Light metering

Discussion in 'Digital Photography' started by alertjean@rediffmail.com, Feb 27, 2007.

  1. Guest

    Hello there,
    I waw wondering how camera is actually doing light metering and
    adjusting proper exposure.

    In TTL metering light from the scene will be entering the camera
    through the lens.
    Once the light hit the light meter (or metering points/area) ,it will
    measure light intensity.
    Now based upon the light intensity it LOOKS UP (Or is it calculating
    something on the fly) the corresponding exposure value which need to
    be executed.
    When we use exposure compensation it uses the exposure value lying
    above or below that of the one it arrived at.
    If that is the case how is the first look up table calculated ? By
    humans through trial and error ?
    And it need to know only the light intensity to do all these ?

    Is this correct ?
     
    , Feb 27, 2007
    #1
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  2. ASAAR Guest

    On 27 Feb 2007 03:38:49 -0800, wrote:

    > I waw wondering how camera is actually doing light metering and
    > adjusting proper exposure.
    >
    > In TTL metering light from the scene will be entering the camera
    > through the lens.
    > Once the light hit the light meter (or metering points/area) ,it will
    > measure light intensity.
    > Now based upon the light intensity it LOOKS UP (Or is it calculating
    > something on the fly) the corresponding exposure value which need to
    > be executed.
    > When we use exposure compensation it uses the exposure value lying
    > above or below that of the one it arrived at.
    > If that is the case how is the first look up table calculated ? By
    > humans through trial and error ?
    > And it need to know only the light intensity to do all these ?
    >
    > Is this correct ?


    See dpreview's Glossary page for Metering. It describes several
    types of metering, and the description for the one that you seem to
    be asking about (Matrix or Evaluative Metering) is this:

    > This is probably the most complex metering mode, offering
    > the best exposure in most circumstances. Essentially, the scene
    > is split up into a matrix of metering zones which are evaluated
    > individually. The overall exposure is based on an algorithm
    > specific to that camera, the details of which are closely guarded
    > by the manufacturer. Often they are based on comparing the
    > measurements to the exposure of typical scenes.


    http://www.dpreview.com/learn/?/Glossary/Exposure/Metering_01.htm
     
    ASAAR, Feb 27, 2007
    #2
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  3. <> wrote:
    > Hello there,
    > I waw wondering how camera is actually doing light metering and
    > adjusting proper exposure.
    >
    > In TTL metering light from the scene will be entering the camera
    > through the lens.
    > Once the light hit the light meter (or metering points/area) ,it will
    > measure light intensity.
    > Now based upon the light intensity it LOOKS UP (Or is it calculating
    > something on the fly) the corresponding exposure value which need to
    > be executed.
    > When we use exposure compensation it uses the exposure value lying
    > above or below that of the one it arrived at.
    > If that is the case how is the first look up table calculated ? By
    > humans through trial and error ?
    > And it need to know only the light intensity to do all these ?
    >
    > Is this correct ?


    There are at least two ways to measure light _correctly_.

    1. Incident metering. This measures the light falling on the subject and
    produces a theoretically/technically correct reading. (A gray card will turn
    your TTL meter into an incident meter.) In even lighting with "normal"
    subjects and no need for artistic intervention, incident metering is the
    right thing.

    2. Spot metering. This measures the intensity of specific areas of the
    subject and can be used (if you know what you are doing) to determined a
    subjectively (artistically) correct exposure. It can also be used to
    determine compromise exposures in uneven lighting. (You don't need a gray
    card, but you need to know what you are doing; this is the metering method
    used in zone exposure systems.)

    The books advertised at this site are good introductions to spot metering.

    http://www.spotmetering.com/

    All the other ways I know of off hand (e.g. center weighting, matrix)
    produce what are essentially random guesses.

    In real life, most people use one of the random guess methods, looks at the
    histogram, and then reshoot with an appropriate exposure compensation dialed
    in.

