Flash exposure lock and flash exposure compensation

Getting the exposure right for flash photography used to involve time-consuming calculations based on guide numbers, flash-to-subject distance and apertures. Thankfully, evaluative through-the-lens (E-TTL and E-TTL II) flash metering has changed all this, and Speedlite flash photography is now as easy as pressing a button. But even so, you'll sometimes get better results by giving the flash metering a little help.

Canon EOS cameras' evaluative metering system, which is used to take ambient exposure readings, is shared with the E-TTL flash system. A low-power pre-flash fires in advance of the exposure, and the light from this flash is metered. The duration of the main flash is then controlled to give correct exposure with the selected aperture.

When you are shooting with daylight, exposure adjustment is sometimes needed with very light or very dark subjects. This is because the camera is calibrated to suit an "average" scene. You can either take a reading from an "average" area of the scene and use the exposure lock function to hold this for the main scene, or use the camera's exposure compensation function.

Since E-TTL and E-TTL II flash use the same metering system, the flash meter readings can also be confused by light or dark subjects. In this case, the adjustment is made using the flash exposure lock (FEL) or flash exposure compensation (FEC) functions.

Flash exposure lock (FEL) enables the camera to remember the exposure for any selected area of the subject while you recompose the image in the viewfinder. To take an FEL reading you bring a mid-tone subject area into the centre of the viewfinder and press either the exposure lock or flash exposure lock button (it varies between cameras). The flash fires, and a reading is taken without an exposure being made. This reading is held for 16 seconds while you recompose the image and take a picture.

FEL gives you total control over the flash exposure. You choose the area from which the reading is taken.

FEL is especially useful when there are highly reflective surfaces, such as mirrors, within the subject area. These can create bright hotspots by reflecting the flash illumination directly back to the camera. A general flash reading will detect this bright light and reduce the flash output to compensate. The result will be underexposure by the flash. This problem is avoided if you take an FEL reading from an average area of the scene which does not include reflective surfaces.

If there is no area of average tone in the subject, you can take an FEL reading from a lighter or darker area and then apply flash exposure compensation to adjust the setting (more about this shortly).

FEL is not limited to a Speedlite attached to the camera. It can also be used with the Speedlite on an off-camera shoe cord or with the Canon wireless flash system.

Flash exposure lock provides a useful "flash in range" check. When you press the exposure lock or FEL button, a green star will appear to the right of the flash bolt icon in the viewfinder. This reminds you that you have locked the flash exposure. However, if the flash bolt icon starts to blink on and off, it means that there is not enough flash power for adequate exposure at the current settings. You should either set a wider aperture or move closer to the subject and take a new reading.

It's worth noting that the green flash confirmation lamp on the back of the Speedlite should be used with caution when FEL is used. This normally lights up for about three seconds after a flash exposure if correct flash exposure has been obtained. However, if FEL is used and there is no out-of-range warning in the viewfinder, the confirmation lamp will always show green, even if you are out-of-range with the recomposed image. The confirmation lamp is picking up its information from the FEL pre-flash, rather than from the main flash exposure.

E-TTL and E-TTL II flash exposure is calculated before the exposure starts. This can be a problem if you use second curtain flash synchronization and the subject distance changes during the exposure. The exposure will be based on the initial subject distance rather than the subject distance at the end of the exposure, when the flash actually fires. The result will be an over- or underexposed image, depending on the direction in which the subject moved after the exposure was underway. If you have control of the situation, the solution is to take the FEL reading with the subjects at their final position, then get them to move to their initial position before starting the exposure.

The metering sensors inside an EOS camera are calibrated for mid-tone subjects (often referred to as 18% grey). When the Speedlite pre-flash fires, it is reflected from the subject back to the camera. If the main area of the subject does not have an "average" tone, the flash exposure will not be correct.

Although flash exposure lock is a very effective method of overcoming the problem, you have to be aware of certain issues. First, the FEL reading is held in the camera for only 16 seconds. You can extend this by keeping partial pressure on the shutter button, but this is not always convenient. Second, you need to take a new FEL reading for each exposure, as the flash data is not stored by the camera after the shutter release button is pressed. Third, there may not be an average area of the subject from which to take the FEL reading.

An alternative technique is to use flash exposure compensation (FEC). Here, you simply enter the amount of compensation you want on the camera and it will be applied to every flash exposure until you reset it to zero. Flash exposure compensation can also be set on some Speedlites, and if this is done, it will override any flash compensation settings on the camera.

Of course, FEC assumes that you know the amount of compensation required for different subjects. As with many aspects of photography, this comes only with experience. However, very light-toned subjects will need an increase in exposure of around +0.5 to +1.5 stops; dark-toned subjects will require a reduction in the flash output of around -1 to -2 stops.

If you are shooting in daylight and using flash to add detail to shadow areas or create catchlights in the eyes of a person or animal, the result can look a little artificial if you leave the camera to calculate the aperture and flash output. If you want a more subtle effect, flash compensation of between -1 and -2.5 stops can be effective.

Flash exposure bracketing (FEB) is a convenient feature offered by the Speedlite EL-1, Speedlite 600EX-RT, 600EX II-RT, Macro Ring Lite MR-14EX II and Macro Twin Lite MT-26EX-RT. It makes it easy for you to shoot a sequence of three pictures, each with a different amount of flash exposure compensation. The variation can be between 0.3 and 3 stops. You must use single frame advance and wait for the flash to recycle after each exposure.

The bracketing sequence is normally Standard, Under and Over, but this can be changed by a custom function on the Speedlite to Under, Standard and Over.

Bracketing is useful when you are not sure exactly how much flash exposure compensation is needed. However, you often know in which direction the compensation should be. By setting exposure compensation on the camera as well as FEC, you can force the flash exposure bracketing to shift the Standard position. For example, a sequence of -0.3 stop, 0.0 stop and +0.3 stop becomes 0.0 stop, +0.3 stop and +0.6 stop if +0.3 stop exposure compensation is set.

There is no need to be concerned if your Speedlite does not offer flash exposure bracketing, or if the calculations have left you confused. You can bracket manually simply by altering the flash exposure compensation setting between shots. Examine each image on the camera's LCD screen immediately after exposure, and base the compensation level on the results you see.

The EOS-1D X Mark III, EOS R5 and EOS R6 have E-TTL Balance as an option in their menus under External Speedlite control. This makes it possible to adjust the balance of brightness of the subject and background between Standard, Ambience priority and Flash priority. Standard gives equal weighting to the ambient light and the flash. Ambience priority is a good choice if you want to create very natural-looking shoots as it reduces the proportion of flash light relative to the natural light. It's especially useful with dark scenes because it prevents your subject from being starkly illuminated.

Flash priority is the option to use if you want the Speedlite to be the main light source. It's useful for filling in any shadows on your subject or the background.

If you're shooting a portrait it can also be beneficial to use these cameras' controls to switch the E-TTL II metering to Evaluative (Face Priority). This instructs the camera metering to prioritise the face of your subject when setting the flash exposure.