why can't we have an 18-200 constant f/2.8?

I only vaguely understand why.

can someone explain in simpler terms, maybe with a diagram :D ?

I am not sure that the statement is correct. Although you would need to stuff a huge amount of glass in it. Would be exceptionally heavy and stupidly pricey. I guess it would be nice to cover such a wide focal range with one fast lens but I suspect that you would need scaffolding to mount it on.

Canon do an inexpensive 28-200mm f/3.5-5.6, and a frighteningly expensive 28-300mm f3.5-5.6L IS. Neither is especially well regarded in terms of optical quality (unless I've missed a good review somewhere).

I suspect that such a lens would involve such compromises at the extremes of focal length that nobody would buy it - it would be just too plain fuzzy. That's just a guess, though, I don't claim to actually know anything much about lens design and manufacture.

I was under the impression that there would be too much glass to carry.

I'm sure I have read that to keep a zoom simple internally it shouldn't have more than a three fold range - i.e. 24-70, 70-200, 100-300 etc.

I'm sure a clever physicist with a massive computer could come up with a design. Probably has. Then the marketing department all pi$$ed themselves at the huge size, weight, cost and poor optical performance of the thing :lol1:

But also, why? You can cover that range with a couple of (relatively) affordable zooms of superb quality. Or primes with even wider apertures and even better performance.

I also have a feeling (sadly) that consumer lens design is moving away from lovely big apertures, in favour of size, price and convenience zooms. It seems to be easier to improve a sensor's high ISO performance and add IS/VR to give good low-light performance.

Richard.

Market forces?

Folks that want/need a constant 2.8 probably want optimal optical performance (bit of a tounge twister that), and don't mind carrying a couple of lenses.

Folks that worry about having to change lenses won't pay what an 18-200 2.8 would cost.

a single 18-200 2.8 would probably weigh more than a couple of alternative lenses - and what's the point anyway? Get over the d*mn superzooms. Get a compact or whatever. Half the point in owning a DSLR is interchangable lenses :)

Go that man! Well said!:thumb:

I think they only come up with such questions because they are afraid to clean the camera sensors. ;)

Get some decent primes already. :D

this was a technical question, btw. i rarely ever feel the need for a zoom.

I just thought it was a more interesting question than the usual and endless drivel.

*edited by me as it's the same as below*

So I suppose the technical answer is...... You could if a manufacturer saw it as necessary to develop :thumb:

I'm sure a clever physicist with a massive computer could come up with a design. Probably has. Then the marketing department all pi$$ed themselves at the huge size, weight, cost and poor optical performance of the thing :lol1:

Richard.

Wouldn't the marketing department request it and then the product engineers piss themselves? :D

this was a technical question, btw. i rarely ever feel the need for a zoom.

I just thought it was a more interesting question than the usual and endless drivel.

Can you give us a link to the dpreview.com debate?

Why not a 18-200 with constant 2.8?: Canon SLR Lens Talk Forum: Digital Photography Review (http://forums.dpreview.com/forums/read.asp?forum=1029&message=21486244)

This is probably similar in size and scale as to what you would need for a 2.8 zoom to cover the focal length.

The lens weighs 564lb 1700mm fixed f4 !!!!!!

Believe it or not that small spec on the end is a medium format Hasselblad. Manufactured for a rich oil sheik.

Zeiss Apo Sonnar T* 1700 mm F4 lens: Digital Photography Review (http://www.dpreview.com/news/0610/06100101zeiss1700f4.asp)

Now THAT's just silly. What's the tripod made out of? Concrete?

More money than you can sheik a stick at.

You would not get away with a Jessops £15 monopod thats for sure

the answer is already posted on the site

"

It has to be retrofocus because the minimum focal length (18mm) is less than the registration distance (44mm) which is the distance from the lens mount to the sensor.

So, you kind of have an ultra-wide front end with a variable teleconverter behind it. That means at the long end you're looking through a wide angle lens. That makes it tough to get aperture which means such a lens as you described would absolutely ENORMOUS!!!

--
Lee Jay


"

This is probably similar in size and scale as to what you would need for a 2.8 zoom to cover the focal length.

The lens weighs 564lb 1700mm fixed f4 !!!!!!

Believe it or not that small spec on the end is a medium format Hasselblad. Manufactured for a rich oil sheik.

Zeiss Apo Sonnar T* 1700 mm F4 lens: Digital Photography Review (http://www.dpreview.com/news/0610/06100101zeiss1700f4.asp)

This specific customer must be very much afraid of Lions ..
or.. he wants to shoot the pictures from home and can therefore save some travelling costs. :D