20D flash underexposure

Discussion in 'Canon EOS' started by sam_eaton, Dec 27, 2004.

  1. I've had a good search around, both here and on google in general, and I can find quite a
    few references to similar problems, but not much in the way of a definitive answer so I
    thought I'd add some observations of my own...

    I've very recently aquired a 20D, having previously used a 5/A2E and a 3. One of the first
    things I've shot with it were a set of indoor photos, with flash, using my 550EX.

    They're consistently underexposed, by about a stop or so - looking at the histogram,
    there's nothing in the rightmost bar of the graph. Pushing up the exposure in the RAW
    convertor works fine, so it's not a tragedy, just puzzling.

    Having had a read around, it seems that many people have similar problems, and one of
    the suggestions is that it's down to doing a 'focus and recompose' manouvre. So I
    checked on DPP where the focus points were during exposure on these shots, and they're
    mostly right on people's faces, so that doesn't seem to be it.

    My next try was to do some experimental shots at home, using both my 550EX and a
    420EX I'd borrowed. I shot a range of subjects inside, in a variety of metering modes (I do
    understand about the difference between background exposure and flash exposure in AV
    mode etc), and tried the Cf.14 settings etc. All seemed consistently underexposed too.

    My next experiment was to shoot my Kodak grey card. I figured that if the flash exposure
    was right (I tried this in P mode) then I should get a nice peak on the histogram right in
    the middle. But no, it's one bar to the left consistently. If I bung on +1 FEC, then it moves
    nicely into the center.

    So currently, it seems that I can 'fix' this 'problem' by just habitually using FEC, but it
    seems a bit tiresome considering all this nifty E-TTL II stuff doesn't seem to get it fairly
    right on its own.

    What I'm wondering is a) is this basically the same for all 20Ds, or do I have a duff one? b)
    if they are all the same, is this intention on canon's part (to avoid blowing the highlights
    maybe?)

    Anyone tried a similar experiment with theirs and see the same results?

    Thanks,

    Sam
     
  2. I've used three 20D's and all have the same problem. I've done basically what you have done and find that I need to either bump up the FEC or pre-meter with the little star button...sorry, I forgot its name. Anyway, hope someone else has something to add. To answer part of your question, I don't think its the camera as the three I have had came from all different sources and at different times over the last 30 days.

    Harry
     
  3. Harry Alberts , dec 27, 2004; 07:41 a.m.
    > pre-meter with the little star button...sorry, I forgot its name.
    It is the flash exposure lock.
     
  4. well if the problem goes away when you use the Flash Exposure Lock...the little button...then to me it's defintitely focus-recompose issue, as that is the whole point of that feature!

    sean
     
  5. OK: Take a deep breath.

    The 20D has ETTL-II, which meters differently than most other Canon bodies which use ETTL.

    When used in "automatic", the 550EX and 420EX should give identical results on a given camera (as long as you don't run out of power) since the body controls flash power and flash exposure.

    ETTL uses a highly focus point biased flash metering algorythm. Works great to properly expose a bright face on a dark background.

    ETTL-II uses an "averaging" flash metering algorythm. Similar to an averaging thyrister in $20 flashes. With ETTL-II, you don't have to do a "flash exposure lock" on a face to ensure proper exposure. You "should" be able to just point and shoot.

    Now, here is the real question: When you shot the grey card, what else was in the frame?

    If you shot a grey card on a white wall, I would expect the grey card to be darker than desired, as the camera will tone down the flash to expose the ENTIRE scene to a neutural grey. Conversely, if the grey card was on a dark wall, I would imagine ETTL-II would try to brighten up the wall and would overexpose the grey card.

    Wouldn't it be nice if BOTH ETTL *and* ETTL-II were offered via a custom function?
     
  6. I have exactly the same experience with my 20D as Sam. At some point I got rid of a couple of DRebels because of this. This is, apparently, a design issue rather than a defect.

    P.S. And the grey card occupied the whole frame, of course.:)
     
  7. Just to reply to a couple of the comments - I've tried using the FEL with no difference in results - as during my tests, I wasn't focusing and recomposing. Also, when I did the grey card tests, I'd zoomed enough to fill the entire frame, as otherwise I wouldn't expect the test to be useful :)

    Basically, it seems like this is a 'feature' of ETTL-II from what others are saying, and all 20D bodies seem to do much the same. Which is wierd, but fair enough, it reassures me that I haven't got a duff one and that what I need to do is check those histograms carefully and bump up the FEC.

