Down to Brass Tacks Let's cut the crap.

Notes on TTL Flash

Since there seems to be a lot of misunderstanding concerning the use of Speedlites (portable electronic flashes), I feel the need to bring some precisions. (Note that although a lot of functionalities, parameters and behaviors are similar from one brand to another, since I am more familiar with Canon equipment, there are times when it will be preferable that you look for additional information.)

What is TTL flash metering?

At first there was manual flash exposure, which meant that the photographer had to know the guide number of his unit and calculate effective distances against apertures and ISO numbers using the inverse-square law bla bla bla... For example, a flash unit with an (imperial) guide number of 100 would correctly expose a subject 25 feet away with an aperture of f/4 (because 100 = 25 x 4). And, oh, that's for ISO 100, and no, you can't see if what you just did was correct, because you're shooting film. Now calculate equivalences along the way, as you're shooting your event. Lots of maths, trial and error (but mostly error). Thank goodness, this era is long gone.

Then there was automatic flash exposure, which was calculated by the flash unit itself using a little photo cell. The flash would emit light and judge if enough had been emitted by the general amount of light reflected back. Needless to say, this was not very precise and could only hope to work when the flash was used on the camera's hot shoe. (Later versions offered a way to separate the photo cell from the flash unit so the flash could indeed be used off-camera, but this only solved part of the problem.)

Then, at last, came TTL flash metering. (TTL metering had appeared long before for ambient light metering, but was now also available for flash exposure.) Instead of letting the flash unit calculate if the proper amount of light had been emitted, it left that job to the camera, using exactly what it had seen "Through The Lens". The thing to remember, though, is that the core addition to TTL flash is not so much the fact that the camera and flash unit now talk to each other using more pins on the shoe (which it obviously has to, to achieve the result), but the fact that the metering is done by the camera, through its lens! The more the camera and flash can talk the better to automate the process and achieve more accurate results, but that doesn't change the core fact of who's doing the metering.

What's the focal length got to do with the flash?

The flash doesn't have to know what focal length is used to work properly, but by concentrating its beam on an area that is not wider than the area covered by the focal length, it can preserve its power that would otherwise be wasted to light areas not even seen by the camera. By doing so, it can actually improve its effective reach and illuminate more distanced subjects. This used to be performed manually with Fresnel adapters installed on the end of flash units, but is now fully automated — insofar as the unit is able to. (As a matter of fact, if you're using very long lenses (in wildlife photography, for example), you might be interested by accessories such as the Better Beamer.)

That's all there is to it! The fact that each flash unit can cover different "zoom" ranges doesn't mean it stops working properly if the lens is set to a longer focal length, it only means it won't be able to concentrate further its beam, therefore not improving its effective range further. There is nothing you can do about it (apart from hooking an accessory to the end of the flash).

Why does the flash ignore the lens' focal length when aiming its beam anywhere but forward?

Well, this should be rather obvious: if you're not beaming in front of you with the flash, then whatever focal length you are using is meaningless for the flash, since its beam is no longer aimed in the same direction.

When you aim the beam to the ceiling, for example, what you're hoping to do is to bounce the light so that the (main) source of light now becomes the ceiling — at this point, changing the flash's zoom would mean beaming a wider or narrower spot on the ceiling, which has absolutely nothing to do with the lens' focal length. Setting the orientation angle of the flash head likely has a much more significant impact on the reach/effect of the bounced light than how wide the beam is on the ceiling, depending on how far it is, etc. Since the camera has no way of knowing how far the ceiling is (or if it is flat, etc.), there is no way it can decide how wide the beam should be. Therefore, it defaults to an average value — not the longest, because a small beam on the ceiling would mean harsher light, which is exactly what we are usually trying to avoid.

You can always change the flash's zoom value manually, if you so wish, but there is no question that linking the focal length to the flash's zoom when beaming anywhere but in front doesn't make any sense.

Additionally, with Canon Speedlites (I cannot comment on other brands, which I don't know as well), when using the included diffuser (the one that retracts inside the flash head), the flash's zoom automatically goes to its widest value — because the point of this diffuser is to achieve an extra-wide angle of 14mm (14mm being the widest lens available in the Canon lineup). This also means that the flash won't be as powerful, since it is spreading its beam much more.

Is the flash metering related to the focus point?

Since I am more familiar with Canon equipment, I cannot confirm for every other brand (check your equipment's documentation), but I would think that the behavior is very similar. With Canon cameras, ever since E-TTL II came along (that's a long time ago, circa 2004), flash metering is no longer linked to the autofocus point selected. This means that you can use the "focus-recompose-shoot" technique freely without worrying that the flash metering will be thrown off.

The metering will also use information from the focus distance (when available — this is a per-lens capability) to calculate its power. This means that if the focus distance is on a subject, it will likely expose the subject properly and pretty much ignore the background (which means potentially leaving it dark). If the focus distance is on a background, it will likely expose the background properly and pretty much ignore the subject (which means potentially overexposing it). Exactly what we would expect, considering that the in-focus area is likely the most important part to light properly.

What is the FEL button doing?

First, we have to understand that regardless of our usage of the Flash Exposure Lock (FEL) function, there is always a pre-flash when using TTL metering. Since the pre-flash occurs right before the actual exposure begins, it is hardly noticeable. (When using rear-curtain synchronization, the two separate flashes will be very obvious, because they will occur before the exposure and right before the end of the exposure, respectively, giving you plenty of time to see the two bursts.)

So the goal of the FEL is not to generate a pre-flash (that, we always get). The point is to lock the flash metering — exactly the way the auto-exposure lock (AEL) works with ambient light, but with flash. Why would you do that? Well, if you don't want the flash to be metered based on the final scene (because there might be something you anticipate will throw off the metering, such as a bright white background or, conversely, lots of black tuxedos in the frame), you can trigger the calculation on a different scene or on a specific area of the scene (by zooming in on a subject's face, or by using a different metering zone pattern, for example) and then recomposing-shooting, which will use the flash metering that was calculated before.

If you're systematically using the FEL before each time you shoot, ad nauseum, without significantly changing the frame, you are wasting your time (and battery power). I hardly ever use that function, but it can be useful in difficult situations — just as much as AEL is with ambient light (which I never use). Instead, I tend to prefer using exposure compensation, which I find more predictable.

Note that here, Canons and Nikons work rather differently. Canon's FEL locks the flash metering only as long as you hold the shutter release half-way (otherwise it forgets the flash metering after about 16 seconds, or very quickly after a shot). A little star (*) appears in the viewfinder as long as the FEL is active, and a new FEL will have to be be performed each following time you need it. Nikon's FEL (at least, the way I was told it worked) locks the flash metering for as long as you don't press the FEL button again — the flash exposure will be the same for all the following frames until you unlock it.

What about exposure compensation?

Like I was saying, I tend to prefer exposure compensation over FEL, because I can better judge how to tweak the exposure than by trying to aim at something that I think would be closer to what I want (good luck!)

There are two ways you can apply flash exposure compensation (FEC). You can either apply FEC on the camera, or on the flash unit itself. With Canon (again, I cannot confirm for other brands), if you apply a FEC on the flash unit, it overrides whatever setting was in the camera. I personally prefer applying FEC with the camera, because I can reach the button right next to the shutter release without even having to move my eye away from the viewfinder, and I see what I'm doing from the display at the bottom of the viewfinder.

To wrap up...

One thing is for sure, working with flash requires practice, because it is much less predictable (especially when bouncing off various surfaces). It gets particularly tricky when mixing flash with ambient light, because now you have two independent exposures to oversee, plus color balancing with gels, etc.

Get to work!