Did you know that you can work out the effective clear diameter of a lens based on its focal length and its aperture according to the formula below.
Focal Length / Aperture = Clear Diameter
For example – if you have a 20 mm focal length lens and you need an aperture of f/2 the diameter of the front element must be at least 10 mm. If the aperture was f/1 then the element would need to be 20mm.
But remember in your calculations to add approximately 5mm for a metal lens mount!
This calculation can be useful when you are specifying a lens that must fit in a known space envelope. For example if you have a 50 X 50 mm space into which the lens must fit, there is little point specifying that you require a focal length of 100mm and an aperture of f/1.4 as the lens just will not fit.
Another rule of thumb that applies more to zoom lenses is that in general the length of a zoom lens will be about the same as its longest focal length. If you have a 10 to 100 (10X) zoom lens the length of the lens will be approximately 100 mm.
The length of a zoom can be squeezed down by design but you increase the complexity of the design and impact performance depending on how much you want the length reduced.