Ever wondered how a zoom works, optically?
I did, because the only optical formula I learned in school was that combined focal length of multiple lenses is the inverse of the sum of the inverse focal lengths of each single lens, i.e.: Add a 500mm close-up filter to a 200mm lens gives a 1/(1/500 + 1/200) = 143mm.
Sooooo, in any zoom with 13 to 23 optical elements the focal length should be fixed and can be calculated by the above formula. And this in turn proves that zooms cannot exist
Well yes, you didn't expect anything less crazy from me, did you?
Now, the other thing to keep in mind is that many optical formula are only approximations of the real behavior of real glass in real life. E.g. some formula assume that you focus at infinity
Well, what happens if you focus closer? You have to extend the lens and thus the geometry of projecting the image on the sensor includes an extended distance of the front-lens from the sensor, which is as good as having a longer effective focal length! Things get larger (whether they are in focus or out-of-focus) if you focus closer. The only problem is to get those subjects sharp that you want in focus.
So in a very simplified explanation a zoom consists of two lens groups:
(A) The front lens-group projecting an image at a fixed focal length (and thus fixed magnification) into the zoom-lens. Let's call this the "primary" image. It's a miniature image of the reality in front of the lens.
(B) A rear lens-group that works like a macro on the primary image and produces a secondary image of varying magnifications by focusing closer or less close to the primary image. And as you can move the primary image (almost) as close or far away from the second lens-group by moving the front lens-group towards or away from the sensor it's easy to see how the second lens-group can focus at varying distances (and thus magnifications) on the primary image.
Well, that's in a nutshell how zooms work in principle! Fascinating, isn't it?
And I hope that you are not disappointed when I tell you that modern lens designs don't really project the primary image in front
of the second lens-group but effectively inside
it. So the second lens group is not like a miniature macro lens as you know it but still can magnify the primary image more or less depending on the distance to the front lens-group. The principle idea is still the same.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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