Sometimes, we may have heard of notions like normal lens and standard lens. What is the actual meaning of these notions? Normal lens is actually a lens that has a focal length (focus distance) all along the diagonal size of image sensor camera. For a camera with full frame sensor (36 x 24mm), the normal sensor is 43mm. on the other hand, on a camera with APS-C sensor size (23x35mm), the normal lens is around 28mm. and for four thirds sensor, the normal lens is about 21.5mm.
Because of the rarity of lenses with the same and exact focal length of the lens, usually the lens that is closer to the normal focal length is considered as the normal lens. For example 50 mm lens when it is attached to full frame camera.
It is important to understand that the normal focal length is relative depending on how big the image sensor of the camera. For instance, 50 mm lens is attached to a camera with smaller or bigger sensor, that lens will become no longer normal. When you use normal lens, the perspective on the photo result will resemble the perspective of human’s eyes. This will appear more natural.
There will be no convex distortion which is usually caused by wide lens and there will be no pincushion distortion that is caused by Telephoto lens. When you take a picture of human face, the result will look more natural similar to the real person. The perspective of normal lens is not only found in the fix lens but also in the zoom lens.
For instance, we got a zoom lens with a zoom range of 18-135mm; the way is to rotate the zoom lens until pointing to the normal focal length. For example, when you use a camera with APS-c sensor, rotate to 28 mm or when we use camera with full frame sensor, rotate the zoom lens on number approximately 43 mm.