Can someone explain EV?

Discussion in 'Olympus Cameras' started by manju69, Oct 12, 2011.

  1. manju69

    manju69 Mu-43 Veteran

    493
    Jul 1, 2011
    Stroud, UK
    Pete
    Hi. I've had point and shots for years, and rarely used exposure compensation, now i am happily using mu43 cameras, I still don't use this much. I have played with it but can't seem to grasp it's use? This may be obvious but...

    How does it work,? I assume it's different from changing the aperture. And if so, is it some digital process that I can do afterwards?

    If not then, how might in use this feature? Am I missing something? Thanks in advance!
     
  2. xdayv

    xdayv Color Blind

    Aug 26, 2011
    Tacloban City, Philippines
    Dave
    exposure compensation basically compensates your current exposure settings either negative (underexpose) or positive (overexpose)...
     
  3. DrLazer

    DrLazer Mu-43 Regular

    93
    Mar 23, 2011
    Sheffield, UK
    It depends on what mode you are shooting in.
    If you shoot in Manual then you have control over aperture, shutter speed and ISO.
    In this case exposure compensation will not be available.

    If you shoot in Aperture priority, you have control over the aperture but you are letting the camera pick an ISO (assuming you haven't locked your ISO) and a shutter speed for you. The camera bases this on the light meter. Sometimes though (quite often for me) you will find the camera doesn't get it right and you may know better. You can use exposure compensation to adjust what settings the camera picks.

    So for example if you are shooting in Shutter priority and you dial in 1/250. You point the camera at your subject. The camera may pick an aperture of f/8 and an ISO of 400. You think it looks overexposed so you go -1 on exposure compensation. As you are in shutter priority the camera will either adjust your aperture to let less light in (f/16 maybe) or the lower the ISO.
     
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  4. Howi

    Howi Mu-43 Veteran

    208
    Feb 23, 2011
    Sheffield
    Howard
    exposure compensation, is just that, allowing one to compensate for any situation that is outside the norm.
    The 'norm' is to evaluate a scene as 18% grey for exposure purposes, anything outside this will tend to either over or under expose the scene.
    Two examples spring to mind, a snow scene which is predominantly white, will tend to under expose giving grey snow rather than white.
    The other is a black dog against a dark background will tend to over expose, this is because the exposure system is trying to evaluate as though it was seeing 18% grey.
    Knowing we are taking scenes outside the exposure systems norm, we can dial in exposure compensation to allow for the under or over exposure the camera is applying.
    Note, we are talking about going up or down by as much as 2 stops, if you are not sure, then bracket your exposures, one of them is bound to be right or near enough.

    Another use of exposure compensation is when you want to expose to the right (you may have heard of the expression).
    I use exp comp in conjunction with the histogram to try and avoid blown highlights.
    Using the exp comp wheel, I adjust (usually down initially) so I can see on the histogram the blips that go past the end marker (GF1). These would normally be blown, then adjust upwards the exp comp so the blips are as close to the R/H side as possible.
    Some cameras are known to blow highlights quite easily, the simple solution here is to permanently dial in 1/3 or 2/3 of negative exposure compensation.
    For a more detailed explanation of 'exp to the right' do a google search, but the basic principle is, under exposure creates noise when post processing, so get the histogram over towards the right hand side (more light/exposure).
    There are pitfalls though, as histograms can be hard to decifer.

    I have the histogram on permanently.
     
  5. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    Exposure compensation compensated the autoexposure when the camera gets it wrong. Usually a high key scene like a polar bear on snow will be underexposed. In that case you set exposure compensation to a positive value. The opposite is true for a low-key scene like a black cat in a coal cellar, where the camera will overexpose and a negative compensation is used. Back light is often another situation where exposure compensation is good.

    I use exposure compensation quite often.
     
  6. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    There are three things that affect exposure, Aperture, Shutter Speed, and ISO sensitivity. We measure the amount of change these factors make by a standard called "Exposure Value (EV)". So one EV Stop of Aperture, Shutter, or ISO will have the same effect on the overall exposure of the image.

    1 stop for aperture would include the following f-stop values (darkest to brightest): f/32, 22, 16, 11, 8, 5.6, 4, 2.8, 2, 1.4, 1

    1 stop for shutter speed would include the following speeds (darkest to brightest): 1/4000s, 1/2000s, 1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, etc.

    1 stop for ISO would include the following ISO values (darkest to brightest): 100, 200, 400, 800, 1600, 3200, 6400, 12800

    So jumping from one stop in any of those three factors will affect the exposure by 1 EV.

    If you are in Aperture Priority, then your Shutter Speed is the first thing that will be changed by Exposure Compensation (ISO will only be bumped after your shutter speed reaches as slow as the camera will allow with your chosen aperture). So if you dial in +1 EV for instance, then the shutter speed could be changed from 1/125s to 1/60s.

