The use of lenses in the Obscura has been common place of over 300 years now, yet to figure out how it is done is still somewhat difficult. For the project I attempted to find out how to use lenses with the Obscura but had many problems. Firstly the intention was to use lenses from cameras which would offer many choices in lenses. This proved to be extremely problematic. As lenses are designed to come to a focus on the picture plain (inside a camera) the projections created were very small and the distance from the lens to the viewing surface was also small. (Often less than 5cm) This would not do and ultimately would not be useable in the project.

The other type of lens that was known to be used in the making of a Camera Obscura was what I knew as a bi-convex lens. This lens (shown to me in A History of Pre-Cinema: Volume 1 by Stephen Herbert) is made of glass and is curved on both sides. After much investigation I found my-self lost as any bi-convex lens I has access to still would not work for the project. After much more research and help from several colleagues I found that a lens blank for the optometrist would work. (Confirmation of this was found in the video below) These lens blanks are the blanks used to create eyeglass lenses. Important items of note here are that these lenses are measured in diopters. You need a positive diopter to get an in focuses image and the formula to find that distance is:

Focal distance (in meters) = 1/diopter
(Thanks to Andrew Wright for the formula and explanation)

These lenses can be purchased at any place that sells glasses for relatively cheap. Also it is important to ask them for no coatings as they can have an effect on the projection. They can be made from glass or plastic, and it is also of note that if needed diopter lenses can be added or subtracted to get the correct focal length. (ie. if you need to focus on something 2m away two +.25 diopter lenses could be used together)

Note: we were able to find lenses at FYI Doctors in Regina and they were very helpful. We are particularly grateful to Nadine Elkew who understood what we were doing and assisted us to understand what could be done and what could be available.

Easy to use chart (Common diopters)

+0.37 diopter: 2.66 meters
+0.50 diopter: 2 meters
+0.62 diopter: 1.6 meter
+0.75 diopter: 1.33 meter
+0.87 diopter: 1.14 meter
+1.00 diopter: 1 meter
Source: http://www.bullseyepistol.com/wong1.htm

Also for people wishing to remove UV coatings from their lenses: (have not tried but its worth a shot)
http://makezine.com/2007/02/17/how-to-remove-eyeglasses/
Another method is the use of a product called Armour Etch