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Thread: need explanation on metering..

  1. #1
    Silhouette
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    Default need explanation on metering..

    Hi all, i read something about metering through the camera TTL metering.

    Said sthg that according to the camera, the correct exposure should be 18% reflected light so the camera actually suggest an exposre that gives the subject a mid- grey tone.

    Does that mean that for white surface which reflects almost 100% light, the camera will "advise" to cut down light to 18%?
    and for black surfaces reflecting almost 0% light, the camera will advise to increase light intensity?

    Another qn is :does that mean that when we use matrix metering, the camera will take the average of the scene light intensity and suggest a certain exposure which will actually yield an 18% reflected light intensity throughout the whole scene?

    Am i making sense?
    Ethan...

  2. #2

    Default Re: need explanation on metering..

    Originally posted by Silhouette
    Hi all, i read something about metering through the camera TTL metering.

    Said sthg that according to the camera, the correct exposure should be 18% reflected light so the camera actually suggest an exposre that gives the subject a mid- grey tone.

    Does that mean that for white surface which reflects almost 100% light, the camera will "advise" to cut down light to 18%?
    and for black surfaces reflecting almost 0% light, the camera will advise to increase light intensity?

    Another qn is :does that mean that when we use matrix metering, the camera will take the average of the scene light intensity and suggest a certain exposure which will actually yield an 18% reflected light intensity throughout the whole scene?

    Am i making sense?
    Ethan...
    If you are talking abt the plain partial or spot metering (non matrix), the camera will assume 18% gray when u meter 100%light(white) or 0% (black)... there fore leading to underexposure and overexposure respectively. It would be your job to add exposure compensation +1 to 2EV, -1 to 2EV respectively.

    But if ure using matrix metering, camera might have some form of inteligence to compensate for you.

  3. #3

    Default

    I am not an expert, but I think that your questions can be answered by understanding the difference between light falling on a subject and light reflecting off a subject.

    Look at the pictures on this site and see the differences between metering incident and reflected light.

    http://www.sekonic.com/BenefitsOfIncident.html

  4. #4
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    Default

    As I understand it, matrix meters don't average the whole scene and try to get it an average 18% grey. Some old SLRs have that sort of meter.

    Matrix meters look at the relative brightness of different parts of the matrix (segments) and try to guess what sort of scene you're trying to take. They have been programmed with exposure comparisons of typical scenes, and they'll try to fit your lighting scene with their 'memory bank' of scenes. They don't really know what you're looking at, but they try to match the lighting pattern with one of their stored patterns.

    So they're supposed to be able to give you accurate exposures for sunsets, portraits, etc. They can be fooled, of course.

  5. #5
    Member Knighthunter's Avatar
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    Default Re: need explanation on metering..

    Originally posted by Silhouette
    Said sthg that according to the camera, the correct exposure should be 18% reflected light so the camera actually suggest an exposre that gives the subject a mid- grey tone.

    Does that mean that for white surface which reflects almost 100% light, the camera will "advise" to cut down light to 18%?
    and for black surfaces reflecting almost 0% light, the camera will advise to increase light intensity?

    Yes that's true. Camera metering always sees 18% grey. So if you meter predominantly white the camera will give underexposed reading, because the camera metering try to bring white scene into 18% gray. For predominantly black the camera will do the opposite way by giving overexposed reading to bring black colour into 18% gray.
    I think now with DSLR metering is not so critical since the final image can be adjusted by PS or even worse with RAW files exposure can corrected after taking picture.
    Knowing the basic of exposure metering is very critical for slides since the film latitude margin very low.
    W204FL

  6. #6
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    Default Re: Re: need explanation on metering..

    Originally posted by Knighthunter


    Yes that's true. Camera metering always sees 18% grey. So if you meter predominantly white the camera will give underexposed reading, because the camera metering try to bring white scene into 18% gray. For predominantly black the camera will do the opposite way by giving overexposed reading to bring black colour into 18% gray.
    I think now with DSLR metering is not so critical since the final image can be adjusted by PS or even worse with RAW files exposure can corrected after taking picture.
    Knowing the basic of exposure metering is very critical for slides since the film latitude margin very low.
    Exposure is still critical with DSLRs. There's only so much PS can do. And if you overexpose with a DC, good luck. You can do anything with blown highlights on digital images, while slightly underexposed images can be more easily corrected.

    Regards
    CK

  7. #7
    Member Knighthunter's Avatar
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    Default Re: Re: Re: need explanation on metering..

    Originally posted by ckiang


    Exposure is still critical with DSLRs. There's only so much PS can do. And if you overexpose with a DC, good luck. You can do anything with blown highlights on digital images, while slightly underexposed images can be more easily corrected.

    Regards
    CK
    Agree CK,
    With DSLR as long as the details not blown off the exposure still can be adjusted, there is forgiveness in DSLR/Digicam even not 100% chance. For 35mm negative/slide......is unforgiven, you are on mercy of film latitude and labs to adjust the compensation.
    W204FL

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