Can TTL be used with bounce flash?

Discussion in 'Digital Photography' started by David Arnstein, Feb 8, 2006.

  1. I have access to two flashes that have a TTL mode: the Metz Mecablitz
    54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
    and in particular if I can use it when bouncing the flash light off a
    ceiling.

    I *think* that TTL means that the camera determines how much light is
    needed, and then signals the flash unit to emit that much light.

    If my understanding is correct, then TTL cannot be used when performing
    "bounce flash." This, because the camera does not know how reflective
    the ceiling is. Therefore, its determination of needed light is not
    possible.

    Am I understanding this correctly? Both of these flash units will do
    bounce flash in TTL mode, without complaining. So I suspect that my
    understanding is faulty.

    These two flash units also offer another automatic mode, wherein the
    camera signals the flash gun what aperture, shutter speed, and ISO
    sensitivity it is using. The flash unit then measures the output of its
    own flash light hitting a sensor on the flash unit itself. The flash
    unit cuts power to its flash tube when its sensor has absorbed the proper
    amount of light for the signalled aperture, shutter speed, and ISO
    sensitivity.

    Right?

    To me, this second mode seems suitable for use with bounce flash.
    Neither the camera nor the flash unit need to know about the optical
    properties of the ceiling. The flash unit uses the good old empirical
    method, measuring light returning from one, two, or more reflective
    surfaces.

    I'd appreciate any corrections to my (very weak) understanding.
    --
    David Arnstein | Have fun with your spams:
    | http://www.bluesecurity.com
     
    David Arnstein, Feb 8, 2006
    #1
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  2. In article <dsbemq$nsd$>, David Arnstein
    <> writes
    >I have access to two flashes that have a TTL mode: the Metz Mecablitz
    >54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
    >and in particular if I can use it when bouncing the flash light off a
    >ceiling.
    >
    >I *think* that TTL means that the camera determines how much light is
    >needed, and then signals the flash unit to emit that much light.


    Exactly how TTL flash metering works depends as much on the camera as it
    does on the flash, but usually it means it meters the light coming back
    from the subject through the lens, so it will take the reflectivity of
    the bounce screen into account. Depending on the camera, this metering
    can be achieved during the actual exposure itself or based on a lower
    power test flash emitted either when the shutter release is half pressed
    or immediately before the shutter opens.
    --
    Kennedy
    Yes, Socrates himself is particularly missed;
    A lovely little thinker, but a bugger when he's pissed.
    Python Philosophers (replace 'nospam' with 'kennedym' when replying)
     
    Kennedy McEwen, Feb 8, 2006
    #2
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  3. David Arnstein

    Bruce Hoult Guest

    In article <dsbemq$nsd$>,
    (David Arnstein) wrote:

    > I have access to two flashes that have a TTL mode: the Metz Mecablitz
    > 54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
    > and in particular if I can use it when bouncing the flash light off a
    > ceiling.
    >
    > I *think* that TTL means that the camera determines how much light is
    > needed, and then signals the flash unit to emit that much light.
    >
    > If my understanding is correct, then TTL cannot be used when performing
    > "bounce flash." This, because the camera does not know how reflective
    > the ceiling is. Therefore, its determination of needed light is not
    > possible.
    >
    > Am I understanding this correctly? Both of these flash units will do
    > bounce flash in TTL mode, without complaining. So I suspect that my
    > understanding is faulty.


    There are (at least) two quite different ways that this works, depending
    on whether you're using film or digital.

    Film cameras will often measure the light being reflected back off the
    film as it is being exposed, and use that to kill the flash at the
    appropriate time.

    Digital SLRs (at least the Nikon ones) tell the flash to fire twice.
    The first time is used to measure the necessary exposure (with the
    shutter closed) and then the second tiem the camera tells the flash how
    much more or less light is needed. That's why the D50, D70 etc need the
    new SB600/SB8800 series of flashes for automatic exposures.

