What is "metering"?

Discussion in 'Beyond the Basics' started by keller, Oct 10, 2005.

  1. keller

    keller TPF Noob!

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    I read that to get a good guide for exposure settings, we have to "meter" the light in an area.

    What does that mean? Do I need any special device to meter? I've currently got a digital camera (with manual settings, but not slr), how do I meter with that?
     
  2. Garbz

    Garbz No longer a newbie, moving up!

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    The camera has a light metre built into it. It basically analyses the light comming into the frame and set's thigns accordingly. In most manual modes this is done via aperature priority. In manual you set the aperature and the light metre takes care of the shutter speed.

    However this works in reverse as well, and if you go fully manual it should give you an indication of how far off the mark you are.

    Light is also metred differently depending on the camera. Some cameras have spot metring where the light is analysed from the very centre of the frame, some are centre weighted where 60% of the weight comes from a small portion in the centre of the frame and 40% from the sides, and some especially new cameras use matrix metring which split the frame into bits and analyse each bit.
     
  3. keller

    keller TPF Noob!

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    So the concept of "light metering" is just setting the shutter speed and aperature?
     
  4. PlasticSpanner

    PlasticSpanner TPF Noob!

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    In it's basic form yes but there's more to it than that! The meter measures the light to determine the exposure level using film speed, aperture and shutter speed.


    For example, if you are taking a photograph of a fast moving car at dusk and your camera automaticaly measures the light levels you may get a small apeture and a long shutter speed resulting in a blured car. (or a high film speed resulting in lots of digital grain)
    In most situations an automatic compact digital would get it right but there will be instances where you will want a certain effect and you'll need to manually set either the apeture or shutter speed and let the meter set the other for you.
     
  5. Garbz

    Garbz No longer a newbie, moving up!

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    the concept metering is internal to the camera. The camera meters the light to determine the correct exposure, and in manual settings some indication of the metre should be visible.

    e.g. in my digicam the information i get is:

    1/750 - f5.6 - 0.0

    So shutter speed, aperature, and relationship compared to ideal setting from light meter.

    If i take aperature up shutter speed goes down:
    1/500 - f8 - 0.0
    If I over expose the image:
    1/250 - f8 - +1.0

    In each mode for manual settings some setting is adjusted and other settings fall into the light meter:
    A: Aperature priority means the shutter adjusts itself
    S: Shutter priority means the apperature adjusts itself.
    M: The Aperature and shutter are manual and the light meter shows if the exposure is correct by adjustingin the 3rd number which ideally should be 0.0
     
  6. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    The most important thing to know is that light meters measure for middle gray. To the meter the world is full of various tones from black to white, with various shades of gray in between. Whatever the scene you point your meter at, the meter gives you the recommended settings to expose the scene as middle gray tone.

    This works pretty well in most situations, as many scenes contain both light and dark parts, and average out to gray somewhere in the middle. But if your composition is mostly full of light tones (like a snowy scene), the recommendation from the meter will under expose. It is trying to turn white snow into gray snow. If your scene is mostly dark it will over expose. It is trying to lighten the dark up to middle gray.

    There are basically 2 types of light meter: reflective and incident. Reflective light meters are used by pointing them at the subject or the scene. Incident measures the light falling on the meter, and is usually used at the subject. In-camera meters are reflective light meters.

    As Garbz posted, there are different reflective metering modes: matrix/segmented, center-weighted, averaging, spot, etc... This is what parts of the scene in the viewfinder are being metered, and what importance the camera is giving those areas. Some cameras only have one metering mode, others have several. It's important to know what your camera uses so you know what it is measuring (and thereby trying to turn middle gray) when you point it at your subject. Most camera manufacturers would probably recommend using matrix (or whatever fancy name your camera manufacturer uses for segmented metering) when in auto exposure modes, but I like spot or center weighted when shooting manual.
     
  7. bobaab

    bobaab TPF Noob!

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    this is a great post, i learned a lot from everyone's comments :) thanks.
     
  8. cumi

    cumi TPF Noob!

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    Let me show you some examples:

    1. Here, I used spot metering on the window (I wanted to have buildings outside correctly, so the room inside is underexponated):

    [​IMG]

    2. Here I used spot metering on furniture in the room (pointed the camera on the furniture, pressed the shutter halfways, recomposed the photo and pressed all the way):

    [​IMG]

    3. Here I used Center-weighted average metering mode, so either the window, nor the room has a correct light:

    [​IMG]

    I can also suggest some links if you want, where this is explained in more detail (I also learned from there). When it is clear, how it works, then you stil have to experiment a lot...
     
  9. cumi

    cumi TPF Noob!

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  10. keller

    keller TPF Noob!

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    Thanks for the advice everyone! One thing I don't get though, is will metering have any effect if you manually adjust the shutter speed, apeture, and ISO? If so, what is the use of metering?
     
  11. Rob

    Rob TPF Noob!

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    Metering is the process of measuring the light.

    Shutter speed and aperture are camera functions affecting the amount of light hitting the film (/sensor).

    ISO would be the sensitivity of a film (/sensor) to light, and a function of the camera which if overruled, affects exposure.

    As a clever light meter yourself, you can take readings from various areas within your scene and make a decision to create a composite "metered exposure" by altering shutter speed, aperture, ISO etc. Or you can use the camera's in-built capabilities, which are quite often very sophisticated.

    The use of metering - the process of measuring light - is essential to manually adjusting the aspects of a shot's exposure. Otherwise, what would you get?

    Rob
     
  12. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    In manual exposure mode the meter does not change the settings; it just displays the setting that would make whatever is in front of it middle gray.

    Using the meter in manual exposure mode can be as simple as pointing it at something that has a medium tone, and is in the same lighting as the subject, to get a recommendation. Or I could point it at my darkest, important shadow detail, and then at my lightest, important highlight detail, and use a setting in the middle.

    It can also be used to determine what tonal value various parts of the scene/subject have. This is important because different films and digital have differing capabilities to record extremes in light and darkness. Digital has a 5 or 6 stop range; gray in the middle, and white or black 3 stops in either direction from that. If I compare something I'm exposing as middle gray to something that is much darker or lighter, and the difference is 3 stops or more, I know that part of the subject is going to turn out as black or white.

    The meter measures light, which you need to know to make a good exposure. Light is measured in stops, which is a doubling or halving of the amount of light. Aperture, shutter, and ISO are also measured in stops.

    Aperture in 1 stop increments: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64...

    Shutter in 1 stop increments: 1 sec, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000...

    ISO in 1 stop increments: 25, 50, 100, 200, 400, 800, 1600, 3200...

    Think of the camera sensor or film as a bucket, and light as water from a faucet. Proper exposure is filling the bucket to the top without overflowing. ISO determines the size of the bucket; an ISO 200 bucket is twice as big as an ISO 400 bucket, and half the size of an ISO 100 bucket. Aperture is how much you turn the faucet on, and shutter is how long you leave the faucet on. You can fill the bucket by turning the faucet to just a trickle, and waiting for a long while. You can also fill it by turning the faucet on full blast for just a little bit. Or you can fill it by doing something in between the two.
     
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