Optics is simple. It is merely the glass that light travels through to take a photo. Be it a professional lens for a Digital SLR or just a fixed lens on a compact camera or smart phone.
There are all types of lenses. They range from:
Extreme Wide Angle
17 - 40mm
70 - 200mm
All lenses have a focal distance. This is the distance that the lens can focus up to. The smaller the focal number on the lens (0.4m∞) the shorter the focal distance. The larger the focal number (58m∞) the longer the focal distance.
How do we measure focal distance? Focal distance is measured from the subject or point that you are focused on to the focal plane indicator on your camera.
Many people think that this distance is measured from your subject to the end of the lens; and this is simply not true. On your lens there will be a focal number (say 0.4m∞) this would be for a macro lens. This is telling us that you can focus on your subject as close as 0.4 meters (40cm) away before your subject becomes blurry and the camera can no longer focus. This measurement is from the focal plane indicator to the focal plane on your subject.
This symbol is in perfect alignment to the focal plane of the camera. On an SLR it would be inline with the film. On a Digital SLR it is inline with the front of your sensor.