There was a time when there was no such thing as autofocus. There was only manual focus. Blasphemy!
What did photographers do? Trust their own eyes!
With that being said, there are now photographers who only choose to use manual focus and are quick and accurate.
But the question is, why use manual focus when autofocus is such a dream?

First of all let’s look at what is happening:
A lens is an optical device that bends the light so all the rays are directed into a single point. This allows the camera to create an image and is the reason why lens design is so important.

When designing a lens, the main consideration will be the refraction index, this determines the angle of incidence, the higher the angle of incidence, the more light bends to focus into one point. Think of wide-angle lens, there needs to be a lot of bending to capture the image, this is why they tend to be so bulky.

When we focus a lens manually, we look through a viewfinder or at an LCD screen and verify, with our eyes, if the subject looks sharp.

If not, we can use Autofocus. In Canon, One Shot or AI Servo. In Nikon, AF-S or AF-C . When using our camera we use our aperture and focus point to get the area we want in or out of focus. One shot or AF-S allows us to focus lock on one subject and re-adjust our cameras by half pressing our shutter release and moving composition and pressing fully down.

Remember, technology is here to help us, but sometimes, even the latest technology is not good enough and we need to go back to basics. For example in situations with low light and/or low contrast, you might find yourself trying to focus, you wait, keep half pressing the button and nothing, the camera lens refuses to focus.

This is a perfect situation to use your very own sophisticated sense of sight. Just make sure you have your glasses!