Understanding digital photo quality and file size: A ready guide
As an editor and professional photographer, up to 50% of the images Safrean Gareth Griffiths gets sent for publishing in print are of the wrong resolution. Here’s how you can be sure you get it right next time.
Resolution means the size of the photo in pixels (very simply, these are ‘dots’ of digital information relating to colour and tone) that are expressed in a horizontal and vertical medium. When you multiply the horizontal by the vertical dimension, you get the digital resolution of the image, expressed in megapixels (what you camera ‘takes’). This should NOT be confused with print resolution, which is the dots per inch analogue equivalent of digital resolution. The trade has adopted two slang terms for the presentation of images. They are ‘high res’ and ‘low res’. Be warned that these are subject to misinterpretation.
It is important for digital camera users to grasp the basics of digital image sizing. Should you be sending photos to a newspaper or even a magazine, then a photo taken in JPEG format at medium resolution (around 5 megapixels) is suitable. However, should you need to keep this photo for high quality printing and possible enlargement, it becomes important to shoot at the camera’s maximum digital resolution.
An additional setting available on your digital camera or smartphone is compression or image quality. This is normally selected by setting the camera to take ‘fine or high quality’ or on the other hand, ‘medium or standard’ quality. Some cameras also have a low quality or high compression option. The significance of this setting is as follows:
Low compression means a large file (around 3 megabytes) on your memory storage card but the photo is rendered at highest quality. There is a minimum of digital noise or blurring of detail.
High compression means a smaller sized file (around 1 megabyte) on memory storage, but the image quality is poorest.
High compression files further deteriorate in quality if they are edited on the computer using a programme such as Gimp or Photoshop Elements. It is a good tip to always take your photos at the highest quality setting, even is taken at a low digital resolution – assuming your camera memory card has available space. Smaller sized files in megabytes are also easier to email, however.
As a very rough guide, many publishers will tell you not to send any photos other than 1 megabyte in size for publishing. This applies to a number of factors but in truth you can get away with a much smaller sized file provided the megapixel size is correct. As an editor, I will publish most file sizes of 500 kilobytes (half a megabyte) – that’s my rule of thumb.
Notice Bentley the Frenchie in both images (same size in height and width). The image at the top is rendered for internet/web usage. The lower image is how that same image would come across in print. Note how the image deteriorates at print resolution, confirming the rule that you cannot send low resolution ‘thumbnail’ images for printing. This is very important when sending a press release with an image to a print magazine.
Nowadays many photos are being presented in .PNG (Portable Network Graphics) and not JPEG format. Be aware that this format takes up more disk space for a lower resolution photograph so a .PNG image sent at, say 500 kilobytes (half a meg) is most likely too small to be published in the print medium, whereas a .JPEG file at 500 kilobytes most often is publishable.
I’ll deal with the other (higher) formats of digital imaging, including RAW, in the next article.
So why do photographers refer to pics posted in emails or on the internet as thumbnails? It’s because when printed at standard print resolution, they come out the size of a thumbnail – too small for a magazine or book (unless you have magnifying glass handy!).