Say you have a lens that is F3.5-5.6 and one that is f2.8. The lower numbers being full open. Now if they are both full open why are the numbers different? The apeture being the hole size inside the lens, are some made partionally closed or is it the diameter of the glass that determines the aperature rating? Seems to me that if they are both full open then the numbers should be the same.

The f-number is the ratio of the focal length of the lens to the apparent diameter, not the physical diameter, of the hole you see when looking through the front of the lens (called the entrance pupil). Different lenses have different sizes of entrance pupils. For example a 50 mm f/3.5 lens has an entrance pupil 50/3.5 = 14 mm in diameter. An f/2.8 50 mm lens has an entrance pupil 50/2.8 = 18 mm in diameter. The rest of the lens has to be sized in a corresponding way, so not only is the entrance pupil larger for lower f-numbers, but so are all the parts of the lens. The largest possible aperture for a photographic lens is f/0.5, but there are no normal camera lenses that reach that limit. F/0.7 is rare, but lenses that fast do exist. Lenses with a maximum aperture of f/0.9. f/0.95 and f/1 are commercially available. Best, Helen

The entrance pupil diameter can change as you zoom. As I mentioned, it is not the physical diameter of the iris, but the diameter of what you see, ie the image of the physical iris formed by the lens elements in front of the iris. (With some lenses the physical diameter of the iris does change as you zoom - you can see the iris blades move.) The reason that the entrance pupil diameter can change even though the iris itself does not change is that the elements in front of the iris move, and change the magnification of the image of the iris. Look into the front of your zoom, and try to measure the diameter of the hole you see at different focal lengths. Hold the lens as far away from your eye as you can. By and large a telephoto lens (a true telephoto, not just a long focus lens) will magnify the iris more than a normal lens, and a retrofocus lens will make it look smaller, when looking into the front of the lens. It is the other way round when looking into the back of the lens (when you see the exit pupil). The ratio between the exit pupil diameter and the entrance pupil diameter is known as the pupil magnification. Does that help? Best, Helen