Difference between F number on lens and camera

The f/stop is a measure of aperture--the size of the lens opening and therefore the amount of light it can admit--relative to its focal length.. Lenses are usually defined by maximum aperture and focal length. Your lens probably is a zoom which has a maximum aperture of f/3.5 at wide angle and f/5 at telephoto. Its diaphragm probably stops down in both cases to f/16 which is a small aperture, and that is controlled automatically by your camera. So there is in practice no difference between the two numbers.

It's hard to address any issues without knowing what camera and lens you are talking about, but usually the lens and the camera sync that information pretty closely since the f number is determined by the ratio of aperture diameter and lens focal length (ex: 50mm lens and ~28mm diameter = f/1.8).
It may be that your camera isn't reading the aperture correctly.

Fraser,
What type of camera are you using? If I understand your question correctly... lens has F of 3.5 - 5 and on camera you can select f/2 - f/22... that seems strange to me at least..
The camera appears to have a f-stop larger then the lens(smaller # larger opening)..
Interesting...

"My lens has a f rating of 3.5 - 5 and i can select an f rating on my camera of f2 - f22 what is the difference between these two numbers?"

The rating (f-stop specification) printed on the lens is some what like a car's gas mileage and horse power. How much you get depends on how you press the gas paddle. The camera's f-stop adjustment is like a gas paddle. It is use to control the f-stop on the lens you put on.

In addition, looks like you have a zoom lens that has the f-stop ranging from f3.5 to f5.0. F3.5 is the best this lens can do at its shortest lens focal length setting and f5.0 at its longest. Kinda like freeway mileage and city mileage. Hope these help.

Agree with Jake that you need to tell what are the camera and lens specifically. There are very strange numbers in your question but usually a very strange question will have a very simple answer

My lens has a f rating of 3.5 - 5 and i can select an f rating on my camera of f2 - f22 what is the difference between these two numbers?

I betcha you cannot select that f/2.0 with that f/3.5 - 5.6 particular lens attached to the camera!...but you would be able to do that if you had a lens capable of going up (or down!) to your f/2.00

The two numbers on the lens implies that it is a zoom lens. The f/numbers on the lens are the widest settings available when the camera is zoomed from its widest to its narrowest fields of view. For example, when it's in wide angle, that lens will be able to be at f/3.5. When it's zoomed to telephoto, it'll be at f/5.

This is because the aperture is mechanically independent of the lens' zooming ability; but, mathematically, the number is computed as a ratio of the size of the aperture against focal length;[definition of f/stop] so, with the hole remaining the same size, but the focal length changing with the zooming action, a range of numbers is yielded as the observed f/stop. In the case of that zoom lens, it'll be a number no wider than f/3.5 to f/5, depending on where the zoom is set.

This is a common design among zoom lenses. Some lenses which will stay at a given aperture, even though they are zoomed from one field of view to another; so that the aperture moves relative to zoom: those lenses are called "constant aperture zoom lenses." Their designs usually cost a little more. [Useful for sports, for example.]

The aperture numbers on the camera are probably electromechanical selectors for choosing aperture through computerized switching. [Auto functions, "digital auto aperture"]. The range of potential apertures that camera body can yield would be f/2 to f/22. What this means is that the camera can, with its own computer and switches, move the aperture's width from a f/2 to f/22 position. This is sometimes done by moving a lever;an eectromechanical relay would do this on a contemporary digital camera, in response to a computer command; it'll translate lateral motion into the action of opening or closing the blades of the aperture a desired amount. Thus, f/2 to f/22 on the camera.

Put the two together, and your equipment will be able to yield the narrowest subset of both groups. Provided that the zoom lens can go to f/22 (probably does; common setting), then the camera and lens together will be able to yield apertures as narrow as f/22 on out to f/5 at telephoto. Or, at the widest field of view, from f/22 to f/3.5

It doesn't matter what camera brand or model of equipment is involved; so long as these two items can mechanically and electro-mechanically work together. That's what I would expect.

The numbers on the camera show how it's going to set the aperture on the lens. The numbers on the lens show the biggest aperture it can be set to.

What John said !

Here's another analogy.

Let's say you have a fast sports car that can go from 0 to 200mph. ( The camera ). As long as the tires you put on it can ALSO go 200mph, you will be able to get the whole range of speed. On the other hand, if you put less expensive tires on it, say ones rated only to 120mph, you car will be limited to that lower speed of 120mph. So, with certain tires ( lenses ) your car will not be able to use all it's potential.

It's the same with a camera body and it's lenses. Some lenses, can only open so wide. Their max opening is the lowest number on their f-stop range. My fastest lens is f1.8 some of my lenses can only go to f3.5. If I put the f3.5 max opening lens on my camera, I can't use the camera's f-stop settings below that. If I put the f1.8 lens back on, then I can use f1.8-f3.5 and all the f-stops that are higher. Of course some lenses can't go as high a number as the camera. Some lenses stop at f-16. Some at f-22. I think I've seen some go to f-32.

The point is that what ever lens you slap on your camera, it limits the range of f-stop settings you can tell the camera to use.

Now, the lens you mention is a zoom lens, that as John so completely explained, has a varied max aperture. At wide angles, it's f3.5, but as you zoom out, it's max is reduced to f5.0. That's a common thing with "consumer" lenses. Making a lens that retains it's max aperture through the whole zoom range costs more. Sometimes a LOT more.

( p.s. So, it's not a silly question , as a number of posters were confused by it. They needed to get this info as well. )