When your DSLR focuses, it typically does so by opening the aperture up as much as possible to allow all the available light to flow to the auto focus system. So if for example you are shooting with a f/1.8 lens at f/8, the camera will open up to f/1.8 to focus and then shift down to f/8 to take the picture.
Some lenses, especially lenses that can go very wide, i.e. a low f-stop number, suffer from focus shift, which means that the lens moves the focal plane as a consequence of changing the aperture.
This is nothing short of a catastrophe for an auto focus system, as the camera gets the focus sharp at f/1.8, but then when the image is taken at f/8 the subject is no longer in tac sharp focus.
There are a few ways to mitigate focus shift, as you can of course shoot at the maximum aperture – then the focus aperture and the shoot aperture is one and the same. You can also shoot in live view on your DSLR, as it the uses a different focus system than when not shooting in live view mode. The best option of course is to get a lens that does not suffer from focus shift!
