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I think I have a Canon E-TTL issue with a specific camera body/lens combination when the flash is fired through a modifier (umbrella or softbox).

When I use my EF-S 17-55 f/2.8 lens on my 60D body, using a Hahnel Tuff TTL trigger to fire a Canon 430 EX II flash, the image is vastly underexposed when the lens is zoomed beyond 35mm. The FEC has to be adjusted to at least +1 FEC to achieve the same exposure at 0FEC when the lens is at 35mm or less.

This only happens when firing through a modifier. Direct or bounced flash works perfectly ok.

Any other lens on the 60D body works fine throughout the zoom range, and the 17-55mm lens on another body works perfectly fine throughout the zoom range, when using E-TTL flash (on or off camera). At 0FEC there is no noticeable difference between exposures at different lens zoom distances.

But with this lens on the 60D beyond 35mm the image is noticeably underexposed at 0FEC compared to the exposure at 35mm (or less) at 0FEC.

I thought the wireless trigger might be the issue, but it works fine with this lens on another body throughout the zoom range through a modifier.

Another thing that tells me the wireless trigger isn't at fault is the fact that I can mount the flash on the 60D's hotshoe with an Ezybox Speedlite attached (not recommended!) and it still underexposes when the lens is zoomed beyond 35mm. Take off the Ezybox and the flash works fine beyond 35mm.

So this only appears to be an E-TTL issue with this body/lens combo when fired through a modifier and the lens is zoomed beyond 35mm. When using manual flash the exposure is consistent from 17mm up to 55mm with this body/lens combo.

I can work around this very specific problem, but it's got me baffled and I wonder if anyone knows why it's happening?

  • If you use a preflash for FE lock does it still underexpose? – AJ Henderson May 13 '14 at 19:56
  • Well, that technique works and gives me a solution for working with this body/lens combo. It's not something I have to do with any other body/lens combo though and doesn't explain why it underexposes when the focal length is greater than 35mm. – oneill17 May 13 '14 at 20:44
  • I have similar issue with the same setup : 60d + efs17-55 New fact: ettl-II works fine witch average mettering mode. Problem is only with multisegment ettl-II metering mode. I replicated issue also with c70-200 4.0 L (all zoom settings) Any ideas? Update dpreview.com/forums/thread/3102349?page=2 Similar case and some explanation concernig ettl2 behaviour... – KrzyK Apr 10 '17 at 19:11
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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.

  • It's not the zoom on the flash it's the focal length of the lens, anything over 35mm is underexposed. But the AE lock suggestion above works in this instance. Doesn't explain the problem though, which I think is probably software related. – oneill17 May 14 '14 at 7:09
  • The fact that the FE lock preflash works but the normal low power one doesn't points to the fact the the light from the low power preflash is not being diffused as much as the light from the fuller power flash during exposure is. It may well be a problem with the camera understanding the focal length of the lens when past 35mm, but the modifier is almost certainly spreading much of the output from the flash to areas outside the FoV that the camera incorrectly believes is falling within the FoV. – Michael C May 14 '14 at 23:37
  • Thus the camera thinks it is supplying enough light (which it is in a way) but most of that light is not being placed within the FoV and reflected back to the camera. – Michael C May 14 '14 at 23:38
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    As to whether it is the zoom in the flash or in the lens is immaterial: they are designed to be one and the same. When the camera senses the lens is at a specific focal length it automatically zooms the flash to match so that the output of the flash is used efficiently by falling within the FoV (unless you have set the flash zoom manually). But your modifier is then spreading that concentrated light out over a wider area than the FoV. – Michael C May 14 '14 at 23:40
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    @oneill17 I don't have a better theory about what is happening here, and Michael's sounds reasonable. The important point, though, is that this really is one of those so, don't do that situations. You should zoom the flash to its widest setting when shooting through a softball, regardless of lens. In fact, you almost certainly want to use the flash's pull-out diffusion panel as well – mattdm Jul 14 '14 at 7:25

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