Day in-day out we use photographic lenses. I think we should have a better understanding about them. A lens is a set of optical elements placed together to give a predefined view of a scene. These optical elements are grouped together inside the barrel of the lens with a fixed minimum and maximum focal length. The scene through the lens can vary based on this focal length.
It is the distance between the sensor and the light nodal point which occurs inside the lens. For example, a 200mm lens on a full frame sensor gives an angle of view of just 12 degrees. Which means, we won’t be seeing more than just a small portion of the entire scene. Where as, an 8mm lens on a full frame camera gives an angle of view of 180 degrees – almost everything in front of it.
There are different types of lenses being manufactured. Primarily differentiated by the factor whether the lens’ focal length can be adjusted or not. If the lens’ focal length cannot be adjusted it is known as a Prime lens aka Block lens.
The lens comes with a couple of switches. One being the auto focus switch. When we turn on the auto focus switch, the communication sent from the lens to the camera is “the scene needs to be focused automatically”. Mostly the only input from the photographer would be the focus points. If this switch is turned off, it means the focus mode is in manual. In this mode, the camera or lens would not do any kind of focusing as it would be entirely done by the photographer manually.
Few lens comes with IS or VR switch. IS stands for image stabilisation on Canon cameras. VR stands for vibration reduction on Nikon cameras. When you turn on the switch, what happens is the camera looks out for any possible shakes while clicking the shutter and introduces counter shakes on the sensor and attempts to give a shake free image.
In general, the lens comes with 2 rings. One being the focus ring which helps in focusing the subject manually. And the other ring being the zoom ring which helps us in adjusting the composition – what to include in the frame and what to exclude out of the frame. The prime lenses does not come with a zoom ring as the focal length is fixed.