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Feb 27, 2007
    #3
  4. Alan Browne Guest

    wrote:
    > Hello there,
    > I waw wondering how camera is actually doing light metering and
    > adjusting proper exposure.
    >
    > In TTL metering light from the scene will be entering the camera
    > through the lens.
    > Once the light hit the light meter (or metering points/area) ,it will
    > measure light intensity.
    > Now based upon the light intensity it LOOKS UP (Or is it calculating
    > something on the fly) the corresponding exposure value which need to
    > be executed.
    > When we use exposure compensation it uses the exposure value lying
    > above or below that of the one it arrived at.
    > If that is the case how is the first look up table calculated ? By
    > humans through trial and error ?
    > And it need to know only the light intensity to do all these ?


    There are various simple to complex metering schemes. The most basic
    TTL is to reflect the scene light off the mirror up to a meter under the
    prism (or on a prism surface).

    Some meter sensors are on the mirror (backside) with a semi silvered
    section to allow the light through.

    From there, it can be as simple as measuring the light and calculating
    the EV (whether done in an analog circuit, dedicated digital circuit or
    soft/firmware), and thence exposure time or aperture (A or S modes) or
    select a P mode pair according to the overall light, ISO and possibly
    sub-sets of the P-mode. The user can select (in P-mode) different A/S
    pairs in "Program shift" or offset the exposure with exposure compensation.

    Meters are weighted from none (spot metering) to full matrix with
    various weights in between. Most up to date metering systems have
    several modes (spot, center, scene). RTFM is quite important to be sure
    you know which mode your camera is in and how that affects the metering.

    When "matrix" metering comes in, then each manufacturer has their own
    approach to the problem. Some meters weight for the "bottom" of the
    scene (Leica R8, R9) and even detect if the camera is held in "portrait
    mode" (and which side) or landscape (Minolta and others).

    Nikon has one of the most advanced high resolution and color sensitive
    metering systems that began with (IIRC) the F5. Looking at 1005 points,
    and evaluating color (not just light level) and using a stored
    "template" memory, the camera would find a best fit and then calculate
    the exposure from that fit and actual measured light.

    Compensation is simply an offset from whatever exposure value is arrived
    at. A few simple guidlines will help you determine the compensation for
    most situations. Chimping allows on the fly compensation with DSLR's.

    Canon are PITA's for amateurs as they only include spot metering in
    their high end (pro) cameras. Nikon, Minolta, Pentax and others include
    spot metering in their mid class and some entry level class cameras.

    TTL Flash metering with digital is a problem, as the Off-the-film method
    does not work with digital sensors. So the camera will "pre-flash" the
    scene with a weak blip, measure the return with the available light
    metering system and then set the flash duration from that measurement,
    then fire the shot. Again, each manufacturer has their own twists on
    this. For example, Minolta (Sony Alpha) cameras with Distance
    Integration (focus distance) lenses will weight their flash throw more
    to the focus distance than to the pre-flash TTL measurement.

    Cheers,
    Alan

    --
    -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
    -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
    -- [SI] gallery & rulz: http://www.pbase.com/shootin
    -- e-meil: Remove FreeLunch.
     
    Alan Browne, Feb 27, 2007
    #4
  5. ASAAR Guest

    On Tue, 27 Feb 2007 21:46:17 +0900, David J. Littleboy wrote:

    > All the other ways I know of off hand (e.g. center weighting, matrix)
    > produce what are essentially random guesses.


