Digital Photography 1.0.1.
A question I am frequently asked by newcomers to digital photography is, “How do I make my image files smaller so I can email them?”
It is a matter of 3 variables…
Every digital image is composed of tiny square blocks called pixels. The more pixels it is composed of, the finer the detail and therefore the higher the quality an image is -- hence camera manufacturers using the selling point of how many megapixels their camera can produce. However, the downside is that every extra pixel increases the size of the file created from the image. The fine detail from a multi-megapixel image is only apparent if you can get it all into view at one time. Resolution enters the equation! The human eye can discern the difference in detail between ca 100 pixels per inch and 200-300 pixels per inch. Above that density, we cannot tell the difference. The “sweet spot” for printers is therefore 200-300 pixels per inch, if you wish to make high quality prints from your digital images.
If you want to make a 7 x 5 inch print at maximum quality you will need to set your camera to capture at (300 x 7 =) 2100 pixels by (300 x 5 =) 1500 pixels, or thereabouts. The file size created by the resultant 3.15 million pixels will be considerable (several megabytes if you choose the low compression option). Save that file and keep it safe for printing purposes.
What you now need is a smaller file to email to friends and family or to post to the web. That’s where your monitor enters the equation. Your monitor is displaying the images it receives at only 72 pixels per inch. A 10 inch wide monitor cannot display any more than 720 pixels horizontally. When you open your original 2100 pixel wide image from the camera, your monitor has to hide 2 out of every 3 pixels to get the whole image on the screen. Depending on the software you are using to view it, you will see that it is being displayed at 33% or a ratio of 1:3 or some other figure depending on the exact arithmetic of the situation. (This is the zoom setting.)
What you need to do it RESIZE it (not crop it) so you throw away all the surplus pixels (1400 of the 2100 pixels along the horizontal axis). Your camera’s image editing software should have the necessary means to resize your image. Pick the resize command and set it to 700 pixels wide (if you keep the ‘maintain aspect ratio’ box ticked, the vertical axis will look after itself). You will now have an image that fits onto your monitor at 100% zoom and is now 700 pixels by 500 pixels. That makes the image a total of 350,000 pixels, compared to the print version of 3.15 million, and it won’t look any different on your monitor or anyone else’s.
There is still work to be done, though. The 350,00 pixel image in uncompressed file format will still be over one megabyte. What makes the file sizes smaller than that is the aspect of saving them in JPEG format (.jpg file extension), which compresses the file, not unlike a Winzip file. You should choose to “export as JPEG” or “save as JPEG” depending on the software you are using. You should be offered differing degrees of control over how much compression you apply when saving the file.
There is a trade off when applying the compression. The higher the compression, the smaller the resulting file, but the more quality you lose when the image is displayed. The application “PaintShopPro” is the best I have seen at offering the chance to fine tune the exact amount of compression, while at the same time being able to see the potential outcome of the effect on quality… and to see the file size that will result from that compression setting.
With practice, you should be able to create a 100 kilobyte JPEG version of any digital image that is 10 inches wide at 72 pixels per inch and that is very nearly as good quality as the original file of several megabytes (when viewed on a monitor).
...and one last point… inkjet printers will boast about the number of dots per inch that they print at (several thousand these day). Remember that dots per inch (dpi) is not the same as pixels per inch (ppi). One pixel can have several dots in its composition. As far as quality is concerned, the important thing with digital images is the density in pixels per inch at which they are printed.