Flash exposure compensation and the distance scale (!)
At a recent Flash Photography Fasttrack workshop, a participating photographer noted that when he dialed the flash exposure compensation (FEC) on his flash down to a negative value, the range shown on the distance scale on his flash increased.
At this point, if you're scratching your head and thinking "Wut?" then you might need a bit of a refresher course in flash speedlight photography. If you do, check out the blockquoted text below. If you don't, then skip it and continue reading...
When you shoot in TTL (through the lens) mode with your flash speedlight, your speedlight uses a pre-flash to work out how much flash light it needs to emit in order to light the subject for a middle grey average exposure (that is, the same way in which your camera's light meter wants to expose scenes for a middle grey average). When you use your FEC to ask your flash to increase or decrease the amount of light it wants to emit, you're effectively asking the flash to over- or under-expose the subject lit by the flash.
Logical thinkers might wonder why the distance range covered by the flash increases when you decrease the flash output using FEC. For example, if you happen to shoot with the FEC set at 0, your flash might indicate a scale of 0.6m - 4.5m, which means that subjects within this distance from the flash will be exposed for middle grey average. Yet, when you dial the FEC down to, say, -1, the distance scale shows 1.4m - 7m. Logically thinking, if you reduce the flash output, shouldn't the distance covered by the light also be reduced?
You would think so, BUT, the distance scale does not directly refer to the distance covered by the light from the flash. What it describes is the distance range in which you will obtain the flash exposure selected.
Now, think carefully on this:
If you have your flash set to FEC of 0, then your flash will want to light the subject up to obtain an average of middle grey. What the distance range on the scale shows you is the area in front of the flash in which the subject will be properly lit for middle grey average. Closer and the subject will be overexposed. Further and it will be underexposed. For example, if the distance range shows 0.6 - 4.5m, then subjects beyond 4.5m will be underexposed; subjects closer than 0.6m will be overexposed in terms of flash exposure.
Now, if you want to underexpose on the flash exposure, then what the distance range presents is the actual range in which the subject will be lit by the flash for the underexposure you have dialed in. If you have dialed in -1, then the subject will be underexposed by 1 stop. What this effectively means is that the range at which subjects will be lit and underexposed by 1 stop will be more extensive because you're no longer interested in obtaining middle grey average exposure. So, a distance scale of 1.4 - 7m means that any subject lit by the flash in this range will be underexposed by 1 stop. Any closer and they will be either properly exposed or over-exposed, and any further and they will be under-under-exposed (you know what I mean).
So, dropping down the FEC means that the area that will be underexposed by the flash will be more extensive. Increasing the FEC means that the area that will be overexposed by the flash will reduce, when compared with exposing at 0 FEC.
Easy, huh? 🙂