Photographers: What are the differences between lenses?

[Nocturnal]

Like say a 50mm f/1.8 II lens, then a 28-135mm, then the 70-200mm L USM/IS lens... then some lenses are just 135mm instead of 28-135... what are the differences? And can anyone explain to me what a "stop" is in photography terminology?

 

[ZeroEffect]

50mm lens is considered a normal lens - almost equivalent to what your eye sees (in 35mm photography)

the other two lenses that you mentioned are zoom lenses. They allow you to have focal lengths from a
moderate wide angle to telephoto all in one lens.

The ratio of the focal length of a lens or lens system to the effective diameter of its aperture. Also called f-stop

a "stop" is really an f-stop. On manual cameras there is a ring that you turn that sets your f-stop. The
ring turns an aperture which will adjust it from a small opening to a large opening. Small f-stops
ie: f/22 let in a pinhole of light, and large f-stops like f/1.8 let in a large amount of light. F-stops also control
depth of field. Most of this you don't need to know since most cameras are point and shoot and set
all of these setting automatically.

 

[Hayabusa Rider]

A lens regulates the amount of light passing through it by means of a diaphragm that is much like the iris of your eye. This is adjustable. Whenever you change the opening size you have opened or closed it a "stop". That is the most basic explanation.