Zooming the lens after you focus
Most of the zoom lenses made today aren’t in fact true zooms, or what are known as ‘parfocal’ lenses; rather, they’re ‘varifocal’ lenses. One of the drawbacks of this type of design is that the focus shifts as the lens is zoomed. This means that if you zoom in to lock the focus on a detail within a scene and then zoom back out to take a shot, there’s a good chance that the detail you want to appear sharp will now be blurred. If the zoom range isn’t too great, the change in focus may be subtle. Using a small aperture to give a large depth of field – the amount of front-to-back sharpness in a picture – can also mask any focus shift. But the easiest way to prevent this is to get into the habit of only focusing after you’ve zoomed. Once it becomes part of your shooting regime you won’t even have to think about it.