Warning this article contains technical stuff and some mathematics!
Simply speaking, digital photographic images are made up of pixels – tiny squares. The resolution is the number of pixels in a particular image. You can describe the resolution in two ways, the one that I find useful to use is the number of pixel columns (width) by the number of pixel rows (height). Your computer will display the pixel information for a particular photo image.
Photos for Web
Generally, images on the web are small in resolution on purpose, so that they will load quickly. A typical image which has been sized and optimized for the internet might be anything from 75 pixels wide to 800 pixels wide or high (the latter is considered quite large for web).
Photos for Print
A useful rule of thumb to use when printing photographs is a resolution of 300ppi (or dpi – dots per inch – effectively the same thing!) which means 300 pixels per inch. Yes photograph printers still use inches! So if you wish to make a 4×6” (10x15cm) print from your photo, just multiply the 4 by 300 to get 1200 pixels along one side, and multiply the 6 x 300 to get 1800 pixels along the other side. (I will make a comment here that a large resolution doesn’t necessarily mean a higher quality image or good sharp focus!)
To figure this out in reverse, you could have an image 600 x 900 pixels which will look great on your phone, tablet or computer screen. To work out what size you should print, just divide the pixel numbers by 300 (most photograph printer’s ideal printing resolution). This photo should best be printed at the size 2 x 3 inches, or 5 x 7.5cm.
Enlarging a photo from a digital file…
It is possible to print a photo bigger therefore enlarging by brute force, for example to print a 600×900 image at an 8×12” size – effectively giving you a resolution of 75ppi. Most people will not be happy with the result! This is when you will see the pixels more clearly, and when the image is described as “pixelated”.
Photos can be enlarged in image editing programs; I personally use Photoshop, but there is a limit to how far the existing pixels can be stretched. An image even 1200 x 1800 pixels will only enlarge so far and a pleasing result will depend as well on whether the photo is well focussed to begin with. Please contact me if you need advice.
The below image has been forcibly enlarged from 75 pixels to 600 pixels.
Enlarging a printed photo
If a client needs a large print then I will scan the original photo at an extra high resolution eg 1200 or 2400 dpi and if the photo needs further enlarging from that starting point this will be perfectly possible.
I would like to add the caution that, any enlargement from a small photo to a bigger one, no matter the depth in resolution, if the photo is blurry/unfocussed or even softly focussed to begin with then this will be more apparent in the enlarged photo on close inspection. Usually however the enlarged photo will be displayed on the wall and viewed from a distance so some softness would be quite acceptable.
As always, please contact me if you have any queries or concerns, I am always happy to discuss how I can help you!