Hyperfocal is the minimal focusing distance that allows you to have everything beyond that point in focus. It is really useful for landscape and street photography. There used to be focusing scales on lenses and some still have them. If you don't have any indication on your lens, there is a way to calculate it. Don't worry, there are apps for that too.
The Hyperfocal Formula is:
The hyperfocal distance is approximately equal to the square of the focal lenght (F) divised by the product of the Aperture (N) and the circle of confusion (c).
The circle of confusion is a hard notion to explain but let's say it is the size of a circle on the sensor that the eye can't differentiate from a point. It varies depending on the size of the sensor. Here are some values for the most used sensors:
- Full Frame = 0.029
- APS-C = 0.018
- Micro 4/3 = 0.015
- Most compact cameras = 0.005
That shows that the bigger the sensor, the smaller the depth of field will be, all other factors being equal.
The minimum acceptable focus distance is about half the hyperfocal distance. If the hyperfocal distance is 2 meters, you will be in focus from 1 meter to infinity (and beyond :P).
Let's look at an example. You want to photograph a person at sunset and you want both in focus. You are using a 12mm on a micro 4/3 camera and you chose to work at f/8 to get good sharpness and be sure not to lose details because of diffraction. The hyperfocal distance will be 12x12/(8x0.015) = 1200mm. So if you focus at 1.2m (~4 feet), everything should be in acceptable focus from 0.6m (~2 feet) to infinity. So as long as your model is further than 2 feet from you, he/she will be in focus and the background too.
You can also use this formula to find out the focus area depending where you focus.
s = Distance to the subject where you focus
H = Hyperfocal Distance
f = Focal lenght of your lens
So if your subject is at 1.5m away from you, the minimal distance that will be in focus will be: 1500mm x ( 1200 - 12 ) / ( 1200 + 1500 - 2 x 12 ) = 669 mm. Your focus will then be from 669mm to infinity.
You can't really start calculating this every time but fortunately, there are apps for that. If you don't have any kind of smart phone or tablet, you can print charts like these.