Confused about exposure and light levels

Discussion in 'Beyond the Basics' started by keller, Mar 21, 2006.

  1. keller

    keller TPF Noob!

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    I've been reading a lot about exposure, light levels, and how colour affects light levels. I understand the basic concepts, but I just don't understand their relationship to each other.

    So the amount of light (ie. time of the day) and the colour of the object both affect how much you need to adjust the camera's apeture/shutter speed. But how does it all work? Say you've got a brown object in bright sunlight - the camera measures the light reflected off the object. But why does the camera try to brighten the object? Wouldn't it also take into account the fact that the object, being dark, gives off less light?

    It sounds confusing to me, but basically I'm trying to figure out the "steps" that a camera's light meter takes to determine the correct apeture/shutter.​
     
  2. ksmattfish

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

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    It's not about color, it's about tone (lightness and darkness). Your meter tells you the settings to use to make your subject come out middle gray tone. The meter can't tell what you are metering; whether your subject is very light, in the middle, or very dark, the meter always tells you how to put it in the middle.

    If you meter a white cat, the meter tells you how to get a gray cat. Gray is darker than white, so it underexposes. To get a white cat overexpose from what the meter recommmends.

    If you meter a gray cat, the meter tells you how to get a gray cat.

    If you meter a black cat, the meter tells you how to get a gray cat. Gray is lighter than black, so it overexposes. To get a black cat underexpose from what the meter recommmends.

    It works the same way with colors. Meter off a light blue subject, and the meter will darken to a middle gray tone. Meter off a dark blue subject, and the meter will lighten to a middle gray tone.

    It can take some practice to learn to see tones in colors. Hot colors tend to look brighter than they really are. Cool colors tend to look darker than they really are.
     
  3. selmerdave

    selmerdave TPF Noob!

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    I think one way to keep from getting confused with this stuff is to keep in mind the difference between incident and reflected light. Incident light is the light falling on a subject, where the reflected light is the amount of (the incident) light that the object then reflects.

    For exposure, our main concern is with the incident light. With a given incident light level, a "correct" exposure would be the same for a black velvet curtain as it would be for a mirror. The problem is, it's not always convenient to walk over to our subject to measure the incident light falling on our subject, say for example when the subject is a deer 100m away. So we use a reflected-light meter in our camera, and with a proper understanding of how it works we can basically use that to determine the incident light level falling on the subject. So it is when interpreting this reflected-light reading that we have to take into account the different amount of light that a black velvet curtain reflects compared to a mirror in the same light. Remember, the incident light is the same, and so the correct exposure will also be the same (camera settings) for both.

    Dave
     
  4. Soocom1

    Soocom1 TPF Noob!

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    Consider this:

    Not only does your camera measure the amount of light going into it from the subject, it is also reading stray light coming in from around the subject.

    Darker colored objects absorb more light and thus there is less light hitting the meter. Black backgrounds are preferable for product photography because black naturally absorbs more light. Thus, there is more appealing contrast, and less unappealing contrast. What I mean by that, is that the lighter the overall subject and the lighter the background, the more radical the bright contrast will appear. (The harder it is to see the difference between colors and tones.) Thus washing out the subject. You will get bright white spots where you want color. In these washed out prints, it is very hard to tell the difference between red and pink.

    Less light going into the camera with a black background, but properly lighted, means good contrast, without additional unwanted light coming in. Thus you can see the red vs. the pink.

    The meter in your camera is set up to find what is known as the ‘gray area’ or what is commonly known as the 18% gray area.

    http://home.nc.rr.com/tspadaro/The_Grey_Card.html

    Gray cards are used to calibrate the meters so they will always have a constant point of reference. So for this discussion, call the 18% gray area
    as the zero point or 0.

    The white cat mentioned above will measure in at say a +4. The meter reads the reflected and incident light as being too high. So it tells you to close or ‘stop down’ the camera to bring the meter back to 0.

    The opposite is true if the cat is black. The black cat will hit at –4 and thus the meter will tell you to open up or ‘stop up’ the camera.


    Now comes the point I can loose you, so I will try hard not to. Any expert in this field, please correct me on this if I am wrong. But please try to roll with the terminology I use.


    The time of day as well as the weather conditions determine the Kelvin Temperature of the light from the sun. The Kelvin temperature can be best described for this discussion as the measure of coolness in the sunlight. When in the early morning or late afternoon, you will notice that the light is obviously more yellow. This is because the light from the sun has to travel through more atmosphere to reach you during the extreme parts of daylight. During these times, the Kelvin Temp of the sunlight will hit around 3000-4800 K. (More Yellow). During the mid-day, being directly overhead the sunlight is at neutral, or about 5500K. This is where the most amount of white light (the entire color spectrum) hits. Overcast days cause a ‘cooling effect’ in the air. Or more accurately, gray. Thus the Kelvin Temp becomes higher (around 6000-6500K).

    http://www.phototechmag.com/previous-articles/2003/ma-lane/lane1.html

    Here is where the fun part comes in. The gray card remains constant in its ambient light reflection through all of this. Yes its tonality will change, but its light reflective-ness does not! THIS IS WHY YOUR CAMERA TELLS YOU TO OPEN UP DURING THE LATER AND EARLY PARTS OF THE DAY!!! This is why you typically over expose during these times.
    The Ev settings are a way of ‘tricking’ the camera to get better exposures.

    But they are tuff to learn quickly.

    Here are some links:
    http://www.kenrockwell.com/tech/ev.htm

    http://www.chem.helsinki.fi/~toomas/photo/ev.html
     

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correct exposure in different light levels