The image of an object formed by a lens is of magnification -1 if the distance between the object and its image is 60 cm what is the focal length of the lens?
Answers
Answer:
A camera lens behaves essentially like a simple convex lens:
In such a lens focal length is the distance of the center of the lens and the focal plane when the lens is focused at infinity. Focal point is the center of the focal plane.
Now actual lenses are more complex for various reasons: one needs to correct the fact that different colors bend differently. The physical dimensions of the camera like the mirror in a DSLR put limits so things are not so clear. The focal length is not anymore any physical measure on the lens. It describes more how the lens behaves.
This is an example of retrofocus lens (a typical wide angle lens)
If you look at the blue rays. They first spread then they combine. The focal length of is the distance between the sensor and the point where the rays are as wide as at the entry. This point can be well inside the actual camera body. Now look at the red rays. They become at a completely different angle as if the lens was a simple lens but the result still is the same. The camera does not register the angle the light comes from.
Focal length determines the angle f the view the lens has though this has to be viewed with the sensor size. Smaller sensors use shorter lenses for same eagle of view. If the focal length is same as the width of the sensor (36 mm on full frame, 22–24 mm on crop) then you get as much on the image as the distance is. This kind of a focal length is god for general photography