Ask the Teaboy

Q: What's this 'dpi' business about, and how does it relate to the 'megapixels' of my digital camera?

A: People often get confused over the dpi issue. After transferring the images from their camera to a computer, many open their files up in a program like Adobe Photoshop or similar and then discover (under the 'Image Size' menu) that the image is 'only' 72 dpi, and begin to despair because they've been told that 300dpi is photo quality, and 72 dpi is only screen quality. They've bought a 5 megapixel camera thinking it's very high resolution, and now feel ripped off!

Here's the scoop - dpi is a printing term, and is quite unrelated to the term 'megapixels'. The relation between printing resolution and pixel count can easily be explained, and I will endeavour to do so here. Please read on and you'll soon grasp the concept (although some neuron-stimulating liquid, like coffee perhaps, might be in order):

Here's an example 5 megapixel image, as opened in Photoshop. You can see that I've opened the 'Image Size' dialog box, where you can view the particulars of the image. Notice that the pixel count confirms the megapixels (2592x1944 = just over 5 million pixels). Also note that the image resolution is showing as being 72 pixels per inch (ppi, or commonly misnamed as dpi). But, and I say again, BUT, please also notice the dimensions of the image - 27x36 inches no less. That's a lot of inches I hear you say, and "indeed it is" is my response.

Now, if we make some adjustments you should start to understand how this whole 'dpi' (or ppi) thing comes into play. On our next image below, notice that I've removed the tick from the 'Resample Image' box, and have then changed the 'Resolution' figure from 72 to 300. Notice what has happened as a result - the printing dimension has gone down from 27x36 inches to 6.48x8.64 inches (a bit over a 6x8" print size). BUT, please also observe that the pixel count hasn't changed at all, and is still 2592x1944 pixels (5mp), and at the top of the image size box you can see that the image size of 14.4 megabytes has also remained unchanged.

We haven't added or subtracted any pixels, as the pixel count and file size has remained the same. So, "what have we actually done?" I hear you ask. Well, all we've actually done is 'rearrange' the pixels we have. By telling the software we want to adjust the printing resolution to 300ppi, it has crammed more pixels into an inch (increased from 72 to 300), which has conversely reduced the printing dimension dramatically - from enormous poster size, to a bit over a 6x8" print size. In other words, by filling each inch with more pixels, the pixel count cannot cover as many inches! Or, in other other words - it's a bit like having a large jug of lemonade to share with the kids, the more you put in each glass, the less kids you can serve, and the sooner the party is over!

Hopefully all the above is all clear, and I can move on to a couple of questions that may have now arisen in your mind as a consequence of what we've covered!

Q: "I've been told my 5 megapixel camera will print well up to A3 size (12x18"), but you've indicated via the above that it'll only be photo quality (300ppi) to just over 6x8"!?"

A: An image from a 5 megapixel camera can certainly be printed very well up to A3 size, through a process called 'interpolation' or 'resampling', which is where extra pixels are cleverly added to an image file to make it larger. In the example above we unchecked the 'Resample' box, but if AFTER removing the tick from this box, and changing the resolution to 300ppi, we had then re-ticked the 'Resample' box we could then resample/interpolate the image to whatever dimensions we wanted via the 'Height/Width' boxes. Just keep in mind that the more you interpolate, the greater the degradation in quality.

For your convenience, here's a chart (which you'll also find on our helpful WebPrints Upload Tips page) that will give a good idea as to recommended print sizes from image files of various resolutions:

The chart above relates only to the JPEG image file type. Note: The 'File Size' column above relates to closed files, and assumes medium compression. These same files are much larger when opened (i.e. a 700kb file can decompress to 5 megabytes when open). For more info on JPEG files (their characteristics, and how to work with them), read a previous article here.

Printing via most labs does not require interpolating however, as modern lab machines do this on the fly. You will get excellent results simply by using the file as it comes from the camera. Some of the more pedantic amongst you may like to play with colour and contrast somewhat, and even pre-interpolate at your end - especially if using dedicated interpolation software like Genuine Fractals, or Photozoom Pro, amongst others - but this activity I would only recommend for those more experienced in manipulating and optimising images for print.

Hopefully you've got to the end of your coffee, and still don't regret reading this page!