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The question is why do 70-300 f/5.6 lenses have worse wide open performance than 70-200 f/4 lenses in the same weight class? And that is not even at 300 mm, where 70-300 lenses tend to be softest. It seems that Canon had to add 300 g to the weight to get good performance in a 70-300 f/5.6 lens, but it is also very sturdy built. On the other hand, on APS-C the Sony E 55-210 f/6.3 is also sharper than the Sony 70-300 wide open, albeit with more haze. And that lens is really a lot smaller and lighter. Yes...Show more →
"Constant" zooms are of a really fixed type of base recipe. You have a fixed position frontmost positive group that concentrates light, behind that you have a negative group that is almost perfectly the inverse of the front, so that by moving that negative group up close to the front you get low magnification - or you move it back to allow the cone to get smaller before you intercept, and get high magnification.
This front part of two groups is almost afocal, that means that it doesn't really have a focal length. It just takes a parallel bundle of rays, concentrates it to a tighter bundle, and then bends it back to parallel rays again. Like a binocular.
The parts behind this is actually the main lens. You have a floating front part just in front of the aperture that moves in a path according to the first zoom group, to set the focal length and "float" the correction. Then you have the aperture, and the main lens.
Focus can be either built into the front group, or you can do it in the rear section (the main lens).
Anyhow, you have at least four, often five internally moving groups (including VR/IS then) in a constant zoom. Seven in the 200-400VR...
This gives ample room for built-in aberration compensation, and the genreal recipe is very well balanced for both SA and LoCA correction - but it makes the lens mechanically expensive. Not necessarily expensive in glass, vari-zooms can be just as glass-intensive as constants - they just have fewer moving parts...
And BTW, lenses like the different 300/2.8 versions are all like this too - they consist of a small rear main lens of about 100-130mm focal length, and then a whopping large front teleconverter (afocal condenser) that makes up most of the bulk of the lens.
Many moving parts, at least one float (two in the rear-focusing lenses like the Canon 70-200/2.8IS2 - plenty of options for corrections, but extreme demands on mounting precision and mechanical stability. Very low zoom-ranges, often less than 3x. More is almost impossible.
Vari-zooms are often "just" three moving parts, no floats. Zoom is done by moving the ENTIRE lens assembly forwards (look at the back of a 70-300 lens as you zoom in, and the entire rear sectio disappears into the lens body...). Keeping focus at least in the ballpark while zooming is done with one zoom correction group, focus is done with one group, IS is one rear group. Nice and simple, cheaper to produce, but a lot fewer points of correction possible.
That's why the 70-300L is relatively expensive, to get that kind of correction with that working principle you actually have to work HARDER on the drawing board and get SMALLER mounting errors than in a constant zoom. But you DO get a 4.5x zoom range - and relativly low weight, and relatively compact barrels (when at the shortest zoom setting, trnsport mode).