Where is infinity on the barrel of your lens focus ring? If you look at the little window on top of your lens you will see this ∞¬†symbol. It means that if you roll the ring until focus line, lines up there your lens is set to infinity focus. Which is the furthest point in which your lens is capable of focusing. But if you do you this you will notice that the ring does not come to a hard stop at the end or middle of the symbol! Why is that? With autofocus being a standard in every modern lens manufactures decided that they no longer need a hard stop for infinity focus and that a soft stop was not only better for the life of the lens but made it cheaper to manufacture. It is cheaper because they don’t need to be so precise and easier on the lens because there is less wear and tear on the gears of the lens. When your autofocus system is constantly hitting the end of your focus a soft stop would soften the impact on the gears. They also discovered that focus changes as your lens moves and flexes with atmospheric changes, making it impossible to dictate exactly where infinity is. Instead autofocus relies on the sensor to look for things like contrast to determine whether or not an image is in focus.

How does this impact manual focus? This means that if you are in low light and your camera is no longer sees enough contrast to focus you must rely on your eyes and if you think just turning the ring to the ∞ symbol will do it, you could be wayyyyyyyy off. So don’t do it. At the end of the day you must still find focus. And btw, closing down to f/22 may help but you may be far off enough that it won’t. So always find focus!