Why can't there be a zoom lens with f2.7 or larger?

[kfeng]

Hi,

does anyone know why even the BEST zoom lenses can only have f2.8 as their maximum aperture, and never any bigger?

Is it because of a rule of thumb? i.e. f2.8 is the sweet spot between light entering lens and DOF which most pro. photographers want?

Or is it because of technical limit of the manufacturing process? i.e. the more glass elements, the more difficult to get max light entering, so f2.8 is largest aperture of a zoom lens?

Is there any article on this?
Or is there any zoom lens with greater max aperture than f2.8?

 

[Kit]

Tokina once made a 28-70 f/2.6 - 2.8 lens. It has since been discontinued.

 

[YSLee]

The small digital cameras have zoom lenses starting at f/2, like the Sony 828 and Canon G series.

CCTV lenses start at about f/1.6.

It's all about designing practical lenses which are within reasonable cost.

 

[kfeng]

It's all about designing practical lenses which are within reasonable cost.

So the manufacturers don't want to make lenses of such because the costs will be too high?

Tokina once made a 28-70 f/2.6 - 2.8 lens. It has since been discontinued.

Why'd they discontinue it? Not popular? Maybe the 2.6 could only work at the widest possible focal length? Hence, it's cheaper to just make the lens with f2.8 constant, so nowadays their ATX-Pro are all f2.8 constant...

 

[Larry]

It's all about designing practical lenses which are within reasonable cost.

yah i agree with that, it's a matter of price, size and weight. i'm sure it's technically possible to design a 28-70mm f/1.8 or bigger, but it'll probably be huge, unwieldy and cost more than a Ferrari. you just need to look at the relative sizes of a 28mm f/2.8 and 28mm f/1.4, or the 85mm f/1.8 vs the f/1.4. the size and weight differences are fairly significant. even the 50mm f/1.4 is quite a bit bigger and heavier than the f/1.8. so when you factor in the zoom elements and mechanism, it'll probably end up like a 2kg lens with a front element the size of a small dinner plate. :sweat:

 

[ST1100]

Actually, since the aperture size is [focal length]/[f-ratio], meaning that shorter lengths need less glass for the same f-stop, i think the wider angle zooms can potentially go faster without excessively increasing the price and weight. Something like a 20-24/f1.8 might still be reasonable.

 

[sriram]

One of the reasons wide angle lenses are not very small is because of the extreme angles, the aberrations are difficult to correct, can't make very tiny glass elements and expect a good lens (at least this is what I read in a book on optics). For SLR's you also have to allow the size of a retrofocus design as opposed to a much simpler and smaller wide angle lens on a direct vision or rangefinder body. This is why the wide and ultra wide rangefinder lenses like the Voigtlander 12mm, 15mm, 21mm etc are small.

 

[Ian]

So the manufacturers don't want to make lenses of such because the costs will be too high?

Apart from the horrendous cost of building say an f2 zoom lens there really is no need for them given the quality of modern film emulsions and digital sensors found in modern SLR's.