Why does Canon E-TL underexpose through an umbrella or softbox when the flash head zooms with the lens?
Asked 5/13/2014
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Using a Canon 60D with an EF-S 17-55mm f/2.8 and a 430EX II in E-TTL, I get strong underexposure only when the flash is fired through a modifier such as an umbrella or softbox. At 35mm and wider, exposure is normal at 0 FEC, but once the lens is zoomed longer than 35mm I need about +1 FEC to match the earlier exposure.
The problem does not appear with direct or bounced flash, and manual flash is consistent across the zoom range. Other lenses on the same body work normally, and this lens works normally on another body. The issue also happens with the flash mounted directly on the hot shoe if a softbox is attached, so it does not seem specific to the TTL trigger.
Why would E-TTL underexpose only when using a modifier and zooming the lens longer?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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The point of most flash modifiers is to spread and soften light from your flash. This includes spreading light over areas that aren't in the field of view of the camera/lens so the light can bounce off other surfaces and illuminate what is in the camera/lens' field of view.
The point of having a flash with a zoom head is to concentrate light in a smaller area when the angle of view of the lens is smaller (set to a longer focal length). This means it is trying to concentrate all of the light emitted by the flash into the area covered by the camera/lens' field of view.
Is there any possible inconsistency there?
When using modifiers it is generally a good idea to manually set the flash's zoom to a wide angle and leave it there regardless of the focal length your lens is zoomed. Here's why: The pre-flash used by E-TTL is a fairly low power pulse compared to the flash's full power. The camera doesn't need to be able to see perfect exposure during the pre-flash, it just needs to measure how much of the light emitted by the pre-flash is reflected back to the camera and then assumes the same percentage will be returned at higher power levels. With the modifier between the flash and the field of view the assumption that the same percentage of light emitted by the flash will be reflected to the camera's sensor may be an incorrect one.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
12y ago
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This is most likely normal behavior from the flash head zoom interacting badly with the modifier, not a body/lens fault.
In E-TTL, when you zoom the lens longer, the flash often zooms its head longer too, concentrating the beam into a narrower area. But a softbox or umbrella is designed to spread and diffuse light. If the flash head is zoomed in, it may illuminate only part of the modifier efficiently, changing how the preflash is measured and reducing effective output through the modifier. That can lead to underexposure and the need for positive FEC.
Direct or bounced flash works because the narrowed beam is still useful there. Manual flash seems consistent because you are not relying on TTL metering behavior.
The usual fix is to manually set the flash zoom to a wide setting and leave it there when using modifiers, regardless of lens focal length. That lets the modifier be filled more evenly and makes TTL results more consistent.
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