Enlarger lens for camera obsura?

[Tempest09]

Over the years, as I've moved from place to place (apartments), I've always made it a point to set up a room (usually my bedroom), as a camera obscure. There's just something I enjoy about waking up to see the world projected on my wall, watching cars zip by, and people mulling around.

I initially began with a hodgepodge of cardboard and poster board gaffed up over a window with a small hole punched in the middle, simple, crude, somewhat effective, though pretty fuzzy.

My most recent set-up evolved into sections of mat board carefully butted together and taped over the window, with a large hole (about the diameter of a toilet paper tube) in the center surrounded by a paper slot designed to hold an index card. I then had a pile of index cards with varying apertures that I would switch out depending on the level of incoming light or desired DoF, along with an unaltered one with which to block everything. This design proved to be effective as well as versatile, although still somewhat fuzzy.

Well it's that time again. I've gone without my camera obscura long enough, and I'm ready to build and improve once again. I've a 50mm enlarger lens laying around that I don't much care for the print quality of. The idea of mounting it on a spare lens board and using it for a (hopefully) sharper projection has been rolling around in my head for a little while now. I'm curious as to whether or not anyone around here has tried using an enlarger lens for this sort of thing, and if the result was worth the effort. I'm considering either projecting onto a wall that is approx 12.5ft away, or combining an angled mirror to allow me to orient the lens upward and project onto the ceiling. The latter would at least allow some latitude to vary the distance between the lens and projection surface, although it would complicate the build design.

Any advice or insights would be greatly appreciated.

 

[Steve Smith]

You will need a lens with a focal length of 12.5 feet or 3810mm. That's quite a lot more than a 50mm lens!

It might be worth contacting a local optician to see if it's possible to get a +0.26 Dioptre lens element (or possibly +0.25 Dioptre).

Another source of simple lens elements is cheap reading glasses. I don't know if they go down to 0.25 Dioptres though. I think they are normally in 0.5 Dioptre increments. Might be worth a look though.


Steve.

 

[Newt_on_Swings]

Maybe with a cheap hand held magnifying glass lens?

edit:scratch that probably not lol, maybe try a large format shutter where you can finely turn the aperture size to get a sharper image instead of set index cards

 

[Steve Smith]

The problem is that the lens needs to be of a much lower power than those generally available.


Steve.

 

[Newt_on_Swings]

Haha yea Steve, I realized that in my cloudy morning mindset right after I hit submit, but went back to edit a simpler solution shutter alternative until a lens could be found.

 

[ic-racer]

Use a big pinhole.

 

[Jim Jones]

Lenses of +.25 diopters are occasionally available in the form of screw-in filters for macrophotography. I bought a Vivitar Fractional Diopter Close-up Lens +1/4 years ago at a photographic flea market, but couldn't fine one online in a somewhat quick search. B&H Photo Video has +1/2 close-up lenses that could project an image on a screen two meters from your window. They are awfully expensive. It is remotely possible that a junked lens could be disassembled for such a low-powered element.

 

[Tempest09]

Appreciate the advice. I may look around for some junk glass to disassemble and play with. (The local thrift store always seems to have something.) Worst case, I'll just do things as I've done before. I thought it might be nice to get the advantage of a sharper image along with a more mechanically controlled aperture. I really do need to brush up on the mathematics for this though. Thanks again!

 

[Matthew Rusbarsky]

Check out the lens finder at surplus shed. Right now they are showing this :SKU: L4639, Lens: PMN, Dia: 35.6, Focal: 4000, Coated $4.00.

 

[Steve Smith]

I really do need to brush up on the mathematics for this though.

The mathematics is easy. A lens power in dioptres is 1 divided by its focal length in metres. So a 300mm lens is 1/0.3 = 3.3 dioptres.

If you have two lenses and want to work out their combined focal length when placed together, just convert to dioptres, add them together then convert back again:

e.g. a 300mm lens and a 400mm lens together. We already know that 300mm = 3.3D. 400mm = 1/0.4 = 2.5D so the combined power is 5.8D.

To convert back to focal length, 1/5.8 = 0.172 metres or 172mm.

Another rule of thumb is that a 1 dioptre lens will focus parallel beams of light at a point 1 metre away. Double the power to 2 dioptres and the focus point halves to 0.5 metre, etc.


Steve.

 

[Jim Jones]

Matthew's suggestion of Surplus Shed is a great one. However, they don't seem to have quite the lens needed for Tempest's camera obscura. The 4000 mm lens is a bit too long, even for infinitely distant subjects. Their L 3855 is too short at 2600mm, but would focus on nearby scenes. It is larger and faster, and would form a much brighter image. It's depth of field would be much less. It could be used to focus on a screen a short distance from the far wall. Perhaps you can find a combination of two thrift store lenses of nearly the same power, one positive and one negative, to combine for the desired focal length.

 

[Maris]

I have set up several walk-in camera obscura rooms by blocking a window and inserting very long focal length lens in the opaque window screen.

Making a long focal length lens is easy and cheap and the image quality is ok because the large focal ratio minimises aberrations. I get two spectacle lenses, say +3 and -3 dioptre, from my local optometrist at about $20 each. When held together the powers cancel and the focal length is infinite. When separated, positive converging power (adjustable!) appears according to the distance between the lenses. The mathematical basis is given in Gullstrand's Equation.

 

[Jim Jones]

Thank you, Maris! That's the best help yet on the subject. I suggest getting a variety of screw-in close-up lenses, all of one diopter rating and size, because of their cheapness and ease of mounting. Probably inconsistancies in their actual focal length will result in a combination with the desired focal length. I have several, and may experiment tomorrow if the weather is decent.

 

[Steve Smith]

When held together the powers cancel and the focal length is infinite. When separated, positive converging power (adjustable!) appears according to the distance between the lenses.

Excellent. I wondered what happened when they are separated. I knew that the method of adding dioptres I posted above was only for lenses next to each other or very close.


Steve.