Why aren't zoom lenses faster than 2.8?

[sandymandy]

Zooms are supposed to be allround lenses, easy to carry and ready for all situations (kind of). Having a bigger aperture would make the lenses big and  bulky and not comfortable to carry around. For example if  you want 2.0 at 200mm the lens would have to be 10cm diameter. 70mm would require a 50mm diameter for f/1.4.

[K3nt]

It is quite easy actually. If you want to know what size it would require just divide the focal length with the required f-stop and you'll see what size the front element diameter would be. So, for a 100-400mm f/1.2 the front element would be 33,3cm in diameter. 

If you divide the focal length with the diameter of the front element you'll get the maximum f-stop for that lens.

[bycostello]

cost, weight and demand

[Hillsilly]

Olympus make a 14-35 f/2 and a 35-100 f/2.  Both are $2k+ and only cover a four thirds size sensor.  These lenses are big, but not that big.  With their superior economies of scale and production capabilities, Canon could make similar EF-S lenses without too much of a size, weight or cost penalty.  But is there a market for $2k+ EF-S lenses? 

[brad goda]

wow... the cost and weight!!!
and everyone is wanting that f1.8 to be dead sharp wide open!!! LOL.
so that means its not easy to build... it would have to be OVER built.!
like the new zeiss prime 55mm 1.4... that is over built... massive image circle + good deisgn =
huge sweet spot for sharpness to cover full frame...
In any form UWA ... standard zoom or telephoto zoom its a reach to think our industry would support it... only cinema zooms have that sort of at whatever it costs... its made... and bought.

besides... at 500.00 for super telephoto replacement 2.8 lens hood... can you imagine for 1.8...??

yah great for event photos... an 8-13lb camera + lens...

[rs]

Zooms are supposed to be allround lenses, easy to carry and ready for all situations (kind of). Having a bigger aperture would make the lenses big and  bulky and not comfortable to carry around. For example if  you want 2.0 at 200mm the lens would have to be 10cm diameter. 70mm would require a 50mm diameter for f/1.4.

That's the simple way of looking at it, but look at the reality of what already exists in terms of zooms and primes - the situation is even worse than you predict, so we end up with something that is all but impossible to afford, carry, or use.

Take the 70-200/2.8L (non IS) vs the 200/2.8L II - when fully zoomed, the 70-200 offers the same unstabilised 200/2.8 as the prime. A 200/2.8 theoretically needs a 71.4mm aperture, and the prime indeed has a 72mm filter thread, and is 136mm length, weighing in at 765g. The zoom has a 77mm filter, 194mm long, and weighs 1310g.

Taking those increases over an existing 200/2.8 and applying them to the 200/2 IS lens to end up with a theoretical 70-200/2.0 IS lens, and you get a 136mm diameter, 296mm length, 4.3kg lens. And then there's the price factor between the prime and zoom too. It would cost (years after the introduction price tax disappears, like it has on the 200/2.8, 70-200/2.8 and 200/2) something in the region of £7000 - so lets call it comfortably more than £10,000 at introduction. There clearly wouldn't be a market for something like that.

However, a very limited zoom range around the 50mm mark would be simpler to make faster - 50mm after all does seem to be the easiest focal length to make fast (think of the 50/1.0 L, and also how small the current 50/1.4 is), so a 40-60/2.0 probably isn't pushing the boundaries of what's possible. But with such a small zoom range, why not just get a much cheaper, smaller and optically better 50 instead?

Having said that, smaller imaging circle lenses exist faster than f2.8 - take the Olympus m4/3 35-100/2.0 lens. But that is an equivalent of a 70-200/4.0 FF lens. And that Canon 25-100/1.8 is a 16mm lens (crop factor of 3.4), so in FF terms its a 85-340/6.1 lens.

[Radiating]

The Canon. 25-100mm lens was a cinema lens designed for a format that was much smaller yet was very heavy, about the size if a 70-200mm f/4.0 is. The sensor size was around 1/4 of full frame so while yes such a lens is possible it would be huge and expensive.

Say if you wanted a 28-85mm f/2.0. It would cost at least $10,000 based on Canons manufacturing costs for lenses of similar size, weigh 4-6 lbs and would use 120mm filters, and be over 12 inches long.

The market would be so small for such a lens Canon would likely change $20,000 or more.

Simply put such a product would be very convention defying.

You can also buy a number of f/3.5 zooms for medium format cameras which are equivalent for f/2.2 lenses on full frame. Leica makes one like that, that's equivalent to a 20-60mm lens.

Personally I think a 70-200mm f/2.0 zoom for 7k is realistic and would sell.

