Flash also has Zoom
When using a Speedlite, it makes sense to match the flash output to the focal length of the lens being used. The autozoom capability of a Speedlite does just this, ensuring the flash coverage matches the angle-of-view of the lens. This means there is no ‘wasted’ light, where the flash illuminates more of the subject than you can see through the viewfinder.
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 15° 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 setting of 105mm when you are using a 28mm lens, you will end up with a light beam that is too narrow, producing a spotlight effect.
The Speedlite autozoom adjusts the coverage of the flash illumination to suit the field-of-view of the camera lens. If you are using a zoom lens, the Speedlite autozoom will react to any change in the zoom setting of the lens. It is possible to override the Speedlite autozoom for unusual situations or special effects.
For the flash to be able to autozoom to fit the lens coverage, the camera and lens must pass on information about the lens focal length to the flash. As the lens is zoomed in or out, or in the case of a prime, when it is attached, the data is transmitted to the Speedlite. This automatically zooms the flash head to the nearest wider setting to ensure correct coverage of the field-of-view.
Not all Speedlites feature the autozoom facility. Some require you to set the focal length manually with either a push button or slider switch. The Speedlites with autozoom are the 540EZ, 430EX, 430EX II, 550EX, 580EX and 580EX II.
If you decide that you would rather set the focal length manually on autozoom models, you can use the zoom button or slider to select the Speedlite focal length setting. If you do set the Speedlite manually, be aware that the next time you switch your flash on it will still be at the same setting.
Even though the maximum flash zoom position is 105mm, you can still use the flash with longer focal length lenses. However, the light coverage will be greater than the field-of-view of the lens − some of the flash power will be wasted as it will not hit the subject.
Remember, though, that there is a limit to how much light the flash can produce. Depending on the conditions and the lens you are using, you may find your images are underexposed because the flash simply isn’t powerful enough to light the scene.
If you are only using your Speedlite 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 with a digital camera, 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.
Canon’s Speedlites, the 580EX and 430EX, introduced a new autozoom feature that optimises the flash output for EOS digital cameras.
Digital image sensors in the EOS range come in three sizes: APS-C (1.6x crop), APS-H (1.3x) and full-frame (1.0x). The sensor size affects the field-of-view of the image. For example, a 50mm lens on an EOS 20D has the equivalent field-of-view of an 80mm lens on a full-frame camera. If you use one of the older Speedlites, like 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 580EX can zoom to cover the actual field-of-view, which means that no power is wasted. This leads to more accurate flash exposures, a quicker flash recycle time and more flashes per set of batteries.