John,
If you consider that
« In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture"). It is also known as the focal ratio, f-ratio, or f-stop, and is very important in photography »
(source: Wikipedia entry on f-number)
You will understand why many *zoom* lenses have variable *max* apertures.
The advantage of having a constant *max* aperture (as Peter already mentioned) is that the camera uses for autofocussing automatically the largest aperture that the lens allows, in order to let in most light while also having a reduced depth of field, ensuring that the focussing distance can be measured precisely. Therefore having a constant max aperture allows you to have at any focal length the same quality of autofocusing, even in less ideal light conditions.
The indication of « constant f2.8 » only applying to the *max* aperture, it does not mean that the lens will not offer you other f-stops. Which brings us to the blades which open/close for the choosen aperture.
As regards the (number of) blades and their shapes: imagine having a lens with 3 blades that are straight - your opening would have the shape of a triangle. Now go on with 4 and more blades and the opening will become closer to look like a circle, which would be the ideal shape, as it has no corners where light would be broken differently. However the more blades you have, the more complex (and expensive) your lens gets, so there is a compromise in the number of blades, also due to what is mechanically feasable.
The second aspect is the shape of the blades which will help to create here smoother pictures for the bokeh, due to the rounded shape.
-
TBonz
and
Johnb
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