Portrait of Tina, a young Swedish student.

When using some form of strobe lighting, there are five basic elements that go into determining the optimum settings for your exposure. Knowing and understanding these variables is essential for any photographer who wishes to achieve accurate, repeatable flash exposures with a minimum of fuss. If you can master these, everything else becomes a matter of minor tweaks and adjustements. The five elements are:

1.Shutter Speed
2.Aperture
3.Flash Power
4.Flash to Subject Distance
5.ISO

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Shutter Speed

The shutter speed controls the ambient light. By ambient, I refer to any light that is already present in the scene. This could be light from the sun, moon, streetlights, candles, volcanoes, room lights and so forth. Basically any light that is already there and will affect the image in some way. Adjusting your shutter speed will govern how much or how little of this light is allowed into the scene. One major limitation however is the camera’s sync speed, usually 1/200 or 1/250. This will be the limiting factor when working with flash (in this case manual flash without any high-sync speed settings). We will be restricted to 1/250 (1/200) or longer. Not great for bright sunlight at midday but not a big problem indoors, in shade, in the evening etc. The following bears repeating because it’s an essential concept to grasp.

Shutter Speed Controls Ambient Exposure

Aperture

The aperture, or f-stop, is what controls the light output from the flash. Light from the flash happens at a very short duration – a fraction of a second. Adjusting the shutter speed has no effect on it because the flash is of such a short duration that shutter speed is always longer. We can adjust our flash by controlling the aperture on the camera. If we need more light, we can open up the lens, for example going from f5.6 to f4 to f2.8 etc. Each stop we open up represents a doubling of light reaching the sensor. The reverse applies equally – if we need less light, we stop down (f2.8 to f4 for example). This will halve the amount of light. As with shutter speed, this should be repeated for clarity.

Aperture Controls Flash/Strobe Exposure

Flash Power

To adjust flash power, you firstly need to set it into M mode (the flash, not the camera). We’re not relying on TTL or any auto modes because we want the consistency that manual gives us. TTL and its varieties (iTTL, E-TTL etc) works fine but we want to know that our flash is behaving in exactly the same way, shot after shot. In manual mode, we can adjust power. Full power is 1/1. This squeezes every bit of juice out of the flash. Half that and we get ½ power. Then ¼, 1/8 and so on. Most modern hotshoe flashes go down to 1/128, older models and studio lights may only go to 1/16. Each change in power represents 1 stop of light, as shown in the example below (it’s hypothethical – 1/1 won’t always correspond to f16).

Flash Power 1/1 ½ ¼ 1/8 1/16
_______________________________
Aperture 16 11 8 5.6 4

Essentially, what this means is that if you are getting a proper exposure at f16 and 1/1 but want a shallow depth of field (f4), you need to change the flash power to 1/16 to keep the proper exposure.

A further consideration is battery life. The lower the power setting, the less drain on the batteries meaning the faster the recycle time. Typically when I begin a shoot, I set my flash to either ¼ or 1/8. From here I can go either up or down depending on the situation. It just a personal preference – starting at 1/1 may work for you.

Flash To Subject Distance

This is the part that a lot of people hate. It can get technical if you let it. To determine the optimum distance, we need to use the inverse square law. You probably learned it in high school physics and probably promptly forgot it the day after the exam. In a nutshell, it says that if you double the flash to subject distance, you reduce to light to ¼.

So what does this mean? Well, for someone who’s doing this strictly for fun and not money, not a lot. Having a vague sense of it is enough. You don’t need to carry a calculator and measuring tape with you – in most cases near enough will be good enough, and you can always check the exposure on the LCD and adjust as necessary. If you’re doing this for a living, then this should be second nature to you anyway. If you’re making the step from hobbyist to working photographer, hire a model or two (or beg your family) and spend a few hours working through different distances until you have an intuitive grasp of the concept.

ISO

The fifth element is ISO. Changing the ISO changes the light sensitivity. Going from ISO100 to ISO200 doubles the amount of light. We get an extra stop of light to work with. From ISO100 to ISO400 gives us two stops of light. So how does this apply to a strobe-lit exposure?

Well, say you find that ISO100 and a 1/1 flash power means you need to shoot at f8. Changing the ISO to 200 (400), and the flash to ½ (¼) will keep you at f8 but you be putting a lot less stress on the batteries, getting quicker recycle times and hence being able to shoot much faster.

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