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E-TTL underexposure in a very specific scenario

oneill17oneill17 Member
edited May 2014 in flash & lighting
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?


  • ZenonZenon Member
    Could be a programming glitch. Have you contacted Canon?
  • I'm wondering if this could relate to the scene being metered changing significantly when you zoom, enough that the TTL metering thinks it needs less light when you zoom in. For example, say you have a model standing there wearing a white shirt. If you shoot wide angle, the frame being metered might involve some dark background items, the model's dark hair, dark jeans, etc., etc. So you take that photo. Then you zoom in and now most of the frame is just the model's white shirt, and you take that shot. The TTL metering will end up throwing less light for the 2nd shot.

    That doesn't explain the other inconsistencies you are seeing in your testing, but it was my first thought when I read your story.
  • ZenonZenon Member
    I was going to mention that but that lens seems to work on other cameras and the other cameras work with that lens but here it goes anyway.

    In ETTL the flash pre fires. In Evaluative mode the camera uses multiple metering zones, takes an ambient reading and compares it to the pre flash when you press the shutter all the way. It measures the reflected light from the flash and isolates the closest object which is typically your subject. Now it exposes for that isolated subject. A bride in a white dress, and groom in a black tux and and the bride and groom together will all reflect light back differently. Lets think of the camera without the flash. This is similar to shooting pure white snow, black tar or and equal amount of snow and tar in the frame. Just in case the cameras light meter is for ambient only and has nothing to do with flash exposure. The flash metering just functions the same way.

    In average the entire scene is metered so usually there is some darker areas around the bride. This mode does not isolate the subject so it is more balanced. Some people prefer to use this mode indoors.

    The only way to switch between the two is via your camera flash menus. ETTL is a ball park and zooming may be effecting it. That is why I use manual flash when I have the time and I'm less mobile. On the go it is ETTL.

    So it may just be what the pre flash is reflecting off when you zoom. Here are a few good videos. ETTL 2 is supposed to ignore highly reflective surfaces like mirrors, etc.

  • Nikonguy, that was my first thought too. But it happens every time regardless of the scene. Even if I set the focal length of the lens to 17mm and move right in to the subject such that the viewfinder shows me roughly the same image when at 55mm, the 17mm image is correctly exposed and the 55mm image is not.

    It's been suggested on another forum that I use the AE lock button to fire a preflash before taking each shot and this technique appears to work for this particular body/lens combination at all focal lengths.

    So at least I've got a working solution, but it still doesn't explain the initial problem. I'm going to put this down to a software issue as has been suggested by Zenon, and not necessarily a fault with the camera body or lens.
  • ZenonZenon Member
    Actually for flash you need FE lock which is achieved by pressing the M-Fn button. You can assign the AE lock to do this. This turns the flash into spot metering which is the 3rd and final option you have with flash metering. If you are using this you still need to use the FEC depending on what you are metering. Photogs who do use it meter off skin tones and usually add about +1 FEC. Of course you can meter off anything you like. I have tried it but don't like it. I always had to tell people the first flash was not the actual shot. I found it was annoying to me and people.
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