Lens Coverage and Speed

[SagnikB]

I have noticed that the rear element of my 18-55 lens is a lot smaller than the sensor. Still how does the image circle cover my sensor? (The light [I]converges[I], right?)
Also, the rear element of my 50mm f/1.8 lens is really large. Does it have anything to do with its aperture?

[GrahamS]

I have noticed that the rear element of my 18-55 lens is a lot smaller than the sensor. Still how does the image circle cover my sensor? (The light [I]converges[I], right?)

1) Wrong. The light path from the rear of your 18-55 is actually divergent from the lens axis outward to the edge of the image circle.

Also, the rear element of my 50mm f/1.8 lens is really large. Does it have anything to do with its aperture?
2) Not necessarily. It depends entirely on the design of the lens.

[Manfred M]

Just to add to Graham's comments.

Light from a lens converge to the focal point and then diverges. Take one of your lenses and a piece of white paper. Point the lens at a well illuminated subject and put the paper at more or less the distance from the flange to the focal plane of the sensor. You will see a round, upside down image that shows the image circle.

2. The maximum f-stop of the lens is ratio of the focal length to the input pupil of the lens. The exit element is not directly part of this, but as a general rule of thumb, the rear element will tend to have a larger diameter, the longer the focal length of a lens, but as Graham has already pointed out, lens design is the real driver here.