    --
    Bruce | 41.1670S | \ spoken | -+-
    Hoult | 174.8263E | /\ here. | ----------O----------
     
    Bruce Hoult, Feb 8, 2006
    #3
  4. David Arnstein

    secheese Guest

    TTL flash works great with bounced light.
     
    secheese, Feb 8, 2006
    #4
  5. David Arnstein wrote:
    > I have access to two flashes that have a TTL mode: the Metz Mecablitz
    > 54 MZ-4 and the Sony HVLF32X. I am trying to understand what TTL is,
    > and in particular if I can use it when bouncing the flash light off a
    > ceiling.
    >
    > I *think* that TTL means that the camera determines how much light is
    > needed, and then signals the flash unit to emit that much light.
    >
    > If my understanding is correct, then TTL cannot be used when
    > performing "bounce flash." This, because the camera does not know how
    > reflective the ceiling is. Therefore, its determination of needed
    > light is not possible.
    >
    > Am I understanding this correctly? Both of these flash units will do
    > bounce flash in TTL mode, without complaining. So I suspect that my
    > understanding is faulty.
    >
    > These two flash units also offer another automatic mode, wherein the
    > camera signals the flash gun what aperture, shutter speed, and ISO
    > sensitivity it is using. The flash unit then measures the output of
    > its own flash light hitting a sensor on the flash unit itself. The
    > flash unit cuts power to its flash tube when its sensor has absorbed
    > the proper amount of light for the signalled aperture, shutter speed,
    > and ISO sensitivity.
    >
    > Right?
    >
    > To me, this second mode seems suitable for use with bounce flash.
    > Neither the camera nor the flash unit need to know about the optical
    > properties of the ceiling. The flash unit uses the good old empirical
    > method, measuring light returning from one, two, or more reflective
    > surfaces.
    >
    > I'd appreciate any corrections to my (very weak) understanding.


    TTL = "thru the lens". The expression came into common parlance when
    light-metering was changed from external, hand-held light meters to built-in
    meters at or near the film plane. Judging by the current demand for old
    Weston Master light meters on E-Bay, it would seem the photographic world is
    going into reverse.

    I can imagine a whole gaggle of digital photographers waving their old Westo
    n Masters around, not believing the aperture/shutter info on the digital
    screen, and going to full manual set-up for each shot.

    Progress? Don't make me laugh!

    Dennis.
     
    Dennis Pogson, Feb 8, 2006
    #5
  6. David Arnstein

    Clint Kirk Guest

    As a related point of interest, how do automatic compact digital
    cameras get the exposure right when the built-in flash is used?
     
    Clint Kirk, Feb 8, 2006
    #6
  7. Clint Kirk wrote:

    > As a related point of interest, how do automatic compact digital
    > cameras get the exposure right when the built-in flash is used?


    My Olympus C310ZOOM uses preflashes to measure the exposure in the same way
    the iTTL and dTTL modes do on Nikon DSLRs.

    Ronnie
     
    Ronnie Sellar, Feb 8, 2006
    #7
  8. David Arnstein

    Tesco News Guest

    "Clint Kirk" <> wrote in message
    news:...
    > As a related point of interest, how do automatic compact digital
    > cameras get the exposure right when the built-in flash is used?



    Hi.

    Some of them do it the old fashioned way. I read it in a manual for one,
    but cannot remember which.

    The Autofocus gives a distance setting, and the camera then sets the
    aperture. Guide Number divided by distance equals aperture is the formula
    which we all used to use before Auto Flash, Flash Meters and TTL became
    common. It works remarkably well.

    We should all have these figures in the back of our minds when using Flash,
    so that we will know what to set when the Automation lets us down, as it
    does.

    Roy G
     
    Tesco News, Feb 8, 2006
    #8
  9. In article <dsd0rq$mof$1$>, Ronnie Sellar
    <> writes
    >Clint Kirk wrote:
    >
    >> As a related point of interest, how do automatic compact digital
    >> cameras get the exposure right when the built-in flash is used?

