Speedlite Flash Basics

Learn how to calculate flash power and how Canon Speedlites can determine the appropriate output to deliver perfect exposures every time.

Speedlite camera flash units enable you to add extra light to a scene and take control of how your subject is illuminated. They're useful in low-light conditions but they can also transform images in bright sunny conditions by filling in shadows and helping to balance the exposure across the scene.

Canon has a range of Speedlites to suit different cameras, photographers and shooting situations. For example, the Speedlite EL-1 is very powerful and well-suited for use on pro cameras such as the Canon EOS 5D Mark IV or full-frame mirrorless EOS R5, whereas the Speedlite EL-100 is very compact and more at home on a smaller camera like the EOS 250D or compact mirrorless EOS M6 Mark II. And for macro photography, there's the Macro Ring Lite MR-14EX II and Macro Twin Lite MT-26EX-RT, which help you get light onto very close subjects.

All Speedlites have a collection of Canon technologies to help you get the best possible results, and it's useful to understand a bit about them to ensure that you select the right Speedlite and get the best from it. So here's a guide to the most important Speedlite basics.

Understanding guide numbers

The guide number of a flash is an indication of its power. The higher the number, the more powerful the flash. Greater flash power means you can illuminate more distant objects.

A flash's guide number is calculated by multiplying the flash-to-subject distance by the aperture required for a well-exposed photograph. So if good results are produced by using f/8 for a subject 7.5 metres away, the guide number is 60 (8x7.5). To ensure good results, you need to know the sensitivity (ISO) setting used, and the distance must be measured in metres, so guide numbers are usually written as "60 (ISO 100, metres)".

You'll find a Speedlite's guide number listed at the top of its specification sheet and, once you know it, you can use the calculation above in reverse to calculate the correct exposure setting. It's just a case of dividing the guide number by the subject distance (in metres) to find the aperture needed for the exposure. For example, the Speedlite EL-100 has a guide number of 26 (ISO 100, metres), so if the subject is 6.5 metres away, you need to use an aperture of f/4 (=26/6.5) at ISO 100.

If you want to use a different ISO value, you just need to calculate the aperture for ISO 100 and then adjust accordingly. For example, if you need an aperture of f/8 at ISO 100 and you want to use ISO 200, which is increasing the ISO sensitivity by one stop, then you must reduce the aperture size by one stop to f/11 to halve the amount of light reaching the sensor and accommodate the doubling of the sensitivity or the image will be too bright.

Canon Speedlites and EOS cameras have built-in autoflash exposure systems, so there is rarely any need to become too involved in the mathematics of guide numbers, but it can be very useful to determine the maximum distance at which you can use flash with your lens using the calculation above.

A diagram of the TTL autoflash system in a Canon film SLR.

The basics of the TTL autoflash system in Canon film SLRs: light from the flash is reflected from the subject to a dedicated sensor, which controls the duration of the flash in real time to provide the correct exposure.

A diagram of the E-TTL autoflash system in a Canon DSLR.

The E-TTL system in DSLR cameras uses a low-power pre-flash, which is reflected to the camera's light sensors in the viewfinder and compared with the ambient light level detected by the same sensors. Using this calculation, the camera determines the intensity of the main flash needed for correct exposure. This is similar to the system used in mirrorless cameras.

Autoflash exposure

Although Canon Speedlites let you take manual control over the flash exposure if you want, they also have a number of autoflash exposure systems to do the job for you with the camera and Speedlite working together. The exposure is determined by measuring the brightness of the flash (or pre-flash) illumination reflected from the subject.

Since 2004, all new EOS cameras have E-TTL II flash metering systems built-in – more about this in a moment. However, all current Speedlites are also compatible with the older E-TTL system.

TTL autoflash

Early EOS cameras use "off-the-film" flash metering. When you press the shutter button, the shutter opens and the flash fires. The flash illumination is reflected back from the subject, through the camera lens and onto the film. From here, the light is reflected down to a sensor in the base of the camera. This sensor measures the brightness of the light and, in real time during the exposure, controls the duration of the flash to provide the correct exposure. This is the basis of Canon's TTL (through-the-lens) autoflash metering used by the built-in flash of most EOS film cameras.

E-TTL autoflash

E stands for evaluative. The flash system uses the EOS camera's light sensors in the viewfinder, which are also used for evaluative metering of the ambient light. As the shutter button is pressed, an ambient light reading is taken. A low-power pre-flash is then emitted by the Speedlite. The sensors measure the pre-flash illumination reflected by the subject, and deduct the first ambient light reading from this second reading to get a flash-only reading. This flash reading enables the camera to determine the intensity of the main flash needed for correct exposure. Finally, the shutter opens and the main flash fires at this predetermined intensity. Unlike TTL metering, the E-TTL system does not monitor the flash output during the actual exposure – the sensors in digital cameras no longer reflect light back as film did, meaning the older TTL system won't work – but the E-TTL system helps provide better fill-in flash performance than the earlier TTL systems.

E-TTL II autoflash

E-TTL autoflash works on the assumption that the subject will be covered by an autofocus (AF) point. However, an AF point does not always cover the area of the subject where you want the flash exposure to be correct. The E-TTL II autoflash system overcomes this problem. When you press the shutter button, the ambient light is metered by each sensor. Then a pre-flash fires and this is also metered by all the sensors. The ambient and flash readings for each sensor are then compared. Because the main subject is likely to be closer to the camera than the rest of the scene and will reflect more of the flash, the area of the scene that shows a significant difference between the readings is the area where the subject is most likely to be. However, if the difference between the two readings is very high, the camera will ignore this area on the basis that the flash is being reflected back to the camera by a very shiny surface, such as a mirror.

The pre-flash meter readings from accepted areas are weighted and averaged, then compared with the ambient light readings. From this, the main flash output is calculated and stored in memory for the exposure. If the lens is able to provide distance information, this is used to determine the closeness of the subject and any highly reflective areas relative to the background. This information is used to refine the flash exposure. The result is better flash exposure for difficult subjects such as white wedding dresses.

A portrait taken on a Canon EOS-1D X Mark III with the E-TTL balance setting on Ambience priority.

The EOS-1D X Mark III introduced an E-TTL balance option to give you more control over the way that flash and ambient light are mixed together. In Ambience priority, natural light is the dominant light source and flash is just a fill light, producing lifelike shots with a natural mood.

A portrait taken on a Canon EOS-1D X Mark III with the E-TTL balance setting on Flash priority.

Flash priority makes the flash the main source of light, which is useful when you want to reduce shadows on your subject and in the background.

A portrait taken on a Canon EOS-1D X Mark III with the E-TTL balance setting on Standard.

In Standard mode, the flash and ambient light are given equal weighting, which is the way other EOS cameras handle the balance.

Face priority

The Canon EOS-1D X Mark III introduced an E-TTL II "Face priority" External Speedlite Control option, which links with the camera's Face Detection technology. When this is activated and a face is detected in the scene, the camera prioritises the face when processing the pre-flash information to ensure that the most important part of the image is correctly exposed.

E-TTL balance

E-TTL balance is another feature introduced by the EOS-1D X Mark III. It offers three modes – Ambience priority, Standard, and Flash priority – which determine the balance between the brightness of the subject and the background without the need to use flash exposure compensation. They make it easier to control the flash and get the results you want.

Angela Nicholson

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