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05-29-2010, 07:51 PM   #1
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Fixed-aperture lenses

I have a technical question about fixed aperture lenses. Let's take the F* 250-600mm as an example. While F5.6 at 600mm is quite respectable, it's not too hot over at the shorter 250mm end. Why, since this is the case, was the lens not made to go from, say, F2.8 to F5.6? Was this by design, simply to have F5.6 throughout the range, or is there some sort of optical/physical barrier in these kinds of lenses?


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05-29-2010, 07:59 PM   #2
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Disclaimer: I have no knowledge of optical design and am making this up.

I believe there are essentially 2 different categories of zoom lens design. One is where adjusting the focal length adjusts the effective aperture. You can note this as you zoom, it continuously gets darker or lighter.

A different optical design is where adjusting the focal length does not change the effective aperture. It generally appears that this design is usually of higher optical quality, but perhaps that is just the norm as pro's and people with flashes much prefer constant aperture as well as IQ.

In order to change a lens like the F* 250-600mm into a variable aperture, they would have to a complete redesign of the element and mechanical configuration.

It's like an internal focusing lens and an external focusing lens, optical design made it such that the front element was required to move in adjusting the focus. In getting rid of that, however, many IF lenses drop in focal length as they focus closer.
05-29-2010, 08:15 PM   #3
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That would make sense, and is actually what I expected to hear. I don't think you're making anything up! Thanks for the insight

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05-29-2010, 08:50 PM   #4
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I've seen this discussed on some other photography forums and as I recall, it is because the position of the aperture is moving inside the lens when you zoom, allowing for the constant aperture.

Even if they did make it a variable aperture zoom, it would probably be an f/4 or f/4.5 at the wide end, similar to the Sigma super zooms or the Canon 100-400. It would be too hard to fit a 250/2.8 inside a 600/5.6 tube without getting vignetting.

05-29-2010, 10:26 PM   #5
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I have no experience with the 250-600mm but:

On my FA 4/28-70 AL I've noticed that when wide open and zooming out the aperture blades actually close to maintain the constant F4. I don't know how common this kind of mechanism is. However it does sort of explain how the IQ "wide open" (actually stopped down a bit) is very good in the short end up to 50mm and after that it gets worse as the aperture really opens up.

And it looks like if I could disable this zoom-aperture linkage the actual aperture at 28mm would be close to 2.8. With degraded IQ I bet.
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