    >
    >My Olympus C310ZOOM uses preflashes to measure the exposure in the same way
    >the iTTL and dTTL modes do on Nikon DSLRs.
    >

    Not surprising really. Olympus pioneered TTL flash metering 30 years
    ago when they introduced the OTF metering OM-2 and the QA-310 flash
    unit.
    --
    Kennedy
    Yes, Socrates himself is particularly missed;
    A lovely little thinker, but a bugger when he's pissed.
    Python Philosophers (replace 'nospam' with 'kennedym' when replying)
     
    Kennedy McEwen, Feb 8, 2006
    #9
  10. Kennedy McEwen <> writes:

    > In article <dsd0rq$mof$1$>, Ronnie Sellar
    > <> writes
    > >Clint Kirk wrote:
    > >
    > >> As a related point of interest, how do automatic compact digital
    > >> cameras get the exposure right when the built-in flash is used?

    > >
    > >My Olympus C310ZOOM uses preflashes to measure the exposure in the same way
    > >the iTTL and dTTL modes do on Nikon DSLRs.
    > >

    > Not surprising really. Olympus pioneered TTL flash metering 30 years ago when
    > they introduced the OTF metering OM-2 and the QA-310 flash unit.


    However on film cameras, TTL flash measures the reflections off of the film, so
    it didn't need a preflash. Digital cameras typically will emit a preflash,
    measure the strength of the pre-flash, and then issue the flash for real.
    Unfortunately if you have slave flashes (or studio strobes), these will get
    confused by the pre-flash. There are slave triggers and slave flashes that
    know about the pre-flash.

    --
    Michael Meissner
    email:
    http://www.the-meissners.org
     
    Michael Meissner, Feb 9, 2006
    #10
  11. In article <-meissners.org>, Michael Meissner
    <> writes
    >
    >However on film cameras, TTL flash measures the reflections off of the film, so
    >it didn't need a preflash. Digital cameras typically will emit a preflash,
    >measure the strength of the pre-flash, and then issue the flash for real.
    >Unfortunately if you have slave flashes (or studio strobes), these will get
    >confused by the pre-flash. There are slave triggers and slave flashes that
    >know about the pre-flash.
    >

    So what's the problem? Obviously if you use a slave that doesn't expect
    a pre-flash you know it won't work. Get the right tools for the job.
    --
    Kennedy
    Yes, Socrates himself is particularly missed;
    A lovely little thinker, but a bugger when he's pissed.
    Python Philosophers (replace 'nospam' with 'kennedym' when replying)
     
    Kennedy McEwen, Feb 9, 2006
    #11
  12. David Arnstein

    Clint Kirk Guest

    Tesco News wrote:
    > The Autofocus gives a distance setting, and the camera then sets the
    > aperture. Guide Number divided by distance equals aperture is the formula
    > which we all used to use before Auto Flash, Flash Meters and TTL became
    > common. It works remarkably well.


    Indeed!

    Now there's just one thing left unanswered for me. My Fuji E550 doesn't
    save the subject distance in the Exif info. I thought this was because
    it didn't know the subject distance. Just because it has autofocus
    doesn't mean it knows the absolute distance, it could work just by
    knowing that moving the lens slightly outwards gives better contrast
    than before, but now it's moved it too far so it moves it back in a
    little bit... in other words, it can work by making relative
    adjustments without knowing the absolute position.

    Assuming it doesn't know the subject distance, it would need a
    pre-flash to get the exposure right. But I don't notice a pre-flash
    when I take a picture.

    Could it be that the pre-flash is in undetectably close temporal
    proximity to the main flash?

    (Wow, I've done it! I've been wanting to use the term "close temporal
    proximity" ever since I saw it in a research paper... now I can get
    back to writing proper English)

    Or could it be that it knows the subject distance, but the camera's
    software developers were too lazy to make it write the distance in the
    Exif info?

    Clint
     
    Clint Kirk, Feb 9, 2006
    #12
  13. "Clint Kirk" <> writes:
    > Could it be that the pre-flash is in undetectably close temporal
    > proximity to the main flash?


