Of all the methods of close-up photography, the single most useful technique is a combination method referred to as "close-up flash". It combines, with other features, speed of operation, extreme depth of field, and the use of slow speed films to get quality images. Although it may appear confusing, it is quite simple to learn. You will need a SLR camera, some type of close-up lens (or extension tube), and a small electronic flash unit. With practice, the whole process of composing, lighting, and exposing can be accomplished in less than a minute.

The first step is to determine the necessary magnification. Since you know the size of the film (about 1 X 1.5 inches), you need only measure the size of your subject and then divide that into the film size, to get the magnification. As an example: if a fly pattern 3 inches long fills the entire frame length (1.5 inches) the magnification is X1/2. The camera can be hand held, or a tripod used for support, the fly can be left in the tying vise.

A number of camera functions are tied to the focal length of the lens. One of these is the f-number, or relative aperture of the lens. That number (engraved on the lens housing) is the ratio between the focal length and the diameter of the aperture. A lens that has an aperture of one-half it's focal length is said to be an f/2 lens. In small subject photography the speed, or largest aperture of the lens, is not important. What is important is how effectively small the f/number of the lens can be made. Producing an image through a small aperture has two basic effects. The subject is focused on the film with the "sharpest" image; and the focus depth of field is greatest.

When doing most types of close-up photography it is nearly always desirable to have the greatest depth of field possible (that is having as much of the subject rendered as sharply as possible). Most normal lenses close down to f/16 or f/22, though some macro-types may go down to f/32. However, the relative aperture (f-number) with the lens abnormally altered by a close-up attachment is called the "effective aperture". This value must be calculated to get the correct exposure setting for using electronic flash. Effective apertures for smaller common f-stops, at frequently used magnifications have been calculated in the table below.

AT MARKED f-NUMBER AT X1/4 X1/2 X3/4 X1

f/16 f/19 f/22 f/27 f/32

f/22 f/27 f/32 f/38 f/45

f/32 f/38 f/45 f/55 f/64

In order to calculate the correct exposure using electronic flash, it is first necessary to know the basic light output of the flash. That value is called the "guide number", and indicates the amount of effect a particular flash will have on a film of given speed.

Electronic flash units will have one guide number for each film speed unless there is more than one power setting. The manufacturer usually only lists one guide number, for a specific film. There is an easy way of determining any other guide number using the exposure calculation dial on the back of the flash unit. Simply follow these steps:

1. Set the film ASA speed on the calculator.
2. Look for the f-number opposite the ten foot distance mark.
3. Your guide number for that film is equal to the f-number times ten.

Knowing the effective aperture and flash guide number will allow you to calculate how far from the subject your flash needs to be for any film you use. The formula below uses an example of a f/16 lens making a X1/4 image.

SYMBOLS DEFINITIONS EXAMPLES

GN Flash guide number 32

EA Effective aperture 19 (f/19)

FD Flash-subject distance ?

FORMULA FD = GN/EA

FD = 32/19

FD = 1.68 feet from subject

In terms of accuracy for exposure purposes a close approximation of the calculated flash distance is all that is needed. If your film, magnification, and flash source never change you only have to do this calculation once.

There are three things to consider when using electronic flash in this manner: the power of the flash unit, proper synchronization of the flash and color correction, and close flash effect correction.

The type of flash used should be the smallest and cheapest electronic unit that is available. Large power output, or automation, is not needed. If you currently have a thyristorized automatic unit, it should be used in the manual (M) mode. Basic flash power is related to your choice of film. For ease of operation it is best to have a flash-to-subject distance similar to the camera-to-subject distance. A high powered flash used with fast film will have an inconveniently long flash distance and exposure will become inaccurate. If your ASA 25 flash guide number is not greater than 45 you will obtain satisfactory results using ASA 64, or ASA 100 speed film. For any guide number greater that 45 it is better to use a slower film like ASA 25. Be sure to synchronize your shutter for electronic flash operation. Choose what ever shutter speed you wish, however, speeds less than 1/30 second should not be hand held.

Color correction of your flash unit may be necessary. Unless the flash tube, or its covering are definitely yellowish in color the output of the unit will almost certainly be too blue to match daylight type films. Film manufacturers recommend a corrective Wratten 81A or 81B filter. I prefer a less expensive CC20Y gelatin filter, cut and taped to the flash head. Either will work.

Two types of exposure compensation may be needed for this type of flash use. If you use a corrective filter (regardless of the type chosen from those listed), a one-third stop exposure correction is necessary. The second correction is needed for small flash units having an ASA 25 flash guide number of 85 or less. These small flash units were not originally designed to be used with suggested slow films and small apertures. Their efficiency is reduced when they are used very close up. That loss amounts to halving the recommended flash guide number of the unit. It doesn't matter what the guide number is, as long as you know its value.

As an example assume you had a flash guide number of 40 for ASA 64 film. Half that to obtain the new guide number of 20. Reduce that by another estimated one-third stop for filter correction (if used). That gives a final working guide number of about 17. If you have an effective aperture of f/22 the formula will give:

FD = GN/EA

FD = 17/22

FD = 0.77 feet (about 9 inches)

Once you have determined the desired magnification it is a simple matter to set the camera to the smallest f-stop, focus on the subject, hand hold the flash at the desired angle and distance, and expose the film. This entire operation can be accomplished with a hand-held camera and flash through experience. For a static subject a tripod is sufficient, the flash is hand held only at any desired angle. You may find that the best photos are made with the light coming from the head of the pattern. If you prefer to have reflected light on your subject, make sure the flash is pointed at a shallow angle and place a piece of crumpled aluminum foil on the opposite side of the fly. In this type of close-up photography back lighting is not necessary, and may actually cause an over exposure of the film.

Because your camera, lens, flash and subject size will not vary the distance calculation need only be done once for your equipment. Getting just the image you want is left to your artistic talent.