Flash Boosters
Close-Up Light Guide Reflective Booster Fresnel Booster Back to Photography Back to Brantacan
The built-in flash source in many cameras may have a guide-number – GN – in the region of 10 or 12 for 100 ISO film. With a GN of 11, this means that f/11 can be used at one metre, and f/5.6 at two metres. These values may not be suitable for some purposes. Given the restriction on shutter speed when electronic flash is used, the aperture may let in so much light that the flash is not ineffective in sunlight. This is a problem if, for example, the flash is being used to freeze motion. This page describes some ways of increasing the effect of a flash source.
In designing a reflector you should remember a basic law of optics – you cannot reduce both the divergence and the diameter of a light beam. If you make a reflector that tapers away from the light source, you will get a smaller source with a greater divergence. If you make a reflector that expands away from the light source, you will get a bigger source, but you may also get a smaller divergence. Click > Pic C
Boosting a separate flash gun
If you have a separate flash gun of the non-dedicated automatic type, not providing through-the-lens metering, a simple trick may give more flexibility. The makers often provide two settings, for example f/4 and f/8, at which the device will set the light output automatically. But if you want to use smaller apertures to get depth of field for close-up work, you have a problem. One solution is to cover exactly three-quarters of the measuring aperture with black tape. A neutral density filter will probably be more accurate, but will certainly be more expensive. The flash gun will then detect only a quarter of the light, and will therefore emit four times as much as it normally would, in order to satisfy the detector. So you can go two stops smaller on the camera. You will still be limited by the maximum output of the unit, but that seldom matters in close-up work.
All the devices described here will give a somewhat artificial effect, but they are lighter and less awkward than a separate flash gun for use in the field. They may at least give service as fill-in devices for harsh shadows. They also reduce the point-source effect of the simple flash source, and therefore produce softer shadows. A very good source of advice on illumination and many other topics is John Shaw’s Closeups in Nature (Amphoto), ISBN 0-8174-4052-6 paperback, -4051-8 hardback. This book includes many photographs of high quality and offers vast amounts of practical advice.
For close-up work the light from the internal flash source is often adequate, so the close-up light guide is really a device to get the light past a long lens. A 90 mm f/2.8 macro lens used at magnifications of more than about X 0.4 will probably prevent some flash light from reaching the lower half of the subject. A simple light guide will prevent this, and it will also enlarge the effective size of the flash source, softening shadows and creating a slightly more natural effect. A simple design uses a rectangular cross-section lined with reflective plastic, such as aluminized mylar. Aluminium foil is hard to keep smooth and flat, though some roughness may help in diffusing the light. The reflector can cover the whole inner surface, or it can be restricted to the half nearer to the flash source, to provide a softer edge to the source. The reflector can rest on top of the lens, as long as it does not impede the motion of the lens barrel when focussing, which could damage an autofocus lens.
Although the flash tube is a small horizontal line, there are three blobby catchlights in the frog’s eye above, photographed using a reflective light guide. Further diffusion could have been obtained by using a piece of tissue paper over the light guide, or by replacing some of the reflector by white card.
This and the Fresnel booster were intended for use with a camera lens in the region of 200 mm, but the principle will work with other lenses. The purpose is to redirect some of the light which would normally be used with a wide-angle subject. A built-in flash source will often be able to cover the cone of a lens of 35 mm focal length, so most of the light will miss the area covered by a telephoto lens. Comparing 200 mm with 35 mm gives a ratio of about (200 : 35)2, which is about 32. If we can redirect some of that light we will have a greater freedom in the choice of apertures. The diagram shows one way of achieving this, using a tapered tube made of white card. The narrow part is lined with reflective plastic. The angles are chosen to create four new images of the flash source. Allowing for some inefficiency we can get about four times as much light as with the bare flash. This represents two stops of aperture at a given distance. It also represents a doubling of the useful range for a given aperture – in effect doubling the guide number.
The first picture below shows how to calculate the angle of the cone. The principle is simple – at the middle of the reflector the two marked angles are equal. The throat of the booster needs to be only a little larger than the face of the flash source. The angle of the reflector should be as wide as possible, subject to being able to fit it above the camera lens in line with the flash source. The second picture shows an example of the patch of light made by a booster. The patch was designed for a 200 mm lens, but photographed using 70 mm to show the whole patch. The yellow outline shows the area covered by a 200 mm lens. The booster needs to give a slightly wider cone of light than this, as there is no margin for error as it stands.
A booster like this may also also be added to a separate flash gun, remembering that the gun is well off the lens axis. If the flash gun is not pointed at the subject, the booster needs to be designed to allow for this, to avoid uneven illumination.
The Fresnel booster is potentially more effective than the reflective type, but it needs more care in design and alignment. The booster uses one or two flat Fresnel lenses. These are plastic lenses sold for use as magnifying or reading. The same type of lenses are used in some overhead projectors and in lighthouses.
The first step is to measure the focal length. A simple method uses a pale wall facing a window. The lens will focus the distant scene on the wall. The rough surface of the lens should be towards the light, and the smooth side towards the wall. When the image is as sharp as possible, the distance from lens to wall will be about one focal length – let’s call this F. If it is around 20 cm or more, two Fresnels should be used, giving about half the focal length of one alone.
The second step is to calculate the position of the lens. The lens will normally be rectangular. It will be placed a little less than one focal length away from the lens. If the height of the Fresnel is H, and the focal length is F, the angular coverage – A – is roughly H / F in radians. If the focal length of the camera lens is f, for example 200 mm, and the height of the film is h, for example 24 mm in a 35 mm camera, the required angular coverage – a – is h / f, which is 24 / f for 35 mm film. We now know A and a and so we can calculate the factor by which the divergence is to be reduced, namely a / A – let’s call that X, which is hF / Hf. In an actual example, H was 59 mm, h was 24 mm, F was 95 mm and f was 200 mm, so X was about 0.2.
From this we can calculate the position of the Fresnel lens. If the distance of the Fresnel lens from the flash tube is D, the approximate formula is D = F (1 – X), which is 0.8 F. A more accurate calculation gives D = F / (1 + X), which is 0.83 F. The focussing of the beam should not be pushed to the limit, because of the difficulty of getting perfect alignment, especially as the flash source is significantly above the axis of the camera lens. We should also remember that the focal length of the camera lens may be a little less than the nominal value, especially with a zoom lens. So the Fresnel lens should be placed at perhaps 0.9 of the calculated distance, which will make the light beam a little wider.
The Fresnel lens can be mounted at the front of a cone of rectangular cross-section, with the rough surface away from the flash source.
In the example a factor of five was obtained, giving a factor of five in the guide number, or about four stops in aperture. In practice the gain will be less, because the flash tube is not a point source, but a horizontal tube. Nevertheless, the Fresnel booster can be quite useful.
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