Copyright Michael Karbo, Denmark, Europe.

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    Chapter 35. Using the flash

    When there isn’t enough light to photograph, then a flash is used to provide extra light. Nearly all cameras have a little flash built in and this can often be extended with a still better external flash. Flash systems are found in many variants from the very simple and primitive to the very advanced and ”intelligent” systems. Furthermore, a flash can be programmed in several ways.

    Figure 172. The flash goes off – but how?

    A flash is an electric lamp, which is designed to let off one powerful light flash on command.

    The flash of light is sent into space where it, for example, hits the people to be photographed. It is the reflection of the flash’s flash of light, which returns to the camera, and gives light to the image. This is why you can’t, for example, use flash for photographing the moon at nighttime; the flash’s light seldom reaches more than 15 meter. There has to be people or things that can reflect the light for it to get back to the image sensor in the camera.


    The flash lamp itself consists of a glass tube containing the gas xenon. As soon as the flash is switched on, the battery starts by charging a condenser, which is connected to the lamp.  You can often hear the charge as a high whirring sound. When the condenser is fully charged, it has a voltage of several thousands of volts, which can be discharged on command.

    At the same moment as the electric charge is sent through the xenon gas, it sends a very brief (ca. 1/1000 second or less) but very powerful flash of light out. This is a full charge and the flash has to be charged again afterwards.

    The trick is getting the cooperation between the camera and the flash to work perfectly. A flash must be dosed correctly; there is no need to discharge the whole charge with every exposure. Ideally, the flash should give precisely the flash of light, which is required for a particular exposure. It appears (logically enough), that the closer the subject is to the camera, the less the flash required. Contrary to this, flash requirement is strongly increased, the further away the subject is.

    Figure 173. The built-in flash can certainly be used, but it’s not ideal. It’s too close t the lens and too weak for most purposes.


    The camera’s shutter has to be synchronised with the flash. This means that the shutter has to be open for as long as the flash takes to go off.  In practice this means that the shutter flies up first. Then the flash is fired. All this happens in the space of 1/1000 second, while the lens is open. Finally the shutter smacks down and the whole exposure has taken about 1/60 second. This synchronising between shutter and flash is done quite automatically.

    Figure 174. Sychronising between the flash and the lens’ shutter.


    Most cameras can synchronise flash with several different shutter speeds. It is worth taking note of the fastest shutter speed that can function with the flash. This could be a shutter speed of 1/60 second, but shorter synchronising speeds are still better if you, for example, use the flash outdoors with a so-called fill-flash, which works as a supplementary source of light. In principle, you take a completely ordinary exposure where there is, in fact, enough light.  The flash is used to give extra light to the main subject, which might be shadowed a little.

    Some cameras can synchronise the flash with a shutter speed of 1/250 second (like Canons G-series), and others can get right down to 1/500 second (like Nikon D70). This is an advantage, if you are working with fill-flash.

    The internal flash

    All cameras have a little flash built-in in the camera body itself. It can be quite good but this is certainly not always the case because it is often much too weak. As it uses the same battery as the camera itself, the flash can easily drain the camera for power. Finally, the internal flash is placed much too close to the lens and this gives the well-known and irritating red eye, which ruins so many exposures.

    The best version of an internal flash has a pop-up function. Most of the time the flash is hidden in the camera body and you tip it up when you want to activate it. This is extremely practical and user-friendly and at the same time the flash is further away from the lens, which means there is less chance of red eye. The pop-up flash is much better than a model that is fixed in the camera house.

    A pop-up flash can be excellent. It is easy to use because the flash is only active when it is tipped up.

    Flash control

    In the ”olden days ” you could only fire a flash in one way, which was with full force. The camera’s diaphragm was the only means of controlling the light; the closer the subject was to the camera and the flash, the smaller the aperture. There was usually a table on the back of the flash according to which you could set the diaphragm.

    Since then flashes have been developed, which can dose light in bigger or smaller flashes. This means that is necessary for communication between the camera and the flash; the camera has to be able to make a request for a flash charge, which fits the subject and the distance there is to it.

    The task consists of measuring the flash’s effect while it is fired. The charging has to be able to be interrupted on time, as soon as the image is well-lit. Several systems have been made that can do this. There can be a little sensor on the front of the flash, which can measure the reflected light. Some Single Lens Reflexcameras have correspondingly small sensors in the camera body, which can measure the reflected flashlight directly from the film’s surface. This system cannot, however, be used in digital cameras.

    Modern cameras have different “intelligent” systems, which control the flash in cooperation with the cameras’ electronics. A camera can, you see, use the image sensor for measuring the light, which is reflected from the flash through the lens. In this way the camera can manage to stop the flash as soon as the image sensor is “filled up” with light.

    E-TTL with pre-flash

    Other flash systems use a pre-flash. A little glint comes from the flash, 1/20 second before the real flash is fired. The effect of the first flash is registered through the lens by the image sensor. The camera can then quickly calculate and set the strength of the real flash charge.

    Figure 175. The best flash systems fire the flash several times to measure and assess the effect.

    This system is called E-TTL measurement (Evaluate Through The Lens). It is an automatic flash system, which cooperates with the camera’s light meter and several programs. The result is a very effective and precise dose of flash in any situation. E-TTL is used, among others, in Canon’s external Speedlite EX-flash-units, which fit to several of Canon’s cameras – also with the compact cameras in the G-series.

    Measuring and dosing flash is an area, which is still being developed; both Canon and Nikon use, for example, the so-called 3D-measurement in some of their models. The autofocusing system is involved here, which measures the distance to the main subject. This value can be a part of the calculation of a correct dose of flash.

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