Human eyes see the world around us at about 40 degree angle of view i.e. without using our peripheral vision. Lenses that could reproduce a field of view that corresponds approximately to that of normal vision with no perspective distortion are called normal lenses.
We have already seen that sensor sizes affect the angle of view and therefore there is no one single focal length that could be termed as normal. Instead normal focal length for each sensor size is different.
Normal Lens and Sensor Size
To find out what focal length should be considered normal for a camera we need to consider the sensor size of the camera. Focal length which is equal to the diagonal length of the sensor will be the normal focal length for that camera.
In case of full frame cameras:
|Normal Lens and Sensor Size|
Sensor size = width 36mm, height 24mm and a diagonal measurement of 43.3mm
So any lens that produces an angle of view of approximately 43 degrees could be considered as normal lens for the full frame format. That is why 50mm lenses which is very close to 43.3mm is considered as normal lenses.
In case of cameras with APS size sensors
Sensor size = width 22.7mm, height 15.1mm and a diagonal measurement of 27.3mm
Lenses close to the angle of view of 27.3 could be considered as normal. That is why 35mm lens which is close to the 27.3mm is considered as normal lenses for cameras with APS size sensors.
In case of cameras using Micro four Thirds size sensor
Sensor size = width 18mm, height 13.5mm and a diagonal measurement of 22.5mm
So lenses close to 22.5mm could be considered normal lens for micro four thirds format. Either 20mm or 24mm which are two common focal lengths could be considered.
Now that we know what normal lenses are it is easy to figure out other types of lenses. Lenses which have an angle of view wider than normal lenses are called wide angle lenses and lenses which have an angle of view narrower than normal lenses are called tele photo lenses.
We will discuss about wide angle lenses in the next article.
NEXT: Wide Angle Lenses