The Flash Output Power

The output power of electronic flash is a bit of an enigma. I searched but cannot find an explanation about this specific term. In electronics, power is the voltage multiplied by current in a circuit which is expressed in watts. In a layman’s language, it refers to the energy produced in an electrical device.

But when photographers engaged in flash photography talk about the powers of electronic flash, we say that – "This unit (or model) is more powerful than the other." Clearly, we meant that "The light output of this unit is greater than the other." But according to wikipedia, energy (light output) is measured in "Joules." Isn’t this confusing?

Let us review some properties of flash output power:

• To increase light output power, flash capacitor value and wattage of the xenon tube must be increased, too. That explains why the high wattage AC powered strobes has huge flash tubes and plenty of flash capacitor stacked together in parallel.

• Portable electronic flash manufacturers do not publish the output power of a given flash model. Instead, they express it in guide numbers. More powerful units have bigger guide numbers.

• Output power of portable electronic flash is also determined by the voltage source or number of cells that powers the unit. See table:

•High output power allows an increased effective working distance of a given flash unit without having to open your lens with large apperture. Increasing the working distance further by opening up the lens will also work, but at the expense of depth-of-field. Another way to increase working distance is to use a higher ISO setting, but it gives you more digital noise in the picture.

• Flash output power weakens as its light travels to the subject depending on its distance. The further the subject is from the flash, the less light intensity it will receive. So the less light will be reflected from the subject back towards the camera.

In an effort to extend the flash to subject working distance, some flash models were pushed to design limit. Design engineers believe that there is no point in lighting up parts of the scene which will not be captured on the final image anyway. Lenses with longer focal length such as 50 mm for digital (roughly 75 to 100 mm for film) have narrower field of view. When using such lenses, much of flash power is wasted to illuminate areas of the scene which are outside of the field of view of the lens. If this light energy would be used to illuminate only the actual picture area, the guide number could be increased without using a larger capacitor.

 Zooming Flash heads - The basic flash unit produces a cone of light with an angle of view as wide as 24 mm. But these flashes with zooming heads have the ability to direct the illumenation into a narrower angle when needed. When zoomed "in" for longer focal lengths, say 100mm, their guide number is increased. When "zoomed out" for shorter focal lenghts such as 24 mm, the guide number decreases.

Tidbits: "Auto Focus" flash unit?

Auto focus cameras are in trouble when focusing under low light. This explains why some camera models are equipped with built-in lights (white or infrared) to light the scene for focusing. But in some extremely dark or distant subjects, this focusing aid proved inadequate. To address this problem, some flash units have similar focusing aid lights built-in. These are typically more powerful than the ones available on the camera bodies. Such flash units are sometimes called "Auto Focus" (or AF) flash units.

Credits : Special thanks to Water Camp Resort in Kawit Cavite for hosting this writer in my preparation of the manuscript in this issue. For comments and suggestions please call at 5278121 local 384 or 5253085 or email at rivera.benjie@gmail.com