When you're using a Canon Speedlite, it's usually recommended to match the flash output to the focal length of your lens to make the best use of the light. Helpfully, some Speedlites have an auto zoom feature that does the job for you, ensuring that the flash coverage corresponds with the angle-of-view of the lens. This means the light spreads across the whole image with no 'wasted' light where the flash illuminates more of the scene than is captured in the image.

For example, if you take an image with a 50mm lens, the horizontal angle-of-view is about 40°. If the Speedlite were set to give coverage for a 28mm lens, the angle-of-coverage of the flash would be 65°. The Speedlite would, in effect, be emitting 25° of 'wasted' light, illuminating an area that is outside the field-of-view of your 50mm lens.

Conversely, if you set your Speedlite to its 105mm setting when you are using a 28mm lens, you will end up with a light beam that is too narrow, producing a spotlight effect with dark edges to the image.

For the flash to be able to auto zoom to match the coverage of the lens, the camera and lens must pass on information about the lens's focal length to the flash. As the lens is zoomed in or out, or when a prime lens is attached, the focal length data is transmitted to the Speedlite. This automatically zooms the flash head to the nearest wider setting to ensure the field-of-view is covered but not overlapped excessively.

Not all Speedlites have the auto zoom feature. Some require you to set the focal length manually with either a push button or a slider switch. The current Speedlites with auto zoom are the 600EX II-RT, 600EX-RT, 470EX-AI and 430EX III-RT.

If you decide you’d rather set the focal length manually on auto zoom models, you can use the zoom button or slider to select the Speedlite focal length setting. If you do set the Speedlite manually, remember that the next time you switch your flash on, it will still be at the same setting.

Canon Speedlites have a zoom range to match them to the lens in use. With compact flashguns like the EL-100 and 270EX II this is limited to 28-50mm, whereas more powerful units like the 470EX-AI and 430EX III-RT have a zoom range of 24-105mm. These more advanced Speedlites also have a diffuser panel to expand the spread of light to cover lenses with focal lengths as short as 14mm. The Speedlite 600EX II-RT and 600EX-RT can cover a lens range of 20-200mm as standard and down to 14mm with the diffuser panel in place.

Even though Speedlites have a maximum flash zoom setting, you can still use them with longer focal length lenses. However, the light coverage will be greater than the field-of-view of the lens, which means that some of the flash power will be wasted and will not hit the subject. Depending on the conditions and the lens you are using, you might find that your images are underexposed because the flash simply isn't powerful enough to light the scene.

That said, if you are only using a Speedlite 470EX-AI, for instance, for fill-in light or to add a catchlight to the eyes of your subject, rather than as a main light source, you shouldn't have any problems even with lenses around 300mm. The flash illumination from the 105mm Speedlite zoom setting will give enough light.

If you want to maximise the light from your flash, you can turn up the ISO setting to make the camera more light sensitive. This will have the effect of increasing the flash guide number.

There are two sensor sizes used in the current EOS camera range, APS-C (1.6x crop) and full-frame (1.0x), and the Canon Speedlite 600EX II-RT, 600EX-RT, 470EX-AI and 430EX III-RT auto zoom feature can optimise the flash output for these different sensors. That's useful because the sensor size affects the field-of-view of the image. For example, a 50mm lens on an EOS 90D has the equivalent field-of-view of an 80mm lens on a full-frame camera. If you use one of the older Speedlites, such as the 550EX, although the field-of-view is equivalent to that of an 80mm lens, the flash setting will still be automatically set to 50mm. This means that there will be light falling on a wider area than needed, wasting flash output.

By detecting the sensor size of the camera, the Speedlite 600EX II-RT, 600EX-RT, 470EX-AI and 430EX III-RT can zoom to cover the actual field-of-view, which means that no power is wasted. This results in more accurate flash exposures, a quicker flash recycle time and more flashes per set of batteries.

The Speedlite 600EX-RT introduced three new custom function options (in C.Fn-21-0) that specify how the light is spread over the scene: Standard Coverage, Guide Number Priority and Even Coverage. These light distribution patterns are also available on the 600EX II-RT and 430EX III-RT. They work as follows:

? Standard Coverage (C.Fn-21-0): the angle of coverage operates as normal, with slight fall-off at the edges of the frame.

? Guide Number Priority (C.Fn-21-1): the light is concentrated around the centre of the frame by moving the flash tube further forward than the setting for the lens's focal length – for example with a 50mm lens, the flash tube is at 70mm. This can be helpful when shooting with a long telephoto lens or for artistic effect and strong vignetting.

? Even Coverage (C.Fn-21-2): the Speedlite spreads the light wider than normal by positioning the flash tube one step back (for example with a 50mm lens, the flash tube is at 35mm) to avoid any vignetting.