EV( Exposure Value)

Discussion in 'Beginner Questions' started by mark_ward|7, Aug 3, 2010.

  1. Hello!!!
    Can someone define EV on a camera? I know about Exposure compensation and Metering, but when it comes to EV(Exposure Value) on a camera I am lost. EV is often referred to when reading about it in HDR books. I want to make sure I am doing my HDRs right.
  2. Read here http://en.wikipedia.org/wiki/Exposure_value
    Basicly, when you meter a scene you will determine a certain exposure value. For example a sunny day will give you EV 15.
    There is are a number of shutter and aperture values that will give you the same EV. So EV 15 (aka sunny 16 rule for a sunny day) will give you f16 with 1/125. or I can dial in f11 with 1/250 or f8 with 1/500. All of these combos with equal EV15, assumeing ISO 100 (or ISO 125, which was more common with film).
    See the chart at the bottom of the page a linked to earlier
  3. EV really is not tied to iso.

    EV really means a combo of f stop and shutter speed

    An EV=0 being 1 second at F1.0 was defined about 60 years ago.

    If one has say bright sun and iso 25 film; then one has say an EV=13; ie 1/125 @F8
    With an EV=13 one can use 1/60 at F11 too.

    1950's cameras often tied the shutter and Fstop together as one EV setting; a Retina IIIc has this feature.
  4. For HDR photography with a modern DSLR with TTL metering, the actual definition of an EV is less important than understanding what happens when you change EV and how this affects the recorded image. A +1 change in EV means a doubling of light hitting the sensor. A -1 change in EV means cutting the amount of light in half. That part is pretty straight forward. The important part is understanding that when you double the amount of light hitting the sensor, you will get more detail in the darker parts of the image, but you will loose definition in the highlights. When you cut the exposure in half, the highlights will have more definition (spectral highlights will always be blown), but the shadows will have little to no definition at all. The point of HDR is to combine exposures so that the highlights, the mid tones, and the shadows are all exposed adequately to give good definition.
    As far as exposure is concerned, it makes no difference how you achieve the change in EV. You could change f-stop, ISO, shutter speed or any combination of the three. However, changing any of the three factors can have dramatic affects on the image that is rendered. Longer shutter speeds allow more motion blur, while changing aperture changes depth of field, and changing ISO can increase or decrease noise in digital cameras. For HDR with static subjects, you would be better served restricting your changes in EV to adjusting the shutter speed as subject motion is not a concern and camera motion can be accounted for with the use of a tripod or image stabilization.
    PS: I would mention that Kelly is correct in that ISO is not directly related to EV (posted while I was typing), it is more a term of the sensitivity of the sensor/film, but doubling or halving the ISO, independent of a corresponding change in shutter speed or aperture, results in the same effect as increasing/decreasing the actual EV. By changing the ISO, you aren't actually changing the amount of light hitting the sensor, you're changing how sensitive the sensor is to that light.
  5. Yes I know that EV is not directly tied to ISO, however in my example I used EV 15 on a typical sunny day (and this will change at different areas of the world and at different seasons). And as Kelly pointed out you would arrive at a different EV if you meter with at a different ISO. EV 15 with ISO 125 and EV 13 with ISO 25. Simply two stops difference in sensitivity so requiring a two stop shift in EV. Not sure if I am saying that just right, please forgive me.

    And Kelly, you are very right about the Retina IIIc tieing the aperture and shutter together. I have one in rather good shape. Well...if you don't count the stuck shutter and sticky rangefinder. I can not afford the CLA on it just yet, and intend to do the rather simple mod to remove the apeture and shutter interlock. I find it to be a pain.
  6. Mark, take a look at the explanations and tables on Fred Parker's Ultimate Exposure Computer. It will help clarify what EV means within the context of the relationship between light, light sensitivity of the recording medium (film or digital), shutter speed and aperture.
  7. SCL


    Think of EV as a basic measure of intensity of light where each increase in numerical value equals a doubling of the previous intensity. Now think of aperture, shutter speed and iso as components (dependent variables) in an equation whose total value = EV. That's the simplest explanation I can think of.
  8. As you know, exposure is a measure of the light energy allowed to play on the film or digital chip. Exposure consists of two elements; 1.Lens aperture 2. Light intensity. There is a third element, the sensitivity of the film or digital chip, ISO (International Standards Origination) quantity of light requirement. When not otherwise stated we assume ISO = 100.
    When we set our camera, we are working with two intertwined setting, lens aperture and shutter speed. This is because exposure E is equal to the intensity I of the light multiplied by the time T the shutter is open. The equation is E-IxT.
    We communicate the exposure we used by writing the aperture and the shutter speed. Since the amount of light a lens will pass is a function of its working diameter and its focal length we must state this as a ratio. We call this value a focal ratio, it is the working focal length divided by the working diameter. As an example, a 50mm lens with a working diameter or 12.5mm is 50 รท 12.4 = 4 written as f/4. The time of exposure is generally written as a fraction of a second such as 1/125 sec. read as one hundred twenty fifth of a second.
    Two examples 1/60 sec. @ f/8 or 1/125 sec. @ f/5.6.
    All this seems like gobbledygook to the beginner. The dream was to simplify and simplify some more. One idea was to assign a integer (whole number) to each of the shutter speeds and a integer to each of the f/numbers. The Ev (exposure value) is simply these two numbers added together.
    Shutter speeds, each twice as fast as the next are assigned an integer value.
    1 second = 0 - 1/2 sec. = 1 - 1/4 sec. = 2 - 1/8 sec. = 3 - 1/15 sec. = 4 - 1/30 sec. = 5 - 1/60 sec. = 6 - 1/125 sec. = 7 - 1/250 sec. = 8 - 1/500 sec. = 9 - 1/1000 sec. = 10 - 1/2000 sec. = 11 - 1/4000 sec. = 12
    Aperture each passes 1/2 as much light as its neighbor going right is assigned an integer value. Note this number set is each f/number going right is its neighbor on the left multiplied by the square root of 2
    (1.4). Such a number set creates a services of circles each with 1/2 the surface area of its neighbor thus a halving of the passage of light energy.
    f/1 =0 - f/1.4=1 - f/2=2 - f/2.8=3 - f/4=4 - f/5.6=5 - f/8=6 - f/11=7 - f/16=8 - f/22=9 - f/32=10 - f/45=11 - f/64=12
    Ev =The integer assigned to the aperture added to the integer of the shutter speed.
    Thus f/4 @ 1/60 = 4 + 6 = Ev 10 (see above table for each integer).
    Another example f/22 @ 1/8 = 9 + 3 = Ev 12
    Cameras of the 50's and 60's and light meters attempted to introduce this system. It's impact on our industry is marginal.

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