External flashes: what you need to know

The popularity of external flashes is relatively recent and is due to the strobist movement born in the United States under the impetus of David Hobby, a photographer who pushed the control of light, natural and artificial, to its maximum, in particular by separating the flash of the device. We will return to this movement in one of our next chapters.

When purchasing an external flash, there are certain aspects that should be highlighted. What should I know before buying a flash and its accessories?

The diffuser panel to soften the flash light

The light emitted by the flash is “hard” because it has not been modified. With the flash alone, the shadows will be harsh, which means that there is a marked contrast between the dark areas and the light areas and thus leads to unsightly effects such as dark circles under the eyes, nose and nose. chin. This is what happens when the flash light is aimed directly at the subject.

If the zoom of the flash is at the minimum, the light it produces is less harsh, softer but it is not yet the miracle solution.

To soften the light, most flashes incorporate, failing that, a series of small diffusers which make the light spread more homogeneously so that the subject is highlighted more.

Here you can find examples of diffusers to associate with your flash to reduce the unsightly effects on your subject.

Diffusing /reflecting white card and catchlights

Small in size, it suggests that it is not of much use. It hasn’t been included in all flashes for a long time, but now it comes with your flash systematically. A good example to demonstrate its usefulness would be the technique that uses the diffuser card for indoor portraiture. To shoot a portrait in a room with low ceilings, the trick is to direct the light from your flash towards the ceiling and the card outside the flash will take care of bouncing the light off your subject so that you get a result soft and seductive.

What about catchlights then? These are the reflections of light in the eyes produced by a light source. These bright little highlights add depth and life to the eyes in a portrait. What everyone calls “bright eyes” in photography is produced by very specific lighting structures that depend on: the main fill light, the height, angle and shape of the light source or even the distance between the subject and the camera.

And that goes for the interior as well as the exterior. By projecting the flash vertically, without the light bouncing on a surface and without affecting the subject, the simple small white card will be enough to obtain a nice reflection in the eyes.

Panels and controls

Simple panels please because sometimes all these buttons, added to those of the camera and the flash make things a bit complicated. With flashes, you need to change settings quickly to take advantage of every opportunity, especially for social photography or during events. A clear screen with the essential control buttons is appreciated when working with the flash.

The zoom of the flash, an element not to be forgotten

Many recent flashes integrate the possibility of modulating the zoom of the flash. In the same way as the zoom of the lens, that of the flash allows a greater angle of vision at the minimum and a closer vision when it is closed at the maximum.

In the “wide-angle” position, the zoom of the flash will produce a softer light while in the “telephoto” position, the light will be much more intense and concentrated and therefore harsher.


A flash that has a TTL function is really essential for social photography, events and reportage. TTL or “through the lens” means in English “through the lens”, a name which designates a function which calculates the flash flash adapted in a completely automatic way.

The TTL mode works like this: before the shot, a small pre-lightning flash is projected, it illuminates the scene and bounces all the way inside the lens to finally reach the camera’s light sensor itself. same. During this brief moment, like a small processor, the device collected the information necessary to modulate the flash.

A large number of brands have installed their own version of the TTL system in their device, each adding its own particular characteristics that make the use of TTL flash impossible without first reading the manual of each brand.

Most brands seek to obtain the same index of light reflected in the sensor, between 18 and 25% of the previous flash. For this reason, when an object, subject or scene is very bright or on the contrary very dark, it is possible that the TTL produced a flash which underexposed or overexposed the subject. No problem, there is a solution because this automatic mode allows corrections which consist of decreasing or increasing stops.

We can anticipate these “errors” and compensate by 1 or 2 stops less (underexpose) when shooting night portraits to overexpose darker objects. In the same way, we can increase by one or two stops (overexpose) when we are in the presence of a lot of light to compensate by underexposing in this case. Flash exposure compensation has a reason to exist, now is the time to use it.

When using TTL, it is recommended to calculate the pre-flash on a relatively small area such as the subject’s body to better control the lighting. This is the ideal mode for social photography, photojournalism or event photography where every gesture of the photographer must be quick and precise. He must move often, be ready at all times by observing his subjects, in case they make an interesting movement, quickly assess the best angles for his shots. Check for more on dzofilm.com.

HSS or FP: synchronize at high speed

If your flash doesn’t have this option, you will be limited when it comes to syncing your flash and device. Indeed, you will only be able to reach a speed of 1/250″. This value appears in the manual of each camera. So what if we are backlit, want to light our subject well, or light the scene using a high shutter speed?

Sometimes we have to use high shutter speeds (1/1000, for example). If our flash doesn’t have HSS, we’ll get like a blackout at the bottom of our shot. This effect happens because our flash doesn’t have HSS sync, so it’s not fast enough to keep up with the camera’s shutter speed.

The HSS or FP system allows many shots because it illuminates the entire surface of the sensor, while it is not completely uncovered at the moment of the single flash shot, it emits a series of very brief bursts of identical power, for the duration of the shutter. Thus, the entire sensor will have received identical lighting, and flash exposure becomes possible again. However, the power of a flash flash is conditioned by its duration, not by a decrease in the power emitted instantaneously. Here, the emission of a series of very short flashes will result in a decrease in the useful power, and the NG of the flash will drop as soon as the shutter speed is increased.

Guide number, more than just information

This is a fundamental notion for the use of flash. This is the power developed by the flash, expressed in theoretical range. In fact, the power of a flash is expressed in watts/second or in joules (equivalents), but this value is not very meaningful and does not allow simple calculations. Most cobra flashes are equipped with a zoom head that allows you to increase the range for the same power. In practice, the power of a studio flash is given in joules, and that of a cobra or macro is given in meters (range according to the guide number method, abbreviation NG). This is the theoretical range (in meters) of the flash at a sensitivity of 100 iso and an aperture of f/1. The mathematical formula to be applied is as follows, giving the aperture of the lens the value “n” and the distance the value “d”:

Guide-Number = Distance in meters from the scene x f-number (with ISO100)

A simple rule to know the guide number when we vary the ISO number is to double the guide number for two stops of sensitivity. A small example, if the guide number is 40 for ISO100, it will be 80 for ISO400. In all the manuals that accompany the flashes, you will find a distance scale that can guide you to know what distance is reached by the flash light based on its power and the zoom position.

I don’t want to bother you with boring mathematical formulas so to simplify, let’s say the higher the guide number, the higher the potency. Having a lot of power is important because when it comes to photographing, there is no question that the lighting leaves something to be desired. The wider the angle of view, the less distance the flash will travel and the smaller the angle of view, the more distance we gain.

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