Standard Camera Focusing Screen

A focusing screen provides a flat surface on which an image can be precisely focused and allows the eye to see the whole image at once for framing and composing. It normally comprises a ground glass screen, a fresnel lens and one or more focusing aids.

A ground glass screen scatters light in all directions allowing the whole image to be seen, but the picture you see is brighter in the centre. Coarse etching of the glass reduces this. A fresnel lens comprises a series of stepped, concentric rings. Each step is a tiny portion of a curved lens (the same curve for each step), and bends light the same degree. Inclusion of a fresnel screen combines maximum brightness with even illumination. A number of stepped concentric rings, moulded in plastic, are in contact with the underside of the focusing screen. The effect is an even image with undisturbed detail sharpness. Most modern SLR focusing screens include a central split-image rangefinder area and a circle of microprisms in the centre of the screen. These are focusing aids, and are designed to work most efficiently with standard focal length lenses. With other lenses, and with extension tubes, part of these central areas may black out, so use the plain area for focusing.

Microprisms: the criss-cross pattern of the prism circle is clearly visible and shimmers when the image is out of focus. The pattern and shimmer disappear when the image comes sharply into focus. Use this area of the screen for high contrast patterned or textured surfaces.

Split-image spot: the split rangefinder area is two semi-circular wedge-shaped prisms. These make an out-of-focus image appear as two off-set halves which apparently join up when sharply focused. A horizontal split is fine for vertical lines, but a diagonal split works equally well with vertical and horizontal lines. Use the area for bold straight lines, and when focusing in dim lighting.

With focusing aids in the centre of the screen you tend to position the main subject centrally too — which may not be the best place from a compositional point of view. Focus before finalizing composition.

A typical screen may include a split–image spot for rapid focusing on straight lines or in dim light. Microprisms enable fast, accurate focusing especially on textured subjects. Use the plain area of the screen for close–ups, with wide angle and telephoto lenses and for general or fine focusing.

Top: a fine ground glass screen scatters light enabling the entire image area to be seen, but the centre is brighter than the edges. A coarser screen would improve this unevenness but only at the expense of screen brightness. Below: a fresnel lens ensures even image brightness throughout. When combined with a fine ground glass screen an improved focusing screen is produced.

Whether horizontal (as illustrated) or diagonal, a split–image centre spot shows an out–of–focus image as two separate off–set halves. Select a part of the subject which contains a distinct line on which you can focus. You can adjust the composition once you have obtained sharp focus.

If the image is in focus the line which you have selected will appear continuous across the centre spot. A horizontal split is best for a vertical line in a landscape composition; sight a horizontal line in an upright photograph. A diagonal split–image is easy to use for either type of composition.

Possibly related posts: (automatically generated)
Standard Camera Focusing Screen

• Cameras Focusing Screens and Viewfinders
• Taking Sharp Pictures continue...
• Digital Electronic Cameras, basic knowledge and care the Lens
• Taking Sharp Pictures
• How to use your Camera to shot good Pictures/Photo (Infra-red index; Mirror lock and Shutter lock)
• Interchangeable Lens Cameras
• How to use your Camera to shot good Pictures/Photo (Depth of field preview)
• Interchangeable Lens Cameras continued
• Camera Exposure metering systems continued
• Camera Exposure metering systems