Here’s a stab at an explanation.
The short answer is that there is no such thing as a high resolution image. As a graphic designer and photographer, I am working on a range of printed material (from ads and brochures to magazines) and am constantly receiving images from various sources for inclusion in material I am producing. And one thing that strikes me about modern digital technology is that it is very confusing for clients to understand the resolution of images they send.
So here (as simply as possible) I will try and explain about resolution.
Unlike the photographic prints of old, where you can physically see the size and definition, digital images are harder to judge. What may look okay on your computer screen is not necessarily good enough for print. A computer screen may be using a resolution of about 72 pixels per inch (ppi) (this varies, but will do for our purposes here), whereas printed matter requires 300 ppi. Now let’s assume you have an images about 8 inches square on your screen, you might consider that is large enough to print.
But if you work it out (and assuming a screen res of 72 ppi) your 8 inch square will comprise (72 pixels x 8 inches) 576 pixels across both width and depth.
However, because a printed image will require 300 ppi, if you divide the 576 pixel width x 300, you will see that the image will only be 1.92 inches square. Okay, if that’s the size you want it (and if that is the case, you could call it ‘hi-res’), but no good if you want it splashed full across an A4 page. And that assumes the original image you have got is good quality and has not been opened and re-saved too often. JPEGS, for example, degrade each time they are re-saved (but that’s the subject for another article).
To go back to basics, the confusion really starts when we try to imagine the size of a digital image. Clients will ask me what seems a reasonable question – is it six inches across, or 12 inches? Fair enough, but the problem is a digital image is not a physical ‘thing’. Its size depends on the number of pixels per inch required (for its specific use). It is a collection of pixels (little dots of colour information, if you like) and that is the only way it can be accurately described. So if you want a picture that almost fills an A4 landscape page in print, the size of the printed image will be (let’s say) 7” x 11”. So the image needs to be (7” x 300 pixels) 2,100 pixels by (11” x 300 pixels) 3,300 pixels.
Since I am using Photoshop, it is a simple matter for me to interrogate an image file and see the dimensions in pixels, but it is not always so simple for those who don’t have that level of software. I use a Mac, so if I go to Finder (Mac’s file manager) and highlight an image file, it displays certain information about that file and gives the dimensions in pixels. I believe you can perform a similar action on a PC – go to Windows Explorer (the Windows file manager), and right click on an image file. Select “Properties” from the fly-out menu, and then the “Details” tab which gives (amongst other things) the dimensions in pixels. That’s with Windows 7 – other versions may vary.
I’ve used inches as the measurement here for illustration purposes (although in fact I work using metric measurements in most cases). If you want to work out dimensions for metric sizes, you substitute 118 pixels per centimetre for the 300 pixels per inch used earlier. It is, to all intents and purposes, the same procedure. (If you really want to be accurate, it is 118.11 pixels per centimetre – but who’s counting?)
One word of warning. All this assumes you start off with a good image – perhaps a first generation file from a camera. As I indicated earlier, if you open and re-save a JPEG it will (almost imperceptibly) degrade because of the compression used in the format. Do that too often, and the loss of quality will become apparent. If you merely drag the file from one place to another in File Manager (thus not opening and re-saving it) there is no problem. If the image comes from another source (not the camera) you don’t know what has been done to it, so you need to view it in some programme at 100%. That will give you an idea of the quality. It may be that, although it physically matches the dimensions and resolution you require, someone further down the line has enlarged it. Not a good idea, because in enlarging an image, the computer is not adding detail – it merely makes a ‘best guess’ at what image information should be there. Sometimes it works – more often it doesn’t, so view your images at 100% to see what you get.
So, to sum up, there is no such thing as a high resolution image – only an image that is physically large enough for the job you want it to do at the resolution required. Simple? Let’s hope so.
*One other bit of information that might help. Say you have a 13.3 megapixel camera. What does that mean in pixel terms? On such a camera, the image size will typically be 4224 pixels x 3158 pixels. Multiply those two together and you get 13,339,392 pixels – i.e just over 13 million pixels, (or megapixels as they have become known).