    Informed guesses subject to some randomness. If matrix metering
    utilized truly random guesses even Ken Rockwell would avoid using it
    as his preferred D200 metering mode. Center weighting is more
    likely to produce bad results in the hands of a poor or unskilled
    photographer. You know, the kind of shooter that would be better
    off with a $150 P&S than a DSLR. :)
     
    ASAAR, Feb 27, 2007
    #5
  6. David J. Littleboy wrote:
    > <> wrote:
    >> Hello there,
    >> I waw wondering how camera is actually doing light metering and
    >> adjusting proper exposure.
    >>
    >> In TTL metering light from the scene will be entering the camera
    >> through the lens.
    >> Once the light hit the light meter (or metering points/area) ,it will
    >> measure light intensity.
    >> Now based upon the light intensity it LOOKS UP (Or is it calculating
    >> something on the fly) the corresponding exposure value which need to
    >> be executed.
    >> When we use exposure compensation it uses the exposure value lying
    >> above or below that of the one it arrived at.
    >> If that is the case how is the first look up table calculated ? By
    >> humans through trial and error ?
    >> And it need to know only the light intensity to do all these ?
    >>
    >> Is this correct ?

    >
    > There are at least two ways to measure light _correctly_.
    >
    > 1. Incident metering. This measures the light falling on the subject
    > and produces a theoretically/technically correct reading. (A gray
    > card will turn your TTL meter into an incident meter.) In even
    > lighting with "normal" subjects and no need for artistic
    > intervention, incident metering is the right thing.
    >
    > 2. Spot metering. This measures the intensity of specific areas of the
    > subject and can be used (if you know what you are doing) to
    > determined a subjectively (artistically) correct exposure. It can
    > also be used to determine compromise exposures in uneven lighting.
    > (You don't need a gray card, but you need to know what you are doing;
    > this is the metering method used in zone exposure systems.)
    >
    > The books advertised at this site are good introductions to spot
    > metering.
    > http://www.spotmetering.com/
    >
    > All the other ways I know of off hand (e.g. center weighting, matrix)
    > produce what are essentially random guesses.
    >
    > In real life, most people use one of the random guess methods, looks
    > at the histogram, and then reshoot with an appropriate exposure
    > compensation dialed in.


    You have been at this too long. In real live most people don't have
    any idea and have never heard of the word histogram. :)

    Like with most things the answer really is "It depends."

    The exposure the camera comes up with is good enough for most people.
    Most people are not likely to read this newsgroup. Around here there are
    mostly more advanced photographers.

    For anyone who has to ask, I would suggest first following the basic
    instructions that came with your camera. Work with that first. Next try
    some adjusting. Get a feel for what works for you and if you and get a feel
    for what those adjustments do to the final results. If you find you do want
    to do some adjusting, then as suggested look up information on histograms,
    they can be a very good tool. If you are experiencing a specific problem,
    then stop by and ask about that specific issue.

    Photography has a lot of general rules and they are good, but they are
    all made for certain levels of photographers and they all need to be broken
    from time to time.



    >
    > David J. Littleboy
    > Tokyo, Japan


    --
    Joseph Meehan

    Dia 's Muire duit
     
    Joseph Meehan, Feb 27, 2007
    #6
  7. <> wrote in message
    news:...
    > Hello there,
    > I waw wondering how camera is actually doing light metering and
    > adjusting proper exposure.
    >
    > In TTL metering light from the scene will be entering the camera
    > through the lens.
    > Once the light hit the light meter (or metering points/area) ,it will
    > measure light intensity.
    > Now based upon the light intensity it LOOKS UP (Or is it calculating
    > something on the fly) the corresponding exposure value which need to
    > be executed.
    > When we use exposure compensation it uses the exposure value lying
    > above or below that of the one it arrived at.
    > If that is the case how is the first look up table calculated ? By
    > humans through trial and error ?
    > And it need to know only the light intensity to do all these ?


    Center weighted average, spot, and so on are only one part of sorting this
    all out. The other is how "middle gray" bears on this topic (the zone
    system). Some feel that Ansel Adam's zone system is dead, but it is
    actually the basis for the algorithms used in modern digital cameras.

    This is a topic that you will not conquer quickly and, due to different
    design philosophies, never exhaust completely.
     
    Charles Schuler, Feb 27, 2007
    #7
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