We all use lenses every day. But most of us don’t really know much about them. In fact, most of us don’t even know what it is we don’t know. Here are a few facts that you may find interesting, useful, or amusing. They probably won’t make you a better photographer, but may help you avoid some bad shots or other frustrations.
Wide-Aperture Lenses May Have Focus Shift
A lot of you already know this one, but if you don’t, read on! Wide-aperture lenses, particularly prime lenses of f/1.4 or f/1.8 may focus very accurately wide open, but if you stop down slightly, they front−or back focus. It’s not something that can be fixed, it’s the way the lens was designed—the designers accepted the focus shift as a trade-off for more sharpness or less distortion. By f/4 or so, the depth of field of has increased enough that the focus shift isn’t noticeable. The end result then is the lens focuses fine wide open, and fine again at f/2.8 or f/4, but in-between doesn’t focus correctly.
Unfortunately you won’t find “has focus shift” in the manufacturer’s description. If you think your lens may have it, take a series of images focused on the same object, stopping the lens down a bit with each. If the lens has focus shift, you will notice the focus has ‘shifted’ in front of, or in back of, the object as you stop down. The experiment will also show you how far you have to stop down to overcome the focus shift (and this may be different at infinity or close up). The simplest work around is to shoot the lens either wide open or at the aperture at which focus shift is no longer apparent.