Can Someone Explain "Metering", "Exposure Compensation", and "Stops"

Discussion in 'Digital Photography' started by SolracSelbor, Dec 24, 2007.

  1. SolracSelbor macrumors 6502

    Nov 26, 2007
    I dont really understand what metering is and when to use the various otptions such as center, partial, or spot metering. Also, I dont understand what exposure compensation is and when to use it. Finally, what are "stops" such as saying "stabilization" only works between 1 and 2 "stops"?

    Thank you all and Merry Christmas!
  2. termina3 macrumors 65816

    Jul 16, 2007
    Metering is measuring the amount of light in a picture.

    The specific types I can't explain myself--surely someone else will.

    Stops are the different levels or places where different settings can be placed (shutter stops are often 60 to 80 to 125 to 160... aperture 2.8 to 3.2 to 4.5...)
  3. klymr macrumors 65816

    May 16, 2007
    First off, metering is meauring light, like it was said before, to determine the correct exposure. A camera will meter for everything to be about an 18% gray, meaning if you took a picture of snow metered properly, it'd turn out slightly gray. When you look through the view finder, you see the thing at the bottom that looks like this: +|...|...0...|...|- You want the bars underneath to be right around 0. You can read more about metering here:

    Exposure compensation is basicly to compensate for when you may not be getting a proper reading with your exposure meter, such as when using filters, etc (at least I think that's what it is). Here is more info on that:


    A stop is pretty much any relative change in light. This site explains it pretty well, and even has a cool little chart to play with to explain ALL of this exposure/metering stuff. Here it is:
  4. Erendiox macrumors 6502a


    Oct 15, 2004
    Brooklyn NY
    Metering is the method by which a camera will determine correct exposure. For instance, stop metering usually means the camera will measure the correct exposure by the brightness of whatever is exactly in the center of the frame. Other settings may take an average of several points within the frame for exposure. Take a look at your camera manual for explanations on how the various settings work.

    Not quite sure what exposure compensation is, but I think Klymr may have hit that nail on the head.

    A stop, more specifically, is a change in the amount of light by a factor of two. In other words, you're either halving or doubling the amount of light during exposure. So dropping the exposure by one stop means you're halving the amount of light. Raising the exposure by one stop is doubling the light. This continues with two stops meaning a factor of four, three stops meaning a factor of eight, etc.
  5. klymr macrumors 65816

    May 16, 2007
    Those are the words I was looking for! On a side note...
  6. compuwar macrumors 601


    Oct 5, 2006
    Northern/Central VA
    As others have said, your camera's built-in meter tries to set the exposure values for an 18% gray exposure (due to historical paper contrast values.) Center-weighted metering just uses the light values from the center of the image, spot uses them from a specific point. In general, you can "trust" that your camera's metering database has seen enough "similar" scenes that it'll pick the right value in matrix or full-scene metering mode, or you can select what to meter based upon the tonal range in the scene. For example, if you have a rock in the scene that's around 18% gray and in the same light as your subject, you can spot meter the rock, and use that to set your exposure value for the entire scene. If there's something with the same tonal value as 18% gray, but in color you can do the same. If you know that say your fresh snow is 2 stops higher than 18% gray, then you can meter off the snow, dial in two stops of exposure compensation (or change either the exposure manually) and shoot away with the snow where you want it. The same goes for a black subject- the camera is going to say "Hey, that's 18% gray!" so you need to dial in negative exposure compensation to compensate for the fact that the black object is darker than the gray. You can purchase 18% gray cards to carry around, then you can spot meter off the card and get the lighting in the scene matched to the camera's meter. Obviously, you have to put the card in the same light as your subject- such as under a tree if you have someone sitting in the shade.

    With digital, it gets a little easier. You can simply shoot something white, and examine your camera's histogram to see where the white part of the exposure lies. You want it as far to the right as possible without going over if you want any detail at all, or right on/over the line if you want to get a completely white, no detail exposure. If your white object is in the same light as your subject, then you can set the exposure that way by spot metering (some advocate using a fuzzy white towel,) but if your subject is in darker light, you'll have to open up the exposure to get a good shot- the easy way is by dialing in an appropriate amount of exposure compensation. Learning to use your histogram is good- if your camera doesn't do separate red, green and blue channel histograms, you may occasionally have an image where you blow out one of the channels inadvertently. Learning what conditions that happens in is a good thing.

