negative in front of camera obscura

[lukeman89]

I'm thinking of placing a 4" x 5" negative a few inches in front of a truck bed sized camera obscura and developing onto cyanotype coated sheets. Does anyone know if putting a negative in front of a camera obscura can be succesful? I'm hoping it'll just function like a giant enlarger.

2nd is cyanotype capable of long exposures (12 hours or so) or will it degrade too significantly over that time.

[TPF Sponsor]

[compur]

Interesting idea.

I assume you mean that you would place a cyano sensitized sheet inside a large camera obscura (turning it into a pinhole camera).

[Helen B]

If you arrange for the neg to be well and evenly illuminated (with UV), and if the focal length of the lens permits, you should be in with a chance, though the UV-absorption properties of the glass in the lens and possibly the illumination method could have a significant effect. My guess is that cyanotype suffers less from LIRF (low intensity reciprocity failure) than silver-based printing, but that really is a guess and it doesn't mean that it will never happen. Of course the neg cannot be closer to the lens than the focal length of the lens, and you would have to try to focus for UV.
Best,
Helen

[Proteus617]

Rig up a UV light source (or use the sun), mix up some chems, coat some paper and contact print your negative as a cyanotype just to get a baseline exposure time. Then check out the plans for your camera obscura. A rough comparison of negative size to aperture and some fun with the inverse square law to compensate for the increased distance to the light source (if you are using artificial light) will give you some idea of your exposure times. A quick guess would be that a true camera with a truly massive lens might give you a 12 hour exposure time. A camera obscura with it's pinhole aperture would result in exposures that last for many months, if not years. The big danger with exposures that last for days and weeks would be spontaneous development due to humidity.

A better idea would be to scan your 4x5 and have a very large digital negative printed then contact print to cyano from that.