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Thread: Exposure Compensation question

  1. #1
    Hazeb1's Avatar
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    Exposure Compensation question

    Aloha all,

    I'm trying to improve my understanding of how to get a proper exposure. I've been having fun, playing with long and short exposures in varying light conditions and getting some mixed results. My question is, how does the EV +/- settings in the exposure compensation function relate to stopping up or down? Is there sort of an accepted industry standard or is it largely mfgr specific? (ie: +1 EV= 1 stop)

    I'm using a Nikon D5100

    Thanks, in advance , for any advice.

    Warren

  2. #2
    herbert's Avatar
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    Re: Exposure Compensation question

    Hi Warren,

    1 stop is a doubling or halving of light so it is standard. This is the same as a 1EV change.

    EV is used to specify a defined amount of light. So 4EV actually means a certain amount of light and is comparable. Given a certain EV value you can use a table to see the ISO, aperture and shutter speeds required to expose the shot. Have a look at this:

    http://en.wikipedia.org/wiki/Exposure_value

    You can easily see that 1 stop is a doubling or halving of light by setting you camera to manual mode and the doubling or halving the shutter speed. You will see the exposure meter go up and down in 1 EV increments. This also applies to the ISO.

    Note that you do not need to double or half the aperture. Aperture values represent a diameter of a circle. To double the light you need to double the area of the circle. So you use the square root of 2 (1.4) to calculate the increase in aperture. This gives the familiar series 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.

    Alex
    Last edited by herbert; 2nd March 2012 at 08:54 AM.

  3. #3
    Hazeb1's Avatar
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    Re: Exposure Compensation question

    Thanks Alex!!

    This is very helpful. Now time to play around some. :-)

  4. #4

    Re: Exposure Compensation question

    I agree with Alex, but I feel that it's worth mentioning that a camera's metering system can meter different types of scenes with varying degrees of accuracy. I would recommend staying in manual mode so that your exposure is more predictable across different scenes. It also helps you get a better feel for the camera's tendencies.

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    Re: Exposure Compensation question

    To understand how to get proper exposure you must understand how the camera metering works. Basically, the camera only sees gray. And not just any gray...it sees 12.7% gray. 12.7% gray means that the surface is reflecting 12.7% of the light that falls on it. A white surface can reflect 90% or more of the light that falls on it, while a black surface reflect 5% or sometimes less (as with black velvet, which seems to just eat light.)

    If you take a picture of a white wall, it comes out gray. If you take a picture of a black wall, it comes out gray. The camera simply presumes that everything in front of it is gray, and adjusts its exposure to give a 12.7% gray scene. This means that the camera meter is basically wrong the vast majority of the time. Understanding this fact is half the journey to understanding exposure. The other half of the journey is learning what to do about it.

    If you take a picture of a white wall and it comes out gray (as expected, now that you know what the camera is up to) then the white wall was underexposed. You need more exposure to get the white wall white.

    If you take a picture of a black wall and it comes out gray, then the black wall was overexposed. You need less exposure to get the black wall black.

    And so here are your guiding principles for exposure:
    Bright objects that reflect lots of light require positive Exposure Compensation.
    Dark objects that reflect little light require negative Exposure Compensation.

    The next question is...how much positive or negative exposure is required? That depends on the brightness, or tone, of the scene. You must learn the basic tones and then expand on those tones through experience. Part of what makes a photographer a photographer is knowledge. Holding a camera makes you no more a photographer than holding a brush makes you a painter. Exposure knowledge is the core knowledge of a photographer, and it's something that you should devote considerable time to learn and understand.

    The basic tones are common tones that can be used to set exposure. You meter the tone, and then you apply a well-known Exposure Compensation for that tone. When you take the picture, you'll have standard exposure (which, actually, may not be what you want...but more on that later.) So here's a list...
    Clear blue sky, about 45 degrees up: 0 EC
    Clear blue sky, at zenith: -1 EC (yes, the sky is darker directly overhead...take a look. Better yet...meter it.)
    Clear blue sky at horizon: +1 EC
    The sunny side of healthy green grass: 0 EC
    Snow: +2 EC (but the brave and precise can take it to +2.7 EC)
    Evergreen trees (no new growth): -1 EC
    Caucasian skin: +1 EC
    Beach sand: +2 EC
    The 18% gray card you should have in your pocket: +.5 EC

    You should use Spot metering when metering your exposure reference to eliminate any influence from the surrounding scene.

    With experience you should build on this list. On his website, photographer Jim Doty gives some EC corrections he's developed for his landscape photography...
    Birch bark: +1.5 EC
    Yellow aspen leaves: +1 EC
    Buffalo mane: -1.5 EC

    So you essentially learn how to compensate the things you like to photograph.

    The EC function affects the meter only. The meter will measure the light, combine that reading with a value that represents the ISO, and arrive at a number called the Exposure Value (EV). When you apply Exposure Compensation, the EC is subtracted from the EV. The result is then passed on to the camera, which operates as it normally would for the given EV. Here's an example using a Nikon D90 and P mode...

    Lets say the camera meters the scene, and with an ISO of 200 arrives at an EV of 16. I'm using a wide angle lens and the P-mode exposure chart (in my camera manual) says that the camera will select 1/500s and f/11 as the exposure for 16 EV. But there's a problem...the scene is a snow scene. So I set my EC to +2 to compensate. The meter will now subtract 2 from 16, and report 14 EV as the exposure value. P-mode, still working from the exposure chart, will now select 1/250s and f/8 as the exposure for 14 EV, and give us 2 stops more exposure...exactly what we need for a bright snow scene.

    My exposure correction has given me standard exposure. Standard exposure basically means that things should (within the limits of your camera) come out as they appear. However, that may not be what you want. You may want to intentionally over or underexpose a scene for artistic reasons. The compensation for your artistic vision should be applied after you make your technical correction. So you first achieve standard exposure, and then you shift the Exposure Compensation up or down to suit your vision. With practice you'll do it in one step, but when starting out I suggest a two-step approach with a picture taken at standard exposure just to remind you of what you were looking at.

    This is old-school exposure knowledge. Today, people will tell you to simply look at the histogram, correct, and take another shot, or to use your bracketing function to take 3 shots and pick the best one. But what if your first shot is your only shot? It's helpful to be able to get the shot right (or at least very close) on the first try.

    Once you understand exposure you can learn the difference between how a human sees and how a camera sees. You can start with this...

    http://web.mit.edu/persci/people/ade..._illusion.html

    Take a picture of it...see what the camera comes up with for A and B.

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    Re: Exposure Compensation question

    Quote Originally Posted by Graystar View Post
    The camera simply presumes that everything in front of it is gray, and adjusts its exposure to give a 12.7% gray scene. This means that the camera meter is basically wrong the vast majority of the time.
    Probably worth mentioning also though that the camera doesn't just evaluate a scene in only one place (with the exception of spot-metering) - thus it's capable of evaluating the contrast of a scene - and thus - assuming the scene is made up of a healthy dose of reflective and non-reflective surfaces ("highlights and shadows") - then it usually does a pretty good job of getting it right; the bigger issues occur when there's a predominance of one or the other (eg white polar bear in the snow or the proverbial "black cat on a black rug"), or a mixture of reflected and incident light (eg back lighting).

    If in doubt, an incident light meter works pure magic

  7. #7
    Hazeb1's Avatar
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    Re: Exposure Compensation question

    Thanks a bunch!! You guys are great. I really appreciated the info.

    Lots to think about and experiment with. ;-)

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