Old-timey camera obscura

[Bill SOOP]

Good morning-
I’m hoping to find some advice. I wish to build an 18th century style camera obscura. I’ve made a prototype, but not knowing much about optics, I need help identifying and obtaining the right lens. I’d appreciate any assistance.
-Bill

 

[MattKing]

Welcome to Photrio.
I've moved your very interesting query to the Camera Building sub-forum, because it is on-point for that sub-forum, and because it will probably have more visibility there.

 

[Bill SOOP]

Welcome to Photrio.
I've moved your very interesting query to the Camera Building sub-forum, because it is on-point for that sub-forum, and because it will probably have more visibility there.

Thank you kindly- I Appreciate you leading me into the right neighborhood!

 

[Mark Crabtree]

Good morning-
I’m hoping to find some advice. I wish to build an 18th century style camera obscura. I’ve made a prototype, but not knowing much about optics, I need help identifying and obtaining the right lens. I’d appreciate any assistance.
-Bill

For drawing? I'd start with the image size you want, then work backward to a focal length for the lens. Maybe a little shorter lens than the diagonal of the image size for general views or environmental portraits, and probably about the diagonal or a touch longer for closer portraits.

Do you want authentic, or simply functional. Lenses would have been very simple at the time and somebody here can probably offer advice on sourcing something from a supply house. Or just any lens that comes up from the photographic era will work even better. Your are probably into focal lengths that aren't pocket change, but still lots of funky stuff out there that is cheap.

 

[Donald Qualls]

Given the size of your enclosure (assuming I'm judging correctly by the manufacturer's printing on the waferboard) a lens from an old overhead projector (and its accompanying mirror!) seems just right. These lenses usually have a focal length around 300-450 mm at infinity focus, and tend to be quite fast. They're not really sharp enough for most folks to put images on film that will be enlarged, and they have some distortion and aberrations, but they should be quite good enough for a camera obscura like this, where image brightness is the biggest thing to make it easy to see.

And because they don't have much other use, plus overhead projectors having been almost entirely replaced with big screen TVs that can do many other things besides display from transparencies, a used or surplus projector is likely to go pretty cheaply.

 

[koraks]

I’ve made a prototype, but not knowing much about optics, I need help identifying and obtaining the right lens.

From a practical viewpoint, I'd approach this the other way around. Get your hands on some lenses and then figure out the dimensions to make a useful camera obscura. You'll face a tradeoff between brightness of the projected image, size of the projected image and angle of view.

Btw, you can fairly easily get a feeling for what a lens will do by holding it at a distance from a flat, white wall that faces a brightly lit window. The lens will project a somewhat dim image that gives an impression of focal length.

 

[kl122002]

You can do it with old brass lens., they should come cheap. Alternatively during the 1900s ealry cameras , like old RB Graflex has some big solid lens which are tessars - these can provide sharper image.

 

[reddesert]

Donald's idea of the lens unit from an overhead projector is good because you would also get the 90 degree mirror unit, and IIRC the lens unit has some means of focusing.

Other than that, you likely need a lens that has focal length at least the diagonal of your piece of paper/viewing surface (shorter focal lengths likely have objectionable light falloff and aberrations at the corners), and as fast (large aperture) as you can get. The box needs to put the lens one focal length away from the paper and have some way of adjusting the length by a small amount (say 1/10 the focal length) to focus.

A simple meniscus or plano-convex lens may work. One source of meniscus lenses is photographic close-up filters. A +3 close-up has focal length about 333mm (13") which may be about right. If you get one in 62mm thread size, say, then it should have clear aperture of almost 60mm so f/5.5 ... not too bad but even faster would be nice. A +2 close-up has focal length 500mm if you want to make a larger camera obscura.

Older photographic lenses are a possibility, but ones in these long focal lengths are hard to find at fast speeds. Petzval lenses are fast but they aren't cheap any more.

 

[Donald Qualls]

Other projection lenses would also work, though you'd likely need to reverse them (i.e. put the end that's normally toward the slide or film strip to the world) in order to get a useful focal length.

Edit: D'oh! Focal length wouldn't change, you'd just be swapping which end is close to what. Never mind, they're fast but tend to be too short and aren't optimized for a large image circle the way overhead or opaque projector lenses are.

 

[Arthurwg]

I have a very interesting pamphlet, ""Sir John Herschel: Camera Lucida Drawings," published by Hans Kraus Jr. It shows several of his drawings with a great commentary by Larry Schaaf. You may be able to find a copy.

 

[Donald Qualls]

The "camera lucida" as used by Herschel and many landscape illustrators in the 17th and 18th century was a different device from a camera obscura. It was a prism used to view the scene with one eye, visually superposed on the paper seen with the other, and allowed for the hyper-accurate drawings that started to appear in the 17th century. I've seen them advertised in various sources to this day.

 

[Bill SOOP]

Thanks very much for the responses! Hate to admit it, but I’m looking to do this the cheapest way I can. Very much a shoestring budget, so I do appreciate the responses made with frugality in mind. As I get closer to getting this project up, I’ll definitely post my progress.

 

[Don_ih]

shoestring budget

Magnifying glass?

I was going to suggest a process lens (something from a repro camera or a Xerox machine). They're fast, sharp, cover a piece of paper...

Overhead projectors show up at thrifty stores for cheap. An old Artograph would be good, too.