I’ve recently been involved with an iPhone project where we are doing a few custom UI controls, and it’s definitely proved a learning experience about the difference between designing for a computer screen and designing for the iPhone screen (either the current one or the upcoming iPhone 4 Retina Display screen).
One thing I’ve learned has to do with the characteristics of the iPhone screen, and how that influences User Interface Design choices. Over the years, I’ve become used to the what it takes to show a change on a computer monitor, which is to say, the degree to much you need to change the colour, shape, or scale so that it’s obvious, even if the user looks away for a second before the change occurs and then looks back. This might apply to an object in its selected and unselected states, or the addition of something new on the screen, or perhaps the enabling or disabling of a button or other element. At first, I thought this was due to the dots (or in this case, pixels) per inch of the iPhone versus computer monitors. Monitors are usually somewhere between 72 PPI (Pixels Per Inch) and perhaps 200 PPI on the best equipment. The IBM T220/T221 LCD monitors marketed from 2001–2005 were 204 PPI, and they probably set the standard for a while. These days, a 20-inch (50.8 cm) screen with a 1680×1050 resolution has a 99.06 PPI, and a garden variety Macbook (not the higher end Macbook Pros) has 113 PPI (Wikipedia has an article on how this is calculated).
However, the iPhone PPI is listed at 163 PPI, which although it’s on the high side, is certainly not significantly higher than a typical computer these days. The difference, then, must be the size of the screen. In the case of any iPhone screen 2G, 3G, 3Gs and 4G, it’s a 3.5 inch screen (compare that to the aforementioned 20-inch, and now we’re talking different.)
It might be obvious, but what I’ve noticed is that the amount of change you have to make in order to be noticeable is far more on the iPhone’s screen. The contrast must be greater, scaling or moving an object between one state and another has to be larger (or farther), and as a corollary to this rule of thumb, it’s easy to miss subtle changes. Several times during development of the app we’re working on, I had to report to the graphic designer that I was working with, that a selection style wasn’t distinct enough, or that a small detail of a button, such as a downward pointing arrow, had to be rendered with higher contrast (the UI had a lot of grey objects, and some of them had white or darker grey overlays).
I think the easy way to think about this is the analogy of greasepaint. What’s greasepaint? It’s the traditional makeup that actors wore (and has now been superseded by more modern stage makeup) that helps to compensate for both the washing out of facial features by the bright theatre lights, as well as help audience members to make out their faces, even though the actors were farther away (and hence, smaller in the eyes of theatregoers – perhaps the equivalent of being 4 or 5 centimeters tall depending on how far away from the stage they were sitting). I remember going backstage to a dressing room after the Play or Opera was over, and was always struck by how odd the performers looked before removing all of that extreme makeup, which brought out cheekbones or encircled their eyes (like a Raccoon, I thought!).
So User Interface Designers working on iPhone apps, remember, the computer screen is the dressing room, and the iPhone screen is the stage. Don’t forget the greasepaint!