With manual focus, you twist the focus ring on your lens until the image you want pops into sharp focus. Many dSLRs have a cool feature: Even if you’re focusing manually, a focus indicator LED may light up when you’ve achieved correct focus, giving you some helpful extra confirmation. Some points to consider about manual focus are these:

• Speed. Manual focus takes more time, compared to the speedy operation of autofocus systems. If you’re shooting contemplative works of art, portraits in which your subjects will generally stay put for a period of time, or close-up pictures, the speed of manual focus may be no consideration at all. For action photography, however, you may not be able to change focus quickly enough to keep up.
• Non-Memory Effect The eye (the brain actually) doesn’t remember focus very well. That’s why you must jiggle the focus ring back and forth a few times using smaller and smaller movements until you’re certain the image is sharply focused. You never really know if optimal focus is achieved until the lens starts to de-focus. So, manual focus may actually be a trial-and-error experience.
• Difficulty. Focusing is most easily done when the image is bright and clear, the depth-offield of the image being viewed is shallow, and there is sufficient contrast in the image to make out details that can be brought in and out of focus. Unfortunately, you’ll frequently encounter scenes that are dim and murky, with little contrast, and be using a slow lens that compounds the problem.
• Accuracy. If you’re using selective focus creatively, manual focus is the only way to go.
• Following Action. Today’s autofocus systems are sophisticated enough that they can use predictive focus to track moving subjects and keep them in focus as they traverse the frame.