[neuroanatomist]

It is quite easy actually. If you want to know what size it would require just divide the focal length with the required f-stop and you'll see what size the front element diameter would be. So, for a 100-400mm f/1.2 the front element would be 33,3cm in diameter. 

Actaully, that 'rule' applies to telephoto lens designs but not all lens designs.  Else, the front element of the 16-35mm f/2.8L would be only 1.25 cm in diameter. 

[JonB8305]

Zooms are supposed to be allround lenses, easy to carry and ready for all situations (kind of). Having a bigger aperture would make the lenses big and  bulky and not comfortable to carry around. For example if  you want 2.0 at 200mm the lens would have to be 10cm diameter. 70mm would require a 50mm diameter for f/1.4.

That's the simple way of looking at it, but look at the reality of what already exists in terms of zooms and primes - the situation is even worse than you predict, so we end up with something that is all but impossible to afford, carry, or use.

Take the 70-200/2.8L (non IS) vs the 200/2.8L II - when fully zoomed, the 70-200 offers the same unstabilised 200/2.8 as the prime. A 200/2.8 theoretically needs a 71.4mm aperture, and the prime indeed has a 72mm filter thread, and is 136mm length, weighing in at 765g. The zoom has a 77mm filter, 194mm long, and weighs 1310g.

Taking those increases over an existing 200/2.8 and applying them to the 200/2 IS lens to end up with a theoretical 70-200/2.0 IS lens, and you get a 136mm diameter, 296mm length, 4.3kg lens. And then there's the price factor between the prime and zoom too. It would cost (years after the introduction price tax disappears, like it has on the 200/2.8, 70-200/2.8 and 200/2) something in the region of £7000 - so lets call it comfortably more than £10,000 at introduction. There clearly wouldn't be a market for something like that.

However, a very limited zoom range around the 50mm mark would be simpler to make faster - 50mm after all does seem to be the easiest focal length to make fast (think of the 50/1.0 L, and also how small the current 50/1.4 is), so a 40-60/2.0 probably isn't pushing the boundaries of what's possible. But with such a small zoom range, why not just get a much cheaper, smaller and optically better 50 instead?

Having said that, smaller imaging circle lenses exist faster than f2.8 - take the Olympus m4/3 35-100/2.0 lens. But that is an equivalent of a 70-200/4.0 FF lens. And that Canon 25-100/1.8 is a 16mm lens (crop factor of 3.4), so in FF terms its a 85-340/6.1 lens.

Thanks for the knowledge, fascinating post.

[Hobby Shooter]

It is quite easy actually. If you want to know what size it would require just divide the focal length with the required f-stop and you'll see what size the front element diameter would be. So, for a 100-400mm f/1.2 the front element would be 33,3cm in diameter. 

Actaully, that 'rule' applies to telephoto lens designs but not all lens designs.  Else, the front element of the 16-35mm f/2.8L would be only 1.25 cm in diameter.

That's typical, I was just starting to get it and now you complicate things again 

[Lawliet]

Canon used to make a 25-100 F/1.8

Keep in mind that this is a cine lens - about the image circle of an EF-s.
If one took a common 70-200/2.8 and welded an inverse TC2 to it he'd get an 35-100/1.4. Makes me think of the fast 4/3rds zooms...
Now the same trick for EF would require a f/2.8 zoom for medium forma (6x9 should work, 6x7 feels like a tight fit)as a baseline. Not pretty.

[East Wind Photography]

It would make them too heavy to be practical.  The 70-200L 2.8 is already two bricks.

[SiliconVoid]

If you research the primary cons for any premium lens, regardless of brand, you will see 'size' 'weight' and 'cost'..
How well received do you think a 70-200mm with an 6" objective lens, ~6lb heft, and ~$5,000 price tag would be??

[RLPhoto]

The 70-200L II is already pretty hefty, I could only imagine a 1.8 lens.

[Mt Spokane Photography]

As lenses get larger apertures, particularly at shorter focal lengths, it becomes difficult to keep the edges sharp, eliminate field of curvature, etc.  This means larger diameter glass and more elements.
Since the cost to grind glass increases with size, cost of large diameter lens elements skyrockets.  Now, add several more expensive elements to try and correct distortions, CA, etc, and the price gets out of control.
For expensive lenses, the market is much smaller, so development and tooling charges are amortized over just a few lenses, maybe 1000 more or less.  Spread that $1 million over 1000 lenses, and just the development cost is $10,000 per lens, and add another $10,000 manufacturing cost, and you have a lens that sells for $40,000, which is what the low volume cinema lenses sell for.
 
For a lens that sells 100,000 copies, the amortization might be $100 each, and the price is more reasonable.