    If you're in Shutter Priority, then the Aperture is the first thing that will be changed (ISO will be bumped only after you reach maximum aperture size for the lens). So if you dial in +1 EV in Exposure Compensation, then your aperture will jump from say f/4 to f/2.8 for instance.

    After the slowest shutter speed (in A mode) or widest aperture (in S mode) is obtained, then the camera will bump up the ISO if it's not locked. To gain +1 EV for instance, it could bump the ISO from 200 to 400.
     
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  7. DDBazooka

    DDBazooka Mu-43 Veteran

    211
    Sep 3, 2011
    The camera automatically decides the exposure for you, too dark? EV up! Too bright? EV down!
     
  8. GaryAyala

    GaryAyala Mu-43 Legend

    Jan 2, 2011
    SoCal
    Manju69,

    Exposure Compensation is used if one desires to consistently over or under expose. So the question which begs to be asked is "Okay, so when would I want to under/over expose on every shot?"

    In order to determine when to implement Exposure Compensation it is best if one has as understanding of how the camera light meter works.

    #1 The light meter is a guide to help the photog determine proper exposure, not a setting determination set in stone.

    #2 The light meter reads the light (either from the light source or reflecting off the subject), evaluates the lights and tosses out a medium gray exposure setting (13%/18% gray).

    So what does that mean? If a photog fills the viewfinder with a black wall, centers the light meter needle and releases the shutter, process/develop normally and makes a normal print, the resulting image will be a medium gray wall (13%/18% medium gray).

    If a photog fills the viewfinder with a white wall, centers the light meter needle and releases the shutter, process/develop normally and makes a normal print, the resulting image will be a medium gray wall (13%/18% medium gray).

    Okay, so what does this all me and why would I care?

    Well the same science works with people, flowers, cars, et cetera. Say you take a meter reading off the skin tones of a light skinned or even dark skinned person. The resulting image will give both a similar skin tone/color. The light skinned person's skin will be rendered a complexion equal in density to dark gray and the dark skinned person will have have pretty much the same dark gray density. Both will look on the sickly side of life.

    The easy retort is "So, that's why I have Photoshop." While yes you can correct the exposure in PS, the problem when you start moving those sliders you start losing details on the ends of the histogram. So if you move a slider to lighten up the light skinned person, details which are lighter toned than the skin tone will start to fade and similarly on the dark skinned person when you start moving the slider to darken up the skin you start losing detail in the areas which are darker than the skin tone.

    So smart exposures will maximize the dynamic range in the image and minimize the alterations, slider movement and time on the computer time processing the image in photo manipulation programs.

    #3
    Okay the meter is just a guide ... how do I shoot armed with this knowledge?
    For starters shoot in 'Manual Mode'. Purchase a Gray Card from a camera store or online. The Gray Card reflects the density/tone of what the meter reads and is a good tool for comparisons and for metering. When possible meter off the Gray Card instead of the subject. So if you're shooting a portrait, have the subject hold up the Gray Card and meter in manual mode off that card (hold the card over the face and fill the viewfinder with the card) and release the shutter. Next, reset the camera to an auto mode, expose as you normally expose and release the shutter. You should see a bit of a difference in the computer.

    Light skin is roughly one stop over-exposed (+1EV) from medium gray and grass is roughly one stop under-exposed (-1EV) from medium gray. Next time you shoot a portrait read off the skin and over-expose by one stop and viola! ... you should have a properly exposed image (well ... properly exposed for the person at least).

    Say you're shooting a picnic with everybody wearing different tones/density shirts from dark to light. Expose for the grass and under-expose a stop and now you're in the ballpark for the entire scene (shadows will always be a problem though).

    When shooting, using the meter as a guide, experience/trial-and-error goes a long long way to dialing in a proper exposure.

    Manual metering is especially effective in tough lighting conditions, i.e. back lighting, spot lighting, extreme subject tones (black cat in snow, white cat in a cave), low lighting situations. Most importantly, in Manual Mode, one can over/under expose quite easily per shot and/or as required.

    Gary

    PS- This is just a cursory overview, there is a lot more than the above. For example, Google Ansel Adams Zone System, just one of many ways to maximize exposure results.
    G
     
  9. manju69

    manju69 Mu-43 Veteran

    493
    Jul 1, 2011
    Stroud, UK
    Pete
    Thanks for explaining this - just so i get this correct, so does this mean adjusting the EV changes different values depending on what mode you are using, (one of three-aperture, shutter speed and ISO) I always imagined it was adjusting some mysterious fourth value. It sounds like an easy way to quickly bump the exposure one way or another without fiddling around with too many settings - is that right?
     
  10. manju69

    manju69 Mu-43 Veteran

    493
    Jul 1, 2011
    Stroud, UK
    Pete
    No, not that hard, but I like real to hear it from real live people, and I am grateful for their time. Sometimes it makes a change to not use google for everything!