FlyingPete wrote in post #19189706
Wikipedia defines a f-number (f-stop) as:
"In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture")."So this got me wondering, in the first instance with a 70-200 f/2.8, so based on the above calculation the entrance pupil is 71mm, so why isn't it nearly f/1.0 at 70mm?
Also my 100-400 f/4.5-5.6 has a rather large front element, similar to my 70-200. f/5.6 seems about right at 400mm, as that is what my 70-200 is with a 2x. In fact 400mm at f/5.6 gives exactly the same size entrance pupil as the 70-200, so why isn't is closer to f/2.8 when at the 100mm end?
What am I missing here?
What you are missing here is that it is a zoom lens to start with.
When zoomed out to a shorter FL, the part of the optics that zoom, only (can) use part of the image that the front lenses can capture. It is a bit like an imaging system in an imaging system, if you look at it in a simplistic way. Effectively the larger front lenses are there to gurantee the maximum aperture at the longest focal lengths. At shorter focal lengths teh front groups that make up the first (afocal) imaging set of optics, are in the way, and take up a lot of space on the optical axis, so that a wider AoV can only be reached at smaller apertures. Irt is a bit like a lens behind a tube of fixed length: you can't really see everything beyond the rim of the tube, from an AoV PoV, and you are limted in AoV by teh distance between the second imaging part and the distance to the front of the lens, much less so by the diameter of the front element.
In addition, theses lenses have a fixed maximum aperture, which makes shooting all that faster and more consistent, as neither the photographer, nor the camera, have to think about changing aperture settings. Often, but not always, this is achieved by a complex second diaphragm in the lens. In addition these lenses are optimized for use right from the largest aperture, and that makes them much more complex than lenes which are not.
Cheaper lenses often do not have a second diaphagm or a more complicated construction, and then are able to have a slightly larger aperture at the short end, mostly with lenses which cover the normal to tele range. With WA zooms it becomes problematic again due to the large AoV. Even then, it often is a difference of 1 1/2 to 1 2/3 of a stop difference, for exactly teh same reasons as described above, even though the simple optical formula, for very simple lenses, BTW, would give you a potentially much larger opening. The 100-400L for example, starts at F/4/5, but if you take the front element into account, and onlu use that, it should be able to go to F/1.4.. It can't, because of the required lens length for 400 mm and the accompanying length of the afocal part at the front of the lens.
Other than that, larger aperture lenses, and especially zoom lenses, are really very hard to make, big, and heavy, and hence very expensive, and often with a more limited reach, because of very complex designs, both in order to guarantee good optical quality at large apertures and over the entire range they cover. A good example of this is teh RF 28-70 F/2L, an exceptional, and very fast lens for a standard zoom. Very heavy, big and expensive for the limited zoom range it has, but it is in a class of its own.
I hope I did not make it too complex with my explanation.
Kind regards, Wim
.
