Digital photography, especially for those who spend plenty of money on cameras and lenses, is all about recording the sharpest and most color-correct images possible (except, of course, when blur or loss of focus, or even muted colors, is used as a creative tool). Both the camera and the lens contribute to that sharpness, but a lens’ f/stop position has an immediate effect, even before the light reflecting from the scene reaches the sensor. Every lens has one position, or a series of nearby positions, that is the perfect f/stop; and, as much of a shock as it may be, that expensive fast lens you bought (because it gives you an f/1.4 or f/1.8 stop) is not its sharpest at its widest aperture.
Now, where the “perfect” f/stop is located, like anything that claims perfection, is not universally accepted. On the Canon 50mm f/1.8, for example, some think it is f/2.2 while others promote f/4. Often, these findings are based on the photographers’ own testing with their lenses, and can even be slightly different because one photographer’s eye and mind recognize sharpness in his or her images shot at one f/stop, while another photographer at a nearby f/stop.
It almost impossible to imagine that two photographers would choose the perfect f/stop as widely spaced as f/2.8 and f/11, for example, so virtually all photographers would find the perfect position, for them, within a very narrow range. The general guideline that is accepted by many photographers is that the perfect f/stop is likely to be two stops more than the widest aperture of the lens. That means your f/1.8 lens probably produces the sharpest images at approximately f/2.2 or f/2.8.
Your easiest solution is to search the Internet for charts and other sources that will list your lenses’ perfect f/stop; however, you’ll learn more about your specific lenses and this concept if you conduct an experiment. First, attach your camera to a tripod, so hand holding your camera doesn’t induce any blur or loss of focus. Use the aperture priority mode on your camera, so it selects all other exposure parameters, while you still control the aperture, or f/stop, of your lens. Then, capture a number of images at various aperture settings of your lens, from the widest to the narrowest. Next, upload all the images to your computer and view them at 100%. After a bit of study, you should be able to recognize the sharpest images, and maybe the very sharpest, and, therefore, discover your lens’ perfect f/stop.
The cold, hard reality of this test is that the money you spent for that very fast lens is actually giving you a bit less than you thought. For example, for the most popular 50mm, fixed-focal-length lenses, an f/1.8’s widest aperture that produces superior sharpness is actually f/2.2; an f/1.4 is f/1.8; and an f/1.2 is f/1.6. The difference is quite minimal, however, at just 0.4 of a stop in all three examples. It’s very unlikely that you’ll ever need that extra bit of aperture width, so it won’t be missed and you’ll still be able to create many interesting wide-aperture photos.