Possible Duplicate:
Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range?
It is rather common with zooms that the maximum aperture changes (in fact, decreases) with increasing focal lenght. At the same time, it is possible to make zooms with constant aperture (typically and not surprisingly, much more expensive).
What is the technical reason for the decreased aperture? After all it seems naively that the blades could be opened independently of what is happening to the focal length.
Is it to mantain the same level of image quality (distortion, aberrations, vignetting) at each focal length?
Or is there some "more fundamental" reason for this engineering decision, which can be overcome only in more complex/expensive designs?
Answer
The notation f/4.0 is usually taken to mean the size of the opening left by the aperture blades is equal to the focal length divided by 4, so for a 400mm f/4.0 you'd expect to find a 100mm opening. So given the maximum size of the opening is clearly fixed, if your lens has a variable focal length you would expect f ratio to change as you zoom.
This is not quite correct however, f/4.0 only means that the image of the opening as seen through the front of the lens appears to be 100mm - if you look at the barrel of such a lens you'll see there's no space for a 100mm iris. In fact the aperture is smaller but is "magnified" by the lens elements.
So via clever (and expensive) optics its possible to make the aperture appear to get larger as you zoom, giving you a constant f number.
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