Why no 28-300/18-200 lenses with lower f-stop?

So I have this Tamron 28-300 (which is 18-200 on a FX body, right?
Sorry if I get that backwards) which is a fine enough lens, but it
goes from f3.5 - f6.3. It's not a huge lens by any stretch.

What I am wonder is why such a lens can't be made that is either 2.8
straight through or has an at least lower f-stop throughout (say 2.8
- 4).

I have the Nikon coffee thermos (i.e. their 70-200/2.8 lens) which in
comparison is huge, so I am assuming that size of the lens is a factor.

My reasoning goes something like the size of the lens is needed for
the f-stop to be so low at higher zoom distance, but the bigger the
lens, the higher the lowest zoom becomes (which is why it's 70-200 and
not 18-200).

Could anyone shed some light on this?

--
Sandman[.net]

On 10/07/2011 2:11 PM, Sandman wrote:

So I have this Tamron 28-300 (which is 18-200 on a FX body, right?
Sorry if I get that backwards) which is a fine enough lens, but it
goes from f3.5 - f6.3. It's not a huge lens by any stretch.

It would be 28~300mm on the FX body, on a DX (APS-C) it acts like a
42~450mm. They could make it a straight 28~300/2.8 constant. It would
need a 120mm lenscap, and would weigh around 3-4 kg 6-10 lbs.

Sandman wrote:
So I have this Tamron 28-300 (which is 18-200 on a FX body, right?
Sorry if I get that backwards) which is a fine enough lens, but it
goes from f3.5 - f6.3. It's not a huge lens by any stretch.

What I am wonder is why such a lens can't be made that is either 2.8
straight through or has an at least lower f-stop throughout (say 2.8
- 4).

I have the Nikon coffee thermos (i.e. their 70-200/2.8 lens) which in
comparison is huge, so I am assuming that size of the lens is a factor.

My reasoning goes something like the size of the lens is needed for
the f-stop to be so low at higher zoom distance, but the bigger the
lens, the higher the lowest zoom becomes (which is why it's 70-200 and
not 18-200).

Could anyone shed some light on this?

Large aperture lenses are harder to design, the edges are always a
compromise and making it a zoom also means a compromise because it means
putting a variable 7x teleconverter on a 28mm lens to make it a 200mm
lens. All lenses involve some compromise, really. Even the very best
compromise on affordability.

Interesting question though, what is the longest zoom range for a fast
lens? Even if you include f/4 (moderately fast)?
slow
18-200 11.1x
28-300 10.7x
50-500 10x

fast
24-70 2.9x -longest range fast lens I can think of
70-200 2.8x
80-200 2.5x
200-500 2.5x
10-24 2.4x

I've heard of some cine lenses with extremely long zoom range but even
those probably aren't fast. Still I'll bet there are cine lenses that
exceed the specs above, which none of us can afford:

9.5-114mm f/1.4 12x for 2/3" $129,430.00
http://www.unitedbroadcast.com/Home/6232-hae12x95.html
The smaller format makes it easier to build crazy long range zooms too,
that's got a 3.9x 'crop factor' or conversion to 35mm equivalent of
37-444mm and the apparent DOF equivalence (whatever you want to call it)
probably works out to slower than f/4.

Here's a super-zoom compact with 30x zoom 27-810 eq. f/2.8-5.6:
http://www.dpreview.com/news/1102/11...hx100vhx9v.asp

Paul Furman wrote:
Sandman wrote:
So I have this Tamron 28-300 (which is 18-200 on a FX body, right?
Sorry if I get that backwards) which is a fine enough lens, but it
goes from f3.5 - f6.3. It's not a huge lens by any stretch.

What I am wonder is why such a lens can't be made that is either 2.8
straight through or has an at least lower f-stop throughout (say 2.8
- 4).

I have the Nikon coffee thermos (i.e. their 70-200/2.8 lens) which in
comparison is huge, so I am assuming that size of the lens is a factor.

My reasoning goes something like the size of the lens is needed for
the f-stop to be so low at higher zoom distance, but the bigger the
lens, the higher the lowest zoom becomes (which is why it's 70-200 and
not 18-200).

Could anyone shed some light on this?

Large aperture lenses are harder to design, the edges are always a
compromise and making it a zoom also means a compromise because it means
putting a variable 7x teleconverter on a 28mm lens to make it a 200mm
lens. All lenses involve some compromise, really. Even the very best
compromise on affordability.

Interesting question though, what is the longest zoom range for a fast
lens? Even if you include f/4 (moderately fast)?
slow
18-200 11.1x
28-300 10.7x
50-500 10x

fast
24-70 2.9x -longest range fast lens I can think of
70-200 2.8x
80-200 2.5x
200-500 2.5x
10-24 2.4x

I think you have to go to 12-24 to get f/4 medium-fast spec, 2x zoom.


I've heard of some cine lenses with extremely long zoom range but even
those probably aren't fast. Still I'll bet there are cine lenses that
exceed the specs above, which none of us can afford:

9.5-114mm f/1.4 12x for 2/3" $129,430.00
http://www.unitedbroadcast.com/Home/6232-hae12x95.html
The smaller format makes it easier to build crazy long range zooms too,
that's got a 3.9x 'crop factor' or conversion to 35mm equivalent of
37-444mm and the apparent DOF equivalence (whatever you want to call it)
probably works out to slower than f/4.

Here's a super-zoom compact with 30x zoom 27-810 eq. f/2.8-5.6:
http://www.dpreview.com/news/1102/11...hx100vhx9v.asp

On 7/10/2011 2:11 PM, Sandman wrote:

What I am wonder is why such a lens can't be made that is either 2.8
straight through or has an at least lower f-stop throughout (say 2.8
- 4).

It could be made. In fact, here's a link to what purports to be
a 1990's Nikon design prototype for a 28-200mm f/2.8. The page
is in Italian:

http://snipurl.com/1jwhqq [marcocavina.com]

Such a lens would, of course, be considerably larger, heavier, and more
expensive than a 70-200mm f/2.8. My own guess is that it would be _so_
big and heavy as to destroy most of the convenience value of a
superzoom, and so it wouldn't sell all that well regardless of optical
performance.

On the other hand, if Nikon were to make the 75-150mm f/2.0 prototype
shown at that same site, I'd almost certainly buy it.

--
Mike Benveniste -- (Clarification Required)
Its name is Public opinion. It is held in reverence. It settles
everything. Some think it is the voice of God. -- Mark Twain