    Once you meter a scene and figure out what your exposure is (because the camera will do the 18% gray thing, you may want a scene to be darker and moodier, or lighter so there's no one 'correct' exposure, you get to decide what's important about a scene. You have three exposure values that you can modify ISO, shutter speed and aperture. ISO is the sensitivity of the recording medium, in the old days it was "film speed." On digital, it's the sensitivity of the sensor to light. All sensors have a "base ISO" that is their natural sensitivity (usually 100 or 200.) above or below that, the camera's electronics modify the signal to gain more sensitivity. This modification comes at the price of sensor noise, which make the pictures grainy. In film days it was the size of the actual grains- so we keep the term "grainy." The closer you are to the base ISO of your camera, the less noise the picture will have, but the more light you'll need. Shutter speed is how long it takes the shutter to expose your medium (film or sensor.) and aperture is how much light is let in by your lens. The aperture number is a ratio, so the smaller the number, the bigger the opening, or the more light that is let in.

    High shutter speeds allow you to freeze subject movement and make it so that camera movement is less apparent in the image. Large apertures have narrower depth of field, or parts of the image that are in apparent focus from the plane of focus, which is parallel to your lens and sensor plane on most cameras (I won't distract you with the exceptions.)

    Each full value of ISO, shutter speed or aperture is what's called a "stop" of light. It halves or doubles the amount of light necessary for an equivalent exposure. So, if you have 1/60th of a second at f/8 at ISO 400, and you change any single value one stop, one of the other values must be changed in the opposite direction to get an equivalent exposure- say 1/30th at f/11 at ISO 400 or 1/125 at f/8 at ISO 200.

    There are a some "rules of thumb" that apply here in terms of getting a sharp image- the first is that it's best to hand-hold a lens at a shutter speed that's 1/focal length or faster. So a 500mm lens needs 1/500th of a second to hand hold for most people (some can go down to 1/250th, some need 1/1000th.) Obviously, this is only in terms of camera/photographer movement- you need fast shutter speeds to freeze a subject's movement. Image stabilization gives you one to three stops of photographer/camera movement above this rule of thumb. So, if you need 1/500th you can shoot at say 1/250th or 1/125th and still not see much photographer-induced vibration. In real tests, you'll often find that using a tripod provides a visibly sharper image than IS/VR does at slower shutter speeds, and as you go down the scale, the effects are less noticeable, as they are on shorter rather than longer lenses. The amount of impact depends highly on the implementation, in body or in lens. Currently, in-lens implementations are better, but obviously have to be purchased for each applicable lens.

    A new rule of thumb with digital is that you should try to keep your aperture at f/11 or wider (lower number.) That's due to the diffraction limits of the physics of light combined with the sensor's photosite size. The smaller the sensor, with equivalent resolution, the more diffraction comes to play, as well as the larger the number of photosites. That's one of the reasons that a 6MP camera is "better" than a 12MP camera with the same sensor size in terms of diffraction- which is not what the megapixel fans will have you believe. With people and animal shots, a shallow depth of field isolates the subject and makes for nice images. With things like landscape shots, generally you want a larger depth of field, and with a larger depth of field you don't have to be as exact with your focusing. With a 6MP 1.5x crop factor (APS-C sized sensor) diffraction starts to impact sharpness at about f/12.9. With 12.4MP, it comes in at f/8.9. So, you get about a one stop difference between a Nikon D40 and a Nikon D2x, and the "pro" camera is more limited by diffraction than the "consumer" one.

    I think that about covers it...
  7. Clix Pix macrumors demi-goddess

    Clix Pix

    Oct 9, 2005
    8 miles from the Apple Store at Tysons (VA)
    In addition to reading and re-reading and memorizing Compuwar's excellent post, I would strongly urge you to go to the library or bookstore and pick up some books on basic principles and techniques of photography.

