I still have difficulty using manual focus, thanks to my glasses and less than perfect eyesight. For anyone who doesn’t have perfect vision, it can be difficult to see if the subject is perfectly in focus, especially when using a large aperture with a narrow depth of field.
However, there are some instances where you have to use manual focus. With my D40X, any non AF-S lens, which lacks the internal AF motor, can only do manual focus (although the focus indicator will light up when it thinks the subject is in focus). I have a love/hate relationship with my 50mm/f1.8 lens for that reason – it’s the sharpest lens I own with the largest aperture that can produce a wonderfully narrow depth of field, yet I still find the manual focus difficult to use.
For certain subjects, like this spider web I photographed this morning, auto focus won’t work, since it will try to focus on a higher contrast object behind the spider web rather than the web itself. I took about 10 shots to get 3 that were perfectly in focus.
For this one I cheated a bit by spraying the web with a fine mist of water to make it stand out more.