THE DEVELOPMENT OF THE OPTICAL SYSTEM

Justin Marshall. August 2000

As an artist I had little technical or theoretical knowledge of optics before starting this project. The optical systems were developed through some research, discussions with optic experts, but predominately through my own practical experiments.

Due to the small size of our obscura (4ftx4ftx8ft) it was not possible to use the most common optical setup in camera obscura designs. This involves a single lens in the roof of a room with a 45û mirror projecting an image onto a central table. In our obscura the single viewer sits in the centre of the room and images are projected onto a tables placed around the edge of the room and back projected onto screens fitted to the walls of the room.

The optical design therefore involved 4 lens and mirror sets fitted into the dome of the obscura and 3 lenses fitted into the arches on the side of the structure. There were a number of factors which I had to consider when investigating the possible optics designs. These were:

- Our limited budget which restricted the size, quality and number of lenses which could be used.

- The desire to create the images of the correct orientation.

- The need to create images which were bright and sharp.

- The position of the optical systems so that the viewer did not interfere with the image.

- The need to project images of an appropriate size onto the tables and screens.

These considerations had to be played off one another when arriving at the final design of optical systems. In order to keep images bright and sharp as few lenses and mirrors and lenses as possible should be used and the distance between lens and projection surface kept relatively small. However, in order to produce images of the correct orientation at least two mirrors are needed. In addition the optics in our obscura had to placed high enough into the dome to stop the viewers obstructing the images or hitting the mirror system.

I spent some time exprimenting with various systems for producing images the right way round. Although a system which corrected the inverted image produced by a single lens was developed, it involved 3 mirrors and 2 lenses. However, this system limited the field of vision and reduced the quality of the image to such a degree it was decided not worth employing.

The shape of the lens also has an impact on the quality of the image. Standard plano-convex or double convex tend to create blurred areas around the edge of the images. Meniscus lens, like the ones used in spectacles, produce a wider area of well focused image and were therefore selected for our obscura. 6cm diameter lenses were selected as they let in a reasonable amout of light and therefore produce a bright image and are relatively cheap. These types of lenses can be obtained from large opticians or from specialist lens producers.

The final optical design is illustrated below in a cross section of our obscura. Two types of lenses are used, the 4 fitted into the dome are 2m focal length, 6cm diameter meniscus lenses, the 3 fitted into the archs are 33cm focal length 6cm diameter meniscus lenses.