen! Still with all the information of the first size just smaller.
Now, again very generally, good quality reproduction in books is printed at about the equivalent of 300dpi and lower quality reproduction like newspapers is down around the 150dpi – so you need a bigger digital file to produce the same size picture in a quality publication. When your printer or designer asks for an image of 300dpi he wants a file size at least big enough to reproduce at the size you want it in the final publication at 300dpi.
So here we have the crux of the matter 300dpi only means something as a file size if you also know how big you want to reproduce it at 300dpi. Because one multiplied by the other gives the file size needed!
The problem normally comes when a person resizes an image on their screen to the size they want it reproduced. Instead of compressing the dots to a per inch ratio that fits the size, they throw away information from the file to display the picture they want at the screen resolution of 72dpi.
This then saved and sent for reproduction at 300dpi doesn’t have enough dots of information to cope, you have not resized the picture dimensions you have desized the file – which is two very different things!
Things to watch out for when resizing include noting the file size before you start and after you finish, this can be seen in windows explorer, any reduction in the file size means you may have thrown information from the picture away.