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alan-salsman
16-Dec-2009, 19:32
Will a lens with larger diameter elements cover a larger area? For instance,if we had before us 2 lens, both are 240mm, both are tessar, and both are say 6.8F. The diameter of one lens is 5cm and the diameter of the second lens is 10cm will the 10cm lens cover more area than the 5cm lens?

Gem Singer
16-Dec-2009, 20:30
Assuming you are referring to large format taking lenses, yes, I believe a larger diameter lens will throw a larger image circle.

Think of a lens like a flashlight that throws a beam of light onto the film, a larger diameter flash light will project a larger diameter beam of light.

alan-salsman
17-Dec-2009, 00:58
I was thinking about large format lens. I have noticed quite a difference in the diameter of lenses, but have never seen it considered when talking about coverage.

dave_whatever
17-Dec-2009, 02:21
For instance,if we had before us 2 lens, both are 240mm, both are tessar, and both are say 6.8F. The diameter of one lens is 5cm and the diameter of the second lens is 10cm

Surely if they're the same optical design, same focal length, same speed, then that would mean that they'd pretty much have to be the same diameter?

Lachlan 717
17-Dec-2009, 14:18
Surely if they're the same optical design, same focal length, same speed, then that would mean that they'd pretty much have to be the same diameter?

I'm also not sure about the flashlight simile. Parabolic reflector design has much more impact on the end light's brightness than the aperture of the front of the product.

JRFrench
17-Dec-2009, 14:22
Surely if they're the same optical design, same focal length, same speed, then that would mean that they'd pretty much have to be the same diameter?

Couldn't you make bigger elements but keep the maximum aperture the same so more of the 'centers' of the elements are used, possibly leading to fewer/less severe aberrations? Like getting a lens with a very wide aperture, and stopping it down a few stops (but without the option to open it up).

Jack Dahlgren
17-Dec-2009, 15:58
Couldn't you make bigger elements but keep the maximum aperture the same so more of the 'centers' of the elements are used, possibly leading to fewer/less severe aberrations? Like getting a lens with a very wide aperture, and stopping it down a few stops (but without the option to open it up).

You could, but why? Theoretically, you could have a lens a mile wide and only use the middle inch. Any glass that doesn't contribute to the image is wasted and might increase internal reflections/flare.

JRFrench
17-Dec-2009, 16:18
You could, but why? Theoretically, you could have a lens a mile wide and only use the middle inch. Any glass that doesn't contribute to the image is wasted and might increase internal reflections/flare.

If it gives you a superior/sharper image, IE possibly the difference between a cheaper lens and a more expensive one.

Back to the OP, you need bigger elements to get more coverage. You can imagine coverage as how big an angle you can swing from the axis of the lens behind it and still see a clear path through the elements to the aperture. Try it out with a 135 format lens, as soon as you go a little bit off axis your view is blocked by the housing and the edges of the elements. With a large format lens you can get a much bigger angle before things interfere.

Gem Singer
17-Dec-2009, 16:35
Where's professor Leonard Evens when we need him?

Dan Fromm
17-Dec-2009, 18:03
Gem, this discussion doesn't need a mathematician. It needs an optical engineer.

OP, think about mechanical vignetting. Simply increasing the glasses' diameters while changing nothing else may reduce mechanical vignetting, won't change the diameter of the circle of good definition, which is what some of us mean when we talk about the circle a lens covers.

Then think about why a lens designer might want to limit a lens' field. When you do that, ponder the local craze for Petzval type lenses. And ponder the 75/4 Apo Rodagon D's MTF curves. You can find them starting from here: http://www.rodenstock-photo.com/ Lastly, find a copy of the Vade Mecum and read what its authors have to say about the several sizes of the 5"/4 Ross Wide Angle Xpres.

GPS
17-Dec-2009, 18:19
Will a lens with larger diameter elements cover a larger area? For instance,if we had before us 2 lens, both are 240mm, both are tessar, and both are say 6.8F. The diameter of one lens is 5cm and the diameter of the second lens is 10cm will the 10cm lens cover more area than the 5cm lens?

As the question is put in your first sentence, the answer would be - not at all. It depends on the optical design of the lens.
As the question is expressed in the second phrase, the answer would be - not necessarily. You have different tessars with different angles of coverage...

GPS
17-Dec-2009, 18:25
Gem, this discussion doesn't need a mathematician. It needs an optical engineer.

...

To my knowledge we don't have any optical designer on this forum. Unfortunately. That's why there is so much nonsense about optical designs on it...:)

alan-salsman
17-Dec-2009, 18:41
Optical engineering is beyond me.It is all I can do to decide when to trip the shutter.I am going to move up to ULF by building my own camera. And in so doing focus my efforts in that area.so I have to wonder if my 220mm by 240mm G Claron with 5 inch lenses will make the jump.This lens in the barrel weights 6 pounds.I do not want to part with it if it will be of use. maybe a large piece of ground glass to do some testing is in order.

GPS
17-Dec-2009, 18:44
Alan, by all means, test it. Good luck with your project!

rdenney
17-Dec-2009, 18:50
I think if you look at the optical design of lenses, you'll discover that with many designs lenses run out of space at some point. Once the surfaces of a convex lens create an edge too thin to mount, or two air-spaced elements run into each other, no more widening is possible. At that point, one needs a different design altogether.

And then there is the notion that some designs are optimal only in the center and degrade with distance from the center (which is true for most photographic lenses). One can draw a boundary around the acceptable limits of performance, and then mechanically vignette it to prevent use outside that range. One might remove those mechanical limits only to discover that the degradation of performance becomes unacceptable. The limits of acceptability have gotten more stringent over time, just as optical designs have incorporated wide coverage into their formulas to compensate for that.

Rick "suspecting the performance rules most often" Denney