The second you decide to start using a speedlight (the flash) you enter a seemingly complicated world, the world of lighting. You are no longer accepting light as it is. You add, mould, redirect and create your own light. What follows is a simple outline of the logic behind flash photography.
There is only one assumption at this point: you have an external flash (speedlight or speedlite depending on whether you are a Nikon or Canon user) with a head that can be tilted and swivelled.
Are you using the flash to augment available light or is it your main light source?
You are using the flash to augment ambient light
- Measure the ambient light like you would for any photo. Set your ISO, shutter speed and aperture.
- Take a shot and look at the image. If you need to or want to make the scene darker or brighter use exposure compensation.
- Decide on the direction, diffusion and colour of the light coming from the flash. Turn and tilt the flash head if you want to bounce it from a nearby surface, use the diffusion dome or a reflector or softbox to change its quality and add gels to change the colour temperature of the light.
- Decide on the intensity of the flash – use flash exposure compensation to create a lighting ratio to your liking.
The flash is your only (main) light source
In this case there is no ambient light to take into account. This is typical for a studio setting, where you start lighting ‘from scratch’. You have two possible approaches:
- You set up your flash first (direction, diffusion and colour) and then you find your ISO, aperture and shutter speed. In other words, first you deal with light and only then you find your camera settings.
- You may e.g. decide first that you want a certain aperture (for depth-of-field) after which you will modify the intensity of the flash to achieve correct exposure.
Corresponding Key Concepts
- Speedlights are very powerful but the power of the unit decreases with the inverse of the square of the distance to the subject. Put in simple terms, when the flash illuminates something that is twice as far, the intensity is going drop to 1/4th of what it was before. Three times as far and it’s 1/9th. Four times as far and it’s 1/16th. This means that relative to your subject that is let’s say 2 metres from you, the light from the flash will have no effect whatsoever on your background 20 metres away, which being 10 times farther away will receive only 1/100th, i.e. 99% less light from the flash than the subject. Exactly because the background is illuminated only by ambient light, you first need to decide how dark or bright you want your background to be. The flash has nothing to do with that.
- When you adjust the intensity of the flash, you are deciding how much lighter or darker you want your foreground to be relative to the background. This is measured – like everything else related to exposure – in stops. You are, in effect, creating a lighting ratio. E.g. you might want your foreground to be one stop darker than the background. Or perhaps you want the opposite and you want the light from the flash to be 2 stops above the background. This is what you achieve with flash exposure compensation.
This, at first, might seem a bit weird, but it makes sense and is the key to understanding flash photography:
Your shutter speed only influences the exposure of your background (illuminated by ambient light).
The exposure your scene receives from the flash is controlled by your aperture only.
Why? Ambient light (be it natural or artificial) is continuous. It’s exposure depends on both your aperture and your shutter speeds. The faster the shutter speed, the darker the scene gets. The slower the shutter speed, the brighter it becomes.
The flash – as its name suggests – produces a very short burst of light (depending on many factors anything between 1/200th and 1/1000th of a second). Imagine a dark night with a heavy storm: lightning lights up the entire sky but only for an extremely short time. You might stare at the sky and it will be all pitch black, the ‘exposure’ your brain receives will come only from the duration of the lightning. The flash is its own shutter speed because it short and very, very powerful. The amount of light reaching the sensor will only depend on your aperture!
Camera settings: Manual or Aperture Priority? Or perhaps something else?
In a way it doesn’t matter. Pick whatever you’re more comfortable with, the advantages or disadvantages are the same as with any other kind of photography.
There’s just one limitation that you have to take into account: for most flash units there is an upper limit to how fast a shutter speed you can use. These days most DSLRs have a flash sync speed of 1/200th or 1/250th of a second. What you cannot forget is that unless you have a special speedlight (often labelled as HSS – High Shutter Speed units) you cannot use faster shutter speeds than your sync speed. Slower speeds, on the other hand, are not a problem. Most cameras allow you to decide when the flash should fire in case you want long shutter speeds: it can either fire at the beginning (called front or 1st curtain) or at the end (called rear or 2nd curtain). The latter usually looks more natural and is a highly-recommended setting.
Flash settings: Manual or TTL?
Most of time that answer will be TTL. TTL means Through The Lens and in practise it means that the big computer inside the camera and the little computer inside the flash are seamlessly integrated into a single unit. The flash receives all the relevant lens information from the camera (distance, focal length, aperture) and the two computers will arrive incredibly fast at an ideal solution to just how much light the flash should produce.
In case you’re not happy with the result, there’s a simple solution: flash exposure compensation. It works exactly like the exposure compensation you know from your camera. There’s just one crucial difference: in TTL mode the camera exposure compensation is a global decision. E.g. an EV -2 setting that you might use in Aperture priority will also reduce the output from the flash. (This, naturally, doesn’t happen if you are shooting in Manual mode as in that case exposure compensation is not a setting but a decision.) Flash exposure compensation only effects the flash.
Using your flash in Manual mode means that you disrupt the flow of lens and exposure data between the camera and the speedlight. Instead of letting your camera control the power of the flash (and overriding it with exposure compensation if you need to) you take control and regulate the light output of the flash ranging from full (1/1) power all the way down to 1/128th (7 stops less) power.
In a studio setting or when your flash is the only light source, the Manual mode is an extremely efficient way of controlling your exposure.