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Question: How do fixed f-stop zoom lenses work!?
How does the maximum aperture stay constant on a fixed zoom lens!? e!.g!. if a lens is f/2!.8 at it's maximum focal length!.!. how does it stay at f/2!.8 at it's minimum focal length!? Does this mean that here is some arbitrary limitation of the widest aperture at the minimum focal length!? The way I see it is that if the lens can open up to f/2!.8 at it's longest focal length!.!. then all it would need to do is keep open the same amount at the shortest focal length -- giving a much larger f-stop!.Www@QuestionHome@Com
Best Answer - Chosen by Asker:
Its simple mechanics, a 200mm lens needs to have its object lens 200mm away from the sensor (roughly) so the lens barrel has to be this length, this sets the angle of view of the lens to that required by the 200mm part of the specification!. The 70mm has a wider field of view and will require the lens barrel length to be shorter (70mm or so) or it would vignette it!. What lens manufacturers do is they distort the image so it projects down the longer tube to the sensor!. Doing this reduces the aperture, lens manufacturers do their maths so it will be the same all the way through the range!.
Why not make the lens barrel length adjustable as in cheaper lenses!? Well with this size of lens you would need a focusing motor powerful enough to move this weight around with the battery draining all the time!. On top of this at the 200mm end with a large aperture the depth of field is small so you would have to build a focusing mechanism that can move all that metalwork and heavy lenses and position them with an accuracy of about 1/100th of a mm! Not really on for a production model and focusing would be slow, and the lens would be very large and heavy!.
Instead the manufacturers go for rear focusing, this has so many advantages, first you are only having to move a few relatively small pieces of glass short distances (albeit with a very high order of accuracy)!. Result very fast focusing, the lens barrel does not vary (with all its attendant weight) and the front element doesn't have to move at all for focusing!.
As for the ad men explanation, believe me if they could produce a 70mm f1 to 200mm f2!.8 lens that wouldn't need a wheelbarrow to transport it they would!
ChrisWww@QuestionHome@Com
I don't know how things work and your idea is interesting!. If a 70-200mm zoom can give us f/2!.8 at 200mm, this makes me think the aperture diameter is 71mm!. If it can achieve 71mm diameter, why can't it give us (nearly) f/1!.0 at 70mm!? The fine print, though, refers to "effective" aperture and this is all created by the way the interior elements slide up and down inside the lens housing!. Simply looking, I would say that the physical measurement of the aperture at the widest angle is close to the definition of a diameter in ratio to the f-stop, but as you zoom out, something else comes into play that I just don't understand!. You're _thinking_, though!Www@QuestionHome@Com
It isn't a fixed f/stop zoom actually, they just have the same f/stop at both ends of the focal range of the lens!. They can still stop down to f/16 or f/22 if necessary!.
While the engineers, could have allowed the aperture to open more as the lens becomes shorter, keeping it uniform seems to have become popular!. For sure I would love my 70-200 mm f/2!.8 to be a f/2 when at 70mm, but the marketing folks seem to think we don't want or need that, so the engineers build what will sell the bestWww@QuestionHome@Com
I doubt there are any lens engineers on this site that can tell you in a few words the immensly complex design of a modern carmea lens of ANY type!. I suggest you do some searching and study on your own if you really want to know about this!. Just accept that YOU cannot build such an item and if you want it, you have to pay the price and be happy that there are others that CAN design and build it!.
steveWww@QuestionHome@Com
The way you see it is bollocks!. Do read up on the laws of optics!.
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