    I'm intending to have a play with a 580EX in the next couple of days and I'll see if that makes any difference at all, although I suspect not from what people have said.

    Thanks all,

    Sam.
     
  8. Hi Sam, You didn't mention which *mode* you were in when taking your shots and that can affect everything. Having cut my teeth, so to speak, on the 10D, when I first used my 20D with my 550EX I was very pleasantly surprised. My exposure was dead-on without any compensation and when I used the flash white balance setting, the colors were even correct for a change. I'm very pleased with the 20D & 550EX combination, but I'm almost always in Manual mode when shooting indoors. Outdoors, for fill-flash, I use Av mode and seldom have a problem now. Here's a Pultizter Prize Losing shot I took this morning of my two cats: 20D, Manual Mode, 550EX flash, 17-40 f/4L, 1/100th @ f/5.6. Good luck!
    00AZWO-21091784.jpg
     
  9. Hello Sam
    I've had my camera i 3 weeks - ALL pictures taken in christmas was underexposed. They were indoors shot in living room. Some also not in focus (1 of 3) - apparantly because of failure to wait until focused. Picture of myself (the only one...) was complete off focus - my wife just pressed the button.
    I shoot in P mode and use an 380 EX flash. I see only one way to go - which I have tested just this afternoon - get rid of the matrix and spot metering and stick to the AVERAGEe metering - and also use ISO 400. It now (seems to) function perfect!
    How it is that Canon can do such bad engineering - is a great mystery. Maybe they shoud get someone else doing the job?
    Regards - Norway
     
  10. John,

    The camera won't take a picture until the focus is achieved. So look elsewhere to explain the lack of sharpness.

    Also, it sounds like your shots might me underexposed simply due to not powerful enough flash for your settings. That's why increasing ISO helped.
     
  11. Beau Hooker Photo.net Patron, dec 27, 2004; 04:36 p.m.
    > Hi Sam, You didn't mention which *mode* you were in when taking
    > [..] almost always in Manual mode when shooting indoors. Outdoors,
    > for fill-flash, I use Av mode and seldom have a problem now.
    Mostly the same here : best results indoors are achieved with M.
    Outdoors for fill flash I use Av or P and both work as expected.
    Av or P indoors have almost always been underexposed
    and I don't use them there anymore.
     
  12. The cameras (300D and 20D) always underexpose even macro shots taken with a ring flash. Actually, the flaw IS more noticable in flash macro(manual mode on the camera, of course). And increase in exposure doesn't change it. FEC has to be used for every single shot.

    10D I tried didn't have this problem.
     
  13. There is a considerable discussion about this issue here:
    http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00AFhw

    Very interesting issues...
     
  14. Mike P , dec 28, 2004; 12:58 p.m.
    > The cameras (300D and 20D) always underexpose even macro shots taken with a ring flash. Actually, the flaw
    > IS more noticable in flash macro(manual mode on the camera, of course). And increase in exposure doesn't
    > change it. FEC has to be used for every single shot.
    My experience is also that ambient light exposure does not impact on the flash exposure. FEC seem to be
    the only way to get around the problem, but my EOS 300D + 380EX can't do that so I'm stuck with playing
    with levels in Gimp way too much... I guess it's time to get that second hand 550EX on Ebay...
     
  15. It seems that to avoid the perils of blown highlights Canon has been overly conservative in making sure that whatever algorithm is used takes highlights heavily into account. The discussion posted by John Baker a couple of posts above provides some interesting insights about the Canon flash exposure calculation algorithms, but I'm still looking for more information about how the damn thing actually works...
     
  16. Sam
    I've read and tried and found out:
    1)Press the info button twice to get the histogram showing
    2)Press the star button/FEC lock before every shot
    NOW IT WORKS.
    Mike - you are completely right about the camera will not take a photo until focus is achieved. Maybe she put her finger on the focus know when the camera was aimed at the roof. But you are wrong about the 380 not powerful enough - now the pictures come out beautifully - even wiht ISO 200 - ISO 100 may be a probem - also when bouncing flash. I WILL NOT upgrade the flash - it will be more bulky.
     

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