Fast lenses

[graybrick]

Question for anyone, mostly for Bob... since he didn't like my bogus 12-600mm

f/1.2 question a few days ago...

 

I'm wondering, theoretically, what the minimum dimensions and weight would be

for a 600mm f/1.8 lens could be. This would have to be a whopper of a lens,

and a specialty item for sure. Would I need a pickup truck to carry it? I

figure that BA, if anyone, would be able to guesstimate the size and heft of

the elements necessary to create such a monster. And what of a zoom covering,

say, 200-600mm at f/1.8 or f/2. This is a theoretical exercise, and you'll say

it's completely academic and impractical, but I'm interested to hear from a

lens guru what this might entail. We'll leave the price range to theory, as

this lens is unlikely to ever exist, and it would be a one-off on the lines of

the <a href="http://www.dpreview.com/news/0610/06100101zeiss1700f4.asp">Zeiss

1700mm f/4</A>, which requires its own mechanical drive system for aiming and

makes the camera look miniature... Thanks for considering answering such a

ludicrous question. I hope a straightforward question will not further my

newfound reputation as a troll.

[graybrick]

http://www.dpreview.com/news/0610/06100101zeiss1700f4.asp

[mendel_leisk]

Maybe the front element would need to be

 

600mm / 1.8 = 333mm

 

wide? Not sure.

[mark_chappell]

An f1.8 lens needs a front-element area about 5.6X larger than that of an f4 lens. That's

obviously the biggest and heaviest piece of glass in the hypothetical lens. Other elements

would also have to be bigger, but probably not in the same ratio. I'd assume that the

element thickness would also have to be increased as well, and there would probably need

to be more elements as well (fast lenses generally are more complex than slow lenses). So

I figure that there might be 10-15 X as much mass of glass in the f1.8 than in an f4.

 

That bigger diameter and mass of glass would require corresponding increases in the size

of the lens barrel, focus motor, tripod mount, and so forth. That stuff would also be

several times as heavy as in the f4 lens. So as a rough guess, figure the the 600/1.8

would be at least 10 and maybe 15 times as heavy as a 600/4. Thus, maybe 110-160

pounds?

 

As a reality check, consider the Sigma 200-500/2.8 zoom recently announced. It weighs

15.7 kg. Compare that to the one-stop slower Canon 500/4 (both lenses have 17

elements, surprisingly), which weighs 3.9 kg. So, ONE stop of speed increase translates

into a 4-fold mass increase. The hypothetical 600/1.8 is about 2.5 stops faster than a

600/4. That's fairly consistent with a 15-fold mass difference, or even more.

[dan_fromm2]

One of my neighbors has a 400/1.5 Zeiss lens. It weighs 35 kg. But its barrel seems to be cast iron and is quite thick.

 

My 100/2 TTH can be scaled up to 600/2. It weighs 500 g. Since the elements' volumes scale with the cube of focal length, a 600/2 version would weigh around 13.5 kg.

 

I have a couple of compendia of aerial camera lens specifications, also the 1963 GOI book of all the lenses designed there up to them. Without checking, I don't think there are any lenses nearly that long and that fast. Since aerial camera lenses, especially the ones that went into space, were made on a 'cost no object' basis, I'm not sure you can find a lens that was actually made to test the calculations you've been offered.

[dan_fromm2]

Oh, my, I erred. 6^3 = 216. Sorry.

[alex_lofquist]

If someone really wanted an objective this large, they would probably consider Schmidt (catadioptric) optics.

[mark_chappell]

<I>One of my neighbors has a 400/1.5 Zeiss lens.</I><P>

 

Yikes! Any chance we can see an image of that thing?

[James G. Dainis]

A lot would depend on the design of the telephoto. You can have a lot of glass, 17 elements in 14 groups or minimal glass, 2 elements in 1 group.

 

Volume or mass varies as the cube. A two inch sphere has 8 times the mass of a one inch sphere and a three inch sphere has 27 times the mass of a 1 inch sphere. That holds true for all shapes.

 

A 600mm f/1.8 lens would be about 13 inches in diameter. A Canon 600mm f/4 lens is about 6.6 inches in diameter (about 1/2 inch is housing, the actual front glass should be around 6 inches) and weighs about 12 pounds. It has 17 elements in 13 groups. I would guess that 8 pounds of that weight is the glass.

 

(13/6)^3 = 10

 

10 x 8 lbs = 80 pounds.

 

The shape of the lenses would have to be the same to maintain the focal length. If a 6 inch diameter lens of the Canon had a 2 inch thickness, then the 13 inch diameter lens would have a thickness of 4.3 inches. I doubt if you would be able to cram all those thicker lens elements (17) and groups (13) into the required 600mm (18 inch) focal length of the lens tube. Better to go with a simple 2 element, 1 group lens that would still be 13 inch diameter, f/1.8 and weigh perhaps 10-15 lbs.

[JDMvW]

There is a rumor about (I know because I started it) that there will soon be Canon official bolt-on wheels available for the larger L lenses.

 

The 600mm f/1.2 would possibly explain this rumor.

 

There may even be a tracked version for nature photography.

[dan_fromm2]

Mark, look here: http://www.luciolepri.it/lc2/marcocavina/articoli_fotografici/Zeiss_UR_40cm_1,5_T/00_pag.htm

[bryantan]

Oh dear...wouldn't that thing rip off the lens mount?

[dan_fromm2]

Bryan, one hangs the camera on the lens, not the lens on the camera, if you see what I mean.

[Brian]

<P>I shouldn't really be encouraging you, but have a look at <a href="http://www.crestock.com/blog/technology/the-worlds-most-extreme-photography-equipment-117.aspx">this link</a>. The 200-500mm f2.8 zoom is possibly the most reasonably sized optic in the list.</P>