    Note to mods: this is such an excellent, succinct explanation that Compuwar's post ought to be "stickied" for future reference!
  8. kitmos macrumors newbie

    Jul 10, 2007
    yes thank you for the excellent post Compuwar. I learned a lot.:eek:
  9. Westside guy macrumors 603

    Westside guy

    Oct 15, 2003
    The soggy side of the Pacific NW
    Sometimes I think we need a stickie specifically about diffraction - a LOT of people don't know about it. There have been several discussions, over in various Flickr groups and Nikonians over the past month or two, where a person recently moving from film couldn't figure out why his/her photos aren't particularly sharp - and when you look at the EXIF data, you see they've shot at f/22, f/27, etc.

    Cambridge in Colour has an excellent piece on diffraction. It's rather technical, but it also has some great visual examples that should make the concept quite clear.

    BTW having hung out at Nikonians a bit recently... we should be thankful for decent forum software over here at MacRumors. :D I know the search function here is pathetic, but all in all the package Arn uses is pretty nice.
  10. SolracSelbor thread starter macrumors 6502

    Nov 26, 2007
    You my friend are a hero! Thank you so much for this and have a merry Christmas and a happy new year!
  11. Padaung macrumors 6502


    Jan 22, 2007
    Fantastic explanation, Compuwar.

    I also ditto Clix pix's comment on reading all the books and magazines you can. And then go out and practice, and have some fun at the same time!

    Happy Christmas everyone, btw :)
  12. Digital Skunk macrumors 604

    Digital Skunk

    Dec 23, 2006
    In my imagination
    Dude you are SO wrong! :mad:

    Just kidding, the most accurate answer I have ever seen on Mac Rumor. Another vote for making this a sticky. You know they will ask that question again.
  13. termina3 macrumors 65816

    Jul 16, 2007
    Great answer, Compuwar.

    That said, can we stop quoting it? It's long enough once.
  14. compuwar macrumors 601


    Oct 5, 2006
    Northern/Central VA
    You're welcome, happy holidays!

    I know, many of the folks who don't know are professional photographers :(
    It used to be that folks tested their equipment, checked their results and discussed them, now in a lot of places they just post mediocre pictures and pat one another on the back. Thankfully the quality here is above the norm.

    I keep telling folks to hold on to those 6MP cameras as long as they can- and I keep telling beginners to start with a 6MP camera, but everyone's in the "more is better' camp. It's got me really wanting a D3 for my landscape and panorama work, since you can't always focus stack images.

    It's one of the best resources around for sure- thanks for referencing it!

    I've pretty-much stopped hanging out there- after the insurance fiasco (which fortunately I didn't sign up for since I found a better policy) I decided to let my membership lapse- any organization that unprofessional doesn't deserve my support. I'm more of a fan of SMF than vBulletin, but vBulletin 's the market leader for a reason. I still haven't figured out why they're using a different domain for their search engine there, but that software is about the worst forum software I've seen on a site that size. Ick!

    Merry Christmas to you as well! You're most welcome!

    Thanks, and a Merry Christmas to you!


    I know it's long- but I didn't want to skip over too much. I'm a big fan of people understanding their equipment- that's the difference between a photographer and a picture taker in my book.
  15. SummerBreeze macrumors 6502a


    Sep 11, 2005
    Chicago, IL
    Thank you to Compuwar for such a great post! I'm relatively new to photography, and yours was one of the best explanations I've seen.

    For the original poster, there are some great exposure resources on this Digital Photography School Post. As far as exposure goes, it's very helpful to look at the EXIF data for photos that you like. It really helps you get a sense of how photographers get perfectly exposed photos.
  16. AndrewMorrell macrumors newbie


    Dec 16, 2007
    Shaker Heights, OH, USA
    Holy cow - that was an enlightening article. Thanks for linking it.
  17. Doylem macrumors 68040


    Dec 30, 2006
    Wherever I hang my hat...
    Some interesting stuff in this thread... though I wonder why the OP asked for the fundamentals of photography to be explained - again - while there's a manual to read, library books to borrow and web-based photo-resources by the dozen.

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