Close-up and macro photography for entomologists

Focusing screens

A typical focusing screen for general purposes has a split-image rangefinder in the center, surrounded by a ring of microprisms. The remainder of the screen appears to be ground glass, but in fact is backed by a fine Fresnel lens (an arrangement of concentric ridges) that helps to keep the image bright all the way to the corners of the screen.

If you use a close-up lens or a macro lens, then this type of focusing screen will probably be satisfactory. However, if you try to use more specialised equipment such as bellows macro lenses for high magnification, then you will almost certainly find that the image in the viewfinder has become much darker than normal, that the split-image and microprisms do not work and obstruct the centre of the image, and that the rings of the Fresnel lens are visible.

Some 35 mm and roll-film cameras have a range of focusing screens made for them, one or more of which may be suitable for use at high magnifications.

Plain screens

Plain screens are available wihout the central split-image and microprisms, and these are suitable for ×1 to about ×4. At higher magnifications, the image will still be too dark, and the grain of the ground glass may make focusing difficult. Examples include the Olympus 1-4, 1-4N and 2-4.

Some plain screens have a grid of horizontal and vertical lines. These screens are intended for architectural photography, but they are also useful for lining up documents, pinned insect specimens and other symmetrical objects. Examples include the Olympus 1-10.

If your camera takes interchangeable focusing screens but the manufacturer does not produce a plain screen, you may be able to obtain a Beattie Intenscreen; they are available with and without grids.

Clear screens

Clear glass focusing screens provide the brightest possible image in the viewfinder, but are only suitable for use at high magnifications, from about ×3 upwards. In the centre of the screen there is a crossed pair of fine vertical and horizontal lines. Examples include the Olympus 1-12.

Clear screens provide a bright image in the viewfinder, but they also make everything look as though it is in focus, when in fact the depth of field is extremely shallow. In order to focus, you need to use a technique called no-parallax focusing.

First, you need to make sure that you can see the double lines clearly, either by adjusting the viewfinder (if your camera has this facility) or by adding a supplementary lens to the viewfinder to correct for your eyesight (most cameras have this facility). Once you can see the double lines clearly, move your head slightly from side to side. If the crossed lines do not move relative to the subject, then you know that your focus is spot-on. If the crossed lines do appear to move, then your focus is not correct. The best way to focus at these magnifications is to move the entire camera+bellows+lens assembly using a focusing rail.

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Close-up and macro photography for entomologists