    It could be. My E-TTL Canon G5 uses pre-flash (it will trigger an
    optical slave before the shutter opens), but this happens to close to
    the main event to be noticable by human perceptuion.

    > (Wow, I've done it! I've been wanting to use the term "close temporal
    > proximity" ever since I saw it in a research paper... now I can get
    > back to writing proper English)


    Congratulations!

    > Or could it be that it knows the subject distance, but the camera's
    > software developers were too lazy to make it write the distance in
    > the Exif info?


    It could be as well. My Canon G5 has manual focusing, and when turned
    on, some sort of yardstick where the focus distance is marked in
    meters and feet show up on the LCD - but still, there is no meaningful
    distance data recorded in the EXIF.
    --
    - gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
    ------------------------------------------------------------------------
    SD10, Kodak DCS460, Canon Powershot G5, Olympus 2020Z
    ------------------------------------------------------------------------
     
    Gisle Hannemyr, Feb 9, 2006
    #13
  14. David Arnstein

    ASAAR Guest

    On 9 Feb 2006 08:50:06 -0800, Clint Kirk wrote:

    > Could it be that the pre-flash is in undetectably close temporal
    > proximity to the main flash?
    >
    > (Wow, I've done it! I've been wanting to use the term "close temporal
    > proximity" ever since I saw it in a research paper... now I can get
    > back to writing proper English)


    Bull! You wrote it because you're a trekkie, Cap'n Kirk. <g>

    BTW, the Canon S10/S20 in no way has "close temporal proximity"
    between pre-flash and the main flash. It's impossible to avoid
    noticing both flashes. But as they're fairly old digital cameras
    now (even though I occasionally use the S20) they can be excused.
     
    ASAAR, Feb 9, 2006
    #14
  15. Clint Kirk wrote:
    >
    > Assuming it doesn't know the subject distance, it would need a
    > pre-flash to get the exposure right. But I don't notice a pre-flash
    > when I take a picture.
    >
    > Could it be that the pre-flash is in undetectably close temporal
    > proximity to the main flash?


    NO! It's because the two flashes fire so close together that you can't
    see it....






    <s>




    That *was* a nice lot of words you put together, tho.

    I am perplexed as to why there's no distance in my EXIF data, either.
    Often I know, but the camera should be able to record precisely what
    it's focussed on....in my case a 20 D with a 50mm Canon lens, for example.

    --
    John McWilliams
     
    John McWilliams, Feb 10, 2006
    #15
  16. In article <>, John
    McWilliams <> writes

    >I am perplexed as to why there's no distance in my EXIF data, either.
    >Often I know, but the camera should be able to record precisely what
    >it's focussed on....in my case a 20 D with a 50mm Canon lens, for
    >example.
    >

    In that particular case, the camera certainly doesn't know what distance
    it is focussed on. Only certain lenses in the Canon range return
    distance data to the camera, and none of the 50mm lenses are included.

    A reasonably up-to-date list of lenses with and without distance
    encoders is at:
    http://photonotes.org/articles/eos-flash/#distancedata
    --
    Kennedy
    Yes, Socrates himself is particularly missed;
    A lovely little thinker, but a bugger when he's pissed.
    Python Philosophers (replace 'nospam' with 'kennedym' when replying)
     
    Kennedy McEwen, Feb 10, 2006
    #16
  17. David Arnstein

    Clint Kirk Guest

    I was thinking of a way to find out, once and for all, whether it uses
    a pre-flash. I was going to take another camera, set on "B" or a very
    long exposure, and in the dark I would use the E550's flash to take a
    picture of a black disc with a white line across its radius, attached
    to a drill rotating at 3000 rpm. If there is a pre-flash, I would get
    two white lines on the disc; otherwise, only one.

    Then I thought of a simpler way. I read the manual. And there it was,
    on its specifications page:

    Flash type: Auto flash using flash control sensor

    So, no pre-flash, no distance measurement, just a dedicated flash
    sensor.

    Clint
     
    Clint Kirk, Feb 10, 2006
    #17
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