Information on simply DIY large lenses

[false_Aesthetic]

Hey,

Is there a forum or information somewhere about constructing large lenses with rather large image circles.

In Photo 1 I often turn the classroom into a camera obscura to help excite students about photography. I think showing them how a lens affects the exposure and the sharpness would help. It doesn't need to be super complex or sharp. just needs to get the basic ideas across.

Thanks

[Hermes07]

If the lens doesn't need to be sharp enough for photography then you don't really need to construct anything - surely a simple lens from Surplus Shed would show the principle well enough? there is a 6 inch, 2600mm meniscus on there which would have a huge image circle at infinity.

[jnanian]

in the "primitive photography" book, there is a section on making lenses.
some lenses are very simple, and others are a bit more complicated.

what sort of coverage + image circle are you looking for ?

i have gotten some very small meniscus lenses from "the shed" and
used them on 11x14 cameras ...

john

[false_Aesthetic]

The bigger the image circle, the better. I think something in the 14" range would be minimum size I'd want . . . just so that the students don't "strain" themselves too much (I wish I was joking but I've had to explain what a ratio is more than once this semester to college level students).

[jnanian]

hi tom

a rr lens is just a pair of groups with a big airspace between them ...
maybe the good folks at the surplus shed could suggest which plano concave + plano convex lenses to use
i have made barrels out of cardboard.

you can also remove one of the groups, to increase the focal length, and coverage since it is convertible

sounds like a fun project !
john

[jp498]

I'd just get the biggest cheap magnifying glass you can find. Put it in a piece of pipe made of cardboard or plastic. Make waterhouse stops if you'd like an aperture. Put it a few inches ahead of the lens.

[E. von Hoegh]

A pair of surplus telescope achromats mounted in a barrel will give you a Rapid-Rectilinear lens of about half the focal length of the single achromat, with a decent image circle. Waterhouse stops could be used, placing them midway between the cells. A R-R is a pretty good lens, too. Very sharp in the central zone, not bad in the corners.

[false_Aesthetic]

Yeah my photo 1 class starts with the very basics of converting a room into a C.O. Then I get them to make their own pinhole cameras. While they're drying, etc. Introduce them to photograms. Then I make them go out and "properly expose" their paper.

It works decently but there's a jump b/n pinhole and actual camera.

Word.