Copyright Michael Karbo, Denmark, Europe.

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    Chapter 37. External flash

    The best lighting is achieved by using an external flash, which is typically installed on the top of a camera. The external models are usually system flash. This means that they cooperate with the camera to find exactly the right dose of light. The larger camera brands all have their own flash systems.

    Flash is an improvised solution – a flash often gives an unnatural, hard and direct light. The ideal thing is and always will be is to use natural lighting. But when there is not enough, we have to turn to a flash. An external flash has many advantages over the small built-in flashes:

  • Powerful light with a big range.

  • Own power (batteries).

  • Good distance between lamp and lens (no red eye).

  • The lighting angle of the flash can be changed so that it follows the camera’s angle viewing (zoom flash).

  • The flash head can be angled and turned (bouncing).

  • System flash with pre-flash, advanced light measurement and control (like E-TTL) and many options for expansion.

    All this means that an external flash is quite a different tool than an internal. The disadvantages are first and foremost the price. A good system flash can cost up to 3000 kr. On the other hand it can be used for years and for lots of different cameras, as long as they are from the same manufacturer.


    An external flash is usually installed in a socket on the top of a camera. It communicates with the camera via some electric contacts:

    Socket for the installing of a flash, also called a hot shoe. (Canon G2)

    The more advanced flash systems are proprietary, so that you can only use Canon’s flash in Canon’s cameras, and so on. The electric contacts are simply designed differently from the one brand to the other. The contacts are used for sending information like ”the flash is ready”, ”select diaphragm”, ”result of TTL-light measurement”, ”zoom setting” etc. from the camera to the flash and opposite. It is these signals that make it possible for us to achieve the good results given to us by modern flash systems.  


    Two different flash sockets. Both have a central contact, which communicates with the camera body, but the small contacts are different. At the left, you can see an Olympussocket with three contacts and at the right a socket on a Sony camera, where there are five contacts for sending and receiving signals.

    So you can’t press any sort of flash down in a system socket – the flash and the camera have to be of the same brand. Some flash manufacturers like the German Metz can, however, supply flash units that can be made to fit the different brands.

    The system flash

    Advanced flash systems can be found for all mirror SLR cameras, but within compact cameras, it is Canon, which has done the most with system flash. The cameras in the G-series can use most of the advanced functions found in Canon’s own flash. You can see here a really good combination; a camera from the G-series with a Speedlite 420EX.

    The camera can, for example, be set to take exposures in 1/250 second lighting time and then the flash and the camera cooperate to find the best exposure parameters. The flash is dosed with just precisely the necessary amount, and it succeeds, by and large, every time.      


    Figure 176. The flash tells us that the exposure is a success with a light in the yellow-green lamp. (Canon 420 EX)

    Power output of the flash

    A flash always has a certain power output. This is technically described with a guide number. This is often used by manufacturers instead of the flash’s range, measured in meters.

    Here are two conditions you have to be aware of. Because the flash’s range changes in relation to:

  • ISO values

  • The angle view of the lens

    The bigger the camera’s sensitivity, the longer the range of the flash.

    When the ISO-values are multiplied by four, the range is doubled. The angle view is also important. Most cameras with internal flash cannot vary the angle of the lighting. This means that the flash has its maximum effect when the lens is set at wide-angle.

    At the same moment as you zoom in, the angle viewing is reduced and a part of the flash’s light is wasted. External flashes usually have a little motor, which moves the reflector behind the flash lamp so that it spreads light over the same angle as the zoom lens is set to. This is really smart. The flash’s range is reduced in compact cameras when you photograph with tele effect.

    The range of the flash, on the other hand, is increased if the camera’s sensitivity is increased. All together this can give the values seen in this table:





    1X (wide angle)

    0,5- 3,2 meter


    1X (wide angle)

    0,5- 4,5 meter


    1X (wide angle)

    0,5- 6,4 meter





    4X (tele)

    0,5- 2,0 meter


    4X (tele)

    0,5- 2,8 meter


    4X (tele)

    0,5- 4,0 meter

    Range of flash in a compact camera. (Nikon 5400)


    If the flash is a zoom flash, then the flash’s force will be increased, the smaller the angle is. Here the force will be more powerful with 4X zoom, as the energy of the flash is collected into a smaller area.


    The best cameras can work with several flash units at one time. There has to be master flash, which controls one or more external slave flashes. Everything is done wirelessly.

    The advantage of using several flashes is that you can produce a more graduated lighting where you can yourself determine where the shadows should fall.

    In Canon’s cameras, the system requires that you invest in a relatively expensive master flash. This can control a cheaper flash like, for example, the one shown earlier 420EX. In cameras like Minolta A1/A2 and Nikon D70 the internal pop-up flash can be the master for one or more external flash units.

    Figure 177. System flash specially designed for Nikon’s DSLR cameras. (SB-600)

    Guide numbers

    The flash’s power ought to be described with GN or Guide Numbers. These are values, which can be used to calculate range with this formula:

    Guide number/diaphragm = distance in meters

    If the flash, for example, has a guide number of 30 (with ISO 100), it can light 7,5 meters forwards with diaphragm 4, as the formula gives: 30/4 = 7,5.


    Guide number

    Max. Distance with f4



    3,9 meters



    5,5 meters



    7,5 meters


    If you fire a flash off right in the face of a person, the result is seldom a success. The light is too hard, so that the skin looks “burnt out” – it looks completely white without any colour nuances.  A direct flash can also give strong shadows, which are visible in the image. You can counteract the effect of the hard flash in several ways. The light from the flash can be softened by bouncing or with diffusion.

    Figure 178. This image is taken with flash and is quite good – apart from the big shadows on the wall.

    With diffusion a plastic hood is put over the flash lamp. This decreases the force of the flash and makes the light much softer.

    You can also soften the light by letting it reflect from a wall, the ceiling or something like that, which isn’t too far away. This is called bouncing. You let the flash, for example, throw its light up to the ceiling. Doing this spreads the light over a greater area and it is softened on its way to the subject, which it hits from several angles.

    An external flash is designed to bounce; the lamp head can be turned in all sorts of directions. But as this weakens the strength of the light, you have to be sure the flash is correctly dosed. E-TTL is a great system here; first the little pre-flash is fired into the ceiling, and the camera registers the power of light, which is reflected from the subject. A fraction of a second later the real flash follows and the result is a perfectly exposed image. The advantage of E-TTL is that the light measurement always works regardless of the angle in which the flash is fired. The camera measures the reflected light, irrespective of where it comes from.

    Figure 179. Canon G2 with external flash and a more extreme diffuser from LumiQuest. 10% of the light is sent forwards while the remaining 90% bounces up into the ceiling so that it reaches the subject in more indirect ways.

    The end

    I hope you found my book useful. Please forgive me the typos, a poor translation and flaws in the layout. The contents, however, should be all right.

    Michael Karbo

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