focus lock + ETTL doesn't work well on EOS 10D/20D (may apply to all DSLRs)

Discussion in 'Digital Photography' started by peter, May 3, 2005.

  1. peter

    peter Guest

    The ETTL on the EOS 10D/20D seems to use spot TTL sensors corresponding to
    the focus zone. In other words, if the camera focuses on the right most
    focus dot, the flash would ensure the area around that dot is correctly

    This creates a problem when I fix the focus zone to the center dot, then
    focus lock on something, and then reframe the subject (or the subject moved
    slightly away from center). The camera/flash is still trying to make the
    center area correctly exposed while the subject may be off center. This
    could create very over exposed subject.

    The reason for fixing the focus in the center is because I shoot people
    dancing. If I let the camera picks focus zone, it may decide to focus on a
    member of the audience around the dance floor instead of on the moving
    dancers. By fixing the focus in the center, I have better control of what it
    focuses on.

    Any suggestions on how to work around the flash or the focus issue?

    I actually get better TTL flash photos from an old nikon 990 camera when
    focus zone is set to the center zone. I assume it does not have spot TTL
    capability like the EOS 10D and is therefore forced to use average or maybe
    center weighted TTL metering.
    peter, May 3, 2005
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  2. You may also manually select the focus zone?
    It does not take so much time... And doing so, the flash should meter
    the right part of the picture.

    Hope this helps
    nikojorj_jaimepaslapub, May 3, 2005
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  3. peter

    Crownfield Guest

    manual focus?
    Crownfield, May 4, 2005
  4. peter

    Sonrise Guest

    Have you tried AI Servo focus? I have not yet used this on my 20D, but the
    manual says this is best for moving subjects. Supposedly, once you focus on
    a subject, the lens will track its focus when it moves. Let us know what
    works. It might be helpful to know when I'm shooting wedding receptions.

    Sonrise, May 4, 2005
  5. peter

    Roy Guest

    Have you ever watched while a Pro Photographer does this sort of job?

    They almost always use Bounce Flash, unless the ceilings are very high.

    Why do they do that? So that the flash spreads around the the area more
    evenly, and while you still get your principle subject correctly lit, others
    near them, both a bit closer and a bit further away will also be fairly well

    The distances involved in bouncing are much greater than with direct Flash,
    which is why the Pros all use powerful Flashguns.

    Like most things in photography nowadays, you do need to know when to do a
    bit of lateral thinking, and not just do the obvious. A good working
    knowledge of the basic principles, also helps.

    Roy G
    Roy, May 4, 2005
  6. peter

    peter Guest

    I tried it for a few minutes, then the constant movement of the lense
    (ocassionally hunting) started to get on my nerves, and the flash exposure
    didn't seem to improve... but I will try it again next time. I think I
    should also give the default focus mode (let the camera decide focus zone)
    another chance in combination with AI servo.

    Very often, the dead center of the image is a space/background between two
    dancers, which serves to throw off the ETTL sensor. So next time I'll try to
    frame a dancer at the center when the focusing zone is fixed at center, even
    if it means the framing is not ideal. More cropping work during
    post-processing though.

    After I re-read the manuals of the 10D/20D, I found two more potential

    The first one is called flash exposure lock. It involves pushing the *
    button after focus lock, upon which a weak pre-flash would be emitted to
    measure the subject. Then it is ok to reframe and take the photo. Although
    this may help, I see two problems: for every photo I ended up taking, I
    usually have focus locked and attempted to shoot several times. If I engage
    flash exposure lock each time, the dancers would be bombarded with several
    times more flashes -- perhaps to the point of irritating. Plus, pushing two
    buttons (focus lock + flash exposure lock) is slower than pushing one, and
    reduces the ability to chase and shoot quickly. If there's a way to make one
    button activates both focus lock and flash exposure lock, that would be

    The 20D has a new custom function #14 that changes the ETTL metering from
    evaluative to average. This may also help some.

    Everyone moves slower at wedding receptions (except throwing bouquet/garter

    peter, May 4, 2005
  7. peter

    slin100 Guest

    Enable C.Fn 4-1 or 4-3. This remaps focus to the * button on the back.
    Focus using the * button and then release it before releasing the
    shutter. With AF decoupled from the shutter release button, ETTL flash
    metering will be averaged over all of the ETTL metering segments. See
    page 75 in the 10D manual for a depiction of the ETTL metering pattern.

    Note: This is not center-weighted averaging, it's unweighted averaging!
    Regardless, this gets rid of the annoying AF-point centric
    spot-metering nature of ETTL-I. Someone else's suggestion of using
    manual focus works, too. It does the same thing.

    BTW, ETTL-II does not exhibit this behavior, so this advice applies
    only to the 10D, 300D, 1D and 1Ds.
    slin100, May 4, 2005
  8. peter

    slin100 Guest

    Bouncing the flash off of the ceiling won't address this peculiarity
    with Canon's ETTL flash system. Flash exposure will still be
    predominantly based off of the metered preflash over the active focus
    point(s). See my other post about enabling C.Fn 4 or switching to
    manual focus to force unweighted averaging.
    slin100, May 4, 2005
  9. peter

    peter Guest

    You are right! I did an experiment with the 10D with a far object and a near
    object both in the viewfinder with the focus point fixed at the far object.

    Using your trick, the near object was correctly flash exposed even though
    the focus is on the far object.

    Good thing you mentioned "release the *" before the shutter because holding
    it would make the near object over exposed and the far object correctly

    I have the 10D manual and I still can't find where it says that ETTL will
    use average pattern if you decouple the AF from the shutter. On p. 146 where
    it talks about C.Fn 4 it does not mention anything about flash. Is it
    mentioned elsewhere? You sure know the 10D. How many times have you read the

    With the 20D it's a little strange. I am unable to make it overexpose the
    near object (i.e. properly expose the far object). What gives?

    peter, May 4, 2005
  10. peter

    slin100 Guest

    Holding the * button down when releasing the shutter reverts back to
    AF-point centric flash metering.
    No, it's not mentioned anywhere in the manual. I learned it from
    someone else. Now, I pass this knowledge unto you. :)
    The 20D uses ETTL-II, which did away with AF-point metering. ETTL-II
    uses evaluative flash metering. It can also use unweighted averaging
    by enabling a Custom Function. I forget which one, though.
    slin100, May 4, 2005
  11. I think the manual specifically tells you to be carefull about that sort of

    I have used the focus lock with E-TTL (the built in pop-up) sucessfully
    though. It all seems to depend on if the camera pre-fires when you push the
    AEL button.
    Lester Wareham, May 7, 2005
  12. As far as I can make out it uses the selected AF point, or nearers of active
    AF points as a spot meter in flash. If the lens is in MF it just uses the
    centre AF point as a point meter.

    Lester Wareham, May 7, 2005
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