Why do zoom lenses not have below 2.8 Aperture?

I was looking at the fixed 2.8 aperture in my lenses the other day and began thinking. If the f/ stop is just a ratio, yet it is constant across focal length, it has to change physical size as the lens zooms in and out. If that is the case, why don't we ever see f/1.8-2.8 or something similar?

you would have to get a second mortgage to buy some of these Zooms at f/1.2 L

It still comes down to lens diameter. In order to get wider than 2.8 you would need a huge lens. And since zooms have multiple elements each having to be considerably larger would cost more than most users would be willing to pay, especially considering the quality of fast primes comparatively.

So there's something magic about f/2.8?

sbddude wrote in post #9841618
So there's something magic about f/2.8?

Not really, its just a good balance between size, cost, and speed in most cases.

I think 2.8 is around the limit of making a zoom that is (relatively) affordable, light, and is profitable.

I wish we had a lens engineer in the forums or something. I am kinda interested in an explanation from an expert. I am guessing zoom lens design becomes more difficult and/or weight and size becomes difficult to deal with past a certain aperture.

Heres a picture of a sigma 200-500 F2.8:

I agree with the above comments. Even a short zoom would be huge. You would need Ahnold's biceps to carry it around and every time you put up to your eye you'd scare the bejeezus out of your subjects.

The lens would be too big and expensive. But I have seen some Canon patents on some site for a EF-S 17-55 f2 or something along those lines.

Boomsnapp wrote in post #9841803
Heres a picture of a sigma 200-500 F2.8:

I think this lens cost something astronomical like 20-30K

what he's asking isn't, why aren't the apertures larger across the board; but rather, in the case of the sigmonster 200-500 F2.8, why don't they make the lens use the max aperture at the low end of the zoom, the sigma looks like at 200mm the aperture at the rear is more than large enough to be nearly F1.2.. If my rudimentary understanding of aperture is correct at 200 mm at 2.8 the aperture has to be 40% the size it needs to be at 500mm, as such cheaper smaller lenses have wider apertures at narrower focal lengths, but they don't make high end lenses the same way.


i think it has to do with making the lens more controllable and "constant" its nice to not worry about your aperture narrowing when you zoom in on a subject, typically i set up all my settings when i initially frame my subject, and once i start shooting from the same angle, i don't really mess with them again until i move to a different lighting situation, if my 70-200 was going from F1.8 to 2.8 by me zooming in i would be annoyed as all hell.

sbddude wrote in post #9841618
So there's something magic about f/2.8?

Nope:

http://www.bhphotovide​o.com …100mm_f_2_0_ED_​Zuiko.html

Since 4/3 image circle is smaller, a constant-F2.0 zoom still portable/affordable/pr​actical enough.

However, for FF/APS-C, ask yourself what you would rather have:
- the 24-70/2.8L
or
- a 35-70/2.0L

I know which one I would rather have....

sbddude wrote in post #9841473
If the f/ stop is just a ratio, yet it is constant across focal length, it has to change physical size as the lens zooms in and out. If that is the case, why don't we ever see f/1.8-2.8 or something similar?

hes not asking about a wider fixed aperture.

I think the OP was asking why lens makers wouldn't make their 2.8 zooms say 2-2.8 instead of a constant 2.8. the f stop is just a ratio between the focal length and diameter, so instead of making the lens smaller at the low end of the zoom range, why not just let it be wide open like at the end?

PS- Not 100% on how zooms work inside, but I think they work like this to get the constant aperture.

monst0r wrote in post #9842054
I think the OP was asking why lens makers wouldn't make their 2.8 zooms say 2-2.8 instead of a constant 2.8. the f stop is just a ratio between the focal length and diameter, so instead of making the lens smaller at the low end of the zoom range, why not just let it be wide open like at the end?

PS- Not 100% on how zooms work inside, but I think they work like this to get the constant aperture.

OP, sources suggest... we don't know