Constant Aperture vs Variable Aperture Zoom (continued)

Started Mar 18, 2013 | Discussions thread
quadrox Senior Member • Posts: 1,112
My Explanation

I have only read some of the posts in the previous thread.

It is my understanding that a constant aperture design is completely different from a variable aperture design, which is visible by the constant aperture lenses not changing length. I believe that this difference in design also makes the lens shorter, but I am not sure.

As to improved IQ due to stopping down in a variable aperture lens, you are completely mistaken I believe. Improved sharpness from stopping down usually comes from using a smaller part of the glass, but this never happens when stopping down due to changing focal length, in that case you are using the lens wide open at all times. The reason you are changing aperture is only because you are using a longer focal length, not using less of the glass.

The above means that it may be possible to construct a fixed aperture lens that has a special compromise between wide and long end that improves overall image quality. Since a zoom lens design is always a balance between the wide and long end, imagine a zoom lens with much better image quality at the long end of the zoom (at f/4), sacrificing image quality at the wide end (at the theoretical f/2.8). But because the lens is fixed aperture, you are stopping down the wide end to f/4 which may negate the previous compromise - and suddenly you have a lens that performs better at both the wide and long end, at the price of not being able to shoot wide open (f/2.8) at the wide end.

Of course you could argue they should still give you the choice of shooting f/2.8 at the wide end, but I believe the overall lens design in such a lens actually makes this impossible, or they may think it would give them negative publicity due to the worse IQ.

 quadrox's gear list:quadrox's gear list
Canon EOS 5D Mark II Canon EOS 7D Canon EF 85mm f/1.8 USM Canon EF-S 17-55mm f/2.8 IS USM Canon EF 55-200mm f/4.5-5.6 II USM +2 more
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