Resolution is a term that gets a lot of print coverage, but it's often misunderstood, creating confusion in the marketplace. HDTVs, digital cameras, computer monitors and printers all tout high resolution as a selling point. But what does it really describe?
To be meaningful, resolution should involve two units of measure. For digital images, this typically means pixels and inches. The more pixels fit into a given space, the higher the resolution of the image. A digital photo comes from the camera at a fixed size -- 3,000 x 2,000 pixels for example. That image would contain six million pixels in total, which is usually abbreviated as six "megapixels." The figure is often labeled as resolution because it governs how much detail the photo can hold. But true image resolution actually depends on how the photo is displayed.
For example, commercial printing processes generally require an image resolution of at least 300 pixels per linear inch. At that resolution, the six-megapixel photo would print 10 inches wide. To reduce the printed size, the image can either be reduced itself (eliminating pixels) or given a higher resolution -- for example, setting a resolution of 480 pixels per inch would create a printed image 6.25 inches wide.
Similarly, a high-definition television signal has a fixed image size -- 1920 × 1080 pixels in the case of 1080p. A small 20-inch television displaying the same signal as a 40-inch model is actually operating at a higher resolution (and thus may appear to have a sharper picture) because it packs the same pixels into a smaller space. On the other hand, the 40-inch HD display operates at a higher resolution than a 40-inch standard-definition device because it packs more pixels into roughly the same space.
Resolution impacts Web development in several ways. First, that six-megapixel image (which is on the low end of today's cameras) contains far more data than will typically fit on a computer screen, which might offer 1280 x 1024 pixels (on a standard LCD display). A website running inside a browser window within that screen is even smaller. To be useful on the Web, digital images usually need to be resized dramatically. It's often said that images for the Web should be saved at 72 pixels per inch (also called dots per inch or dpi), but this is only partially true. 72dpi can be a reasonable rule of thumb if the print size is kept constant (comparing a 10-inch image at 72dpi to a 10-inch image at 300dpi), but an image can print at 72dpi and still contain six million pixels -- it would just be 42 inches wide and look a bit fuzzy.
What people really mean when they make this recommendation is that an image printed at 72dpi would have a similar size on screen as it does on paper. That was a much more accurate guide when there was less diversity in computer monitors. But today, display sizes can vary wildly. A 13-inch laptop and a 30-inch desktop monitor will likely display the same photo at very different sizes. Preparing an image for use on the Web means understanding how it will be used and adjusting its actual pixel dimensions accordingly.
Because screen sizes can be so different, it's also challenging to make a website layout that works well on all displays. Designers typically approach the problem in two ways. One method is to create a design that expands to fill all available space in the browser window (or a fixed percentage of that space). But this can be a problem for graphic elements, which can't be scaled, and for running text, which is more difficult to read when the lines are very long or short. The more typical solution is to give the website a fixed width that anticipates the smallest screen size likely to be used. That varies depending on the website's audience, but widths of 800 or 1024 pixels are the norm. The drawback to this "lowest common denominator" approach is that the layout may appear fairly compact on a larger display -- or even a smaller laptop screen with high resolution. But just as doorways must be made wide enough to accommodate wheelchairs, a website needs to be accessible to people using a variety of technology.
Labels: design, photography, resolutions, web design