Optical viewfinders are generally based on the reversed Galilean telescope principle, shown right. The design consists of a concave object lens and a convex eye lens. Light from an image scene passes through the lenses, as shown, and by arranging for the positions of the focal points of the two lenses to coincide, the viewer sees a reduced, erect, image of the scene. Clearly, when used the other way round, the Galilean telescope gives a magnified image.

A particular embellishment of this design is the Van Albada configuration. This is where a mask, or reticle, is imaged onto the object scene to assist in framing the image. In this type of viewfinder, a reflective reticle is deposited onto the flat side of the convex lens, or sometimes onto an intermediate glass layer between the lenses. Light from the object scene is reflected by the reticle back towards the concave lens. The concave lens has a semi-reflecting layer on it so that part of the light from the reticle is reflected back towards the eye lens. By careful selection of the radius of curvature of the two lenses, and the distance between them, an image of the reticle is formed at infinity, framing the object scene.

A CAD model of a Van Albada viewfinder is shown right. The lenses, which are designed to be moulded from acrylic, have mounting lugs for fitting inside the camera housing. The reticle is typically bright chrome and the reflective layer on the concave lens may be metallic or dielectric, with typically 50% reflectivity.

This design is now an integral part of the iCAM501 digital stills camera manufactured by Extronics Ltd.

Requirement:  Optical viewfinder for an intrinsically-safe camera

  Home    About    Consultancy    Projects    Testimonials    Contact    Newsroom