How do I know...? (Lighting/Exposure)

Discussion in 'Digital Photography' started by AxelMonkey, Apr 25, 2010.

  1. AxelMonkey macrumors regular

    Mar 1, 2010
    Somewhere between Heaven and Hell, Oregon
    how do I know if my exposure is going to be correct, I read "Understanding Exposure" but im still kinda confused on light metering to get a correct exposure for example Bryan Peterson says " I had set my exposure by aiming my tripod-mounted camera and lens towards the blue sky. With my shutter speed set to 1/500 sec., I adjusted the aperture until f/8 indicated a correct exposure"

    I dont understand "I adjusted the aperture until f/8 indicated a correct exposure"

    can you help me out?

    Please and Thanks:D
  2. toxic macrumors 68000

    Nov 9, 2008
    "proper" exposure (you might want it to be brighter or darker) is determined by a gray card being exposed as middle gray in a scene. some objects give the same reflectance as a gray card, and I believe a blue sky (away from the sun) is one of those things, as is green grass.

    built into your camera is a reflective light meter. when you start metering, usually by half-pressing the shutter button, an arrow shows up in the viewfinder and points at some value on the meter. if you are using an auto mode (P, Av or A, Tv or S), it is set to 0 by default. in other words, it meters the scene so that the region you are metering will be exposed as middle gray (this is assuming you are using partial or spot metering). in manual, you have to vary your f-stop or shutter speed to put the arrow where you want it to be.

    so what Peterson means is that he pointed his camera meter at the blue sky with the shutter set to 1/500. since the sky has the same reflectance as a gray card, he knows for a proper exposure, he needs to center the arrow on the "0" on the metering scale. to do that, he either stopped down (if the arrow was above 0, i.e. overexposure) or opened up (if the arrow was below 0, i.e. underexposure) to f/8.

    the same idea applies if you know the reflectance of any other object or surface. for example, caucasian skin is usually ~1 stop brighter than middle gray. so if you are metering off a white person's skin, you would place the arrow at +1, either by using exposure compensation in the auto modes or doing it yourself in manual.

    center-weighted average and evaluative/matrix metering act a little differently than partial and spot, so the above method may not work. center-weighted does what it sounds like - it averages the scene to middle gray, but biases the calculation to the center. to have the above method work using CWA, the whole frame must be filled with the sky, rather than just the middle.

    evaluative/matrix metering takes the exposure data from the metering zones and active AF point, goes through a database of scenes, guesses what the scene looks like, and selects an appropriate exposure. this mode might not work with Peterson's in I've never tried, so test it out yourself if you use it.
  3. AxelMonkey thread starter macrumors regular

    Mar 1, 2010
    Somewhere between Heaven and Hell, Oregon
    gotcha, so would there be anything on my camera(Canon Rebel XS) that I need to look for to help get a correct reading?
  4. gnd macrumors 6502a


    Jun 2, 2008
    At my cat's house
    I always expose to the right. That is, I go as high with the exposure as possible, without clipping the highlights (not counting reflections on chrome and stuff like that). It is very simple to check that with the histogram.
  5. Edge100 macrumors 68000

    May 14, 2002
    Where am I???

    The bit that says "Exposure level". The indicator will be at the inverted triangle when the exposure is "correct". To the right of that means overexposed, and to the left means underexposed.

    Picture courtesy of
  6. toxic macrumors 68000

    Nov 9, 2008
    not particularly. if you decide to use partial metering (your camera doesn't have spot), you'll have to draw out the metering circle on your own, though, since there isn't one in the viewfinder. it should be a circle that covers the area just within the four AF points surrounding the center.

    remember that 0 on the exposure meter means the average value of the metered area will be neutral gray. if the area you're metering happens to be the same value as neutral gray, then 0 is properly exposed, to the right is overexposed, and to the left is underexposed...but usually that's not the case, and you have to use exposure compensation to correct for it.

    and again, all bets are off if you use evaluative. you'll have to learn how it works over time so you can predict when it will over/underexpose.
  7. HBOC macrumors 68020

    Oct 14, 2008
    TBH, i have never shot in anything other than full manual.

    Edge, thanks for posting that shot.

    Hey Axel, you will get it down. Takes practice. What are you shooting? Or are you just trying to get a grasp on exposure?

    The hard thing with the cameras' metering system is that it isn't as good as our eyes:). So it will gather info on the brightest part of what you are shooting, which causes problems. There are work arounds, of course. I shoot primarily landscapes, so a grad filter can help with that, as can bracketing, although i really have no experience in blending. Reflectors help too! You will learn. I am still learning, and i have been at this forever. :)

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