LV, Light Value and EV, Exposure Value, are terms used to allow easy discussion of exposure and light without the confusion of the many equivalent combinations shutter speeds and apertures.
LV refers to how bright the subject is. EV is the exposure setting on the camera.
You may have seen them if you like to read the fine print of camera specifications. They are used to specify ranges of light levels for metering and autofocus.
EV and LV follow an open-ended scale. Each one is one stop away from the next. In photography values of about 0 to 18 are commonly used. Negative values are perfectly valid, just very dark and only occur in night photography. LV 15 is full daylight, for example.
Each Exposure Value, or EV, represents any of many different but equivalent combinations of f/stop and shutter speed. For instance, 1/250 at f/8 is EV14, and so is 1/125 at f/11. 1/125 at f/8, one stop more exposure, is EV13, and 1/250 at f/11, one stop less exposure, is EV15. You don't need to remember these, they are on the dial of your exposure meter.
Understanding them will allow you to recognize common lighting values and guess correctly at exposures even without a meter.
This system is the correct way to discuss photographic light and exposure because it avoids all the confusion of f/stops and shutter speeds, if all you really want to discuss is light and exposure levels. it replaces the idiotic question I get all the time while shooting, "what f/stop are you using," which of course means nothing by itself.
LV, or Light Values
An LV, or Light Value, is a number that represents how bright a subject appears in absolute terms. It does not take film speeds or exposure into account. LVs are very handy photographic terms to use to describe lighting levels.
LVs measure light coming from a subject, or "luminance." They are not a measure of how much light is falling on a subject. In other words, the same light falling on a black object will have a lower LV than the same light falling on a white object.
Some light meters, especially spot meters like the wonderful Pentax Digital Spotmeter and analog Pentax Spotmeter V, read directly in LV. You transfer this number to a dial that, along with your film speed, reads out all the combinations of aperture and shutter speed that will give the correct exposure.
Here's a table of common Light Values associated with common situations. If you use one of the Pentax meters you will quickly start to learn these without even needing the meter after a while. This is because the same number pops up for each subject each time. LVs eliminate the confusing issues of film speeds and f/stops that hide these simple truths when using SLRs or other light meters:
LV18 and above: Bright reflection off a sunlit object, including reflections off the sea
LV17 White object in full sunlight
LV16 Light gray object or skin in full sunlight
LV15 Gray card in full sunlight; typical exposure for ugly front-lit noon daylight photos
LV14 Typical light level for side-lit daylight shots in good afternoon light
LV13 Typical shadow cast in a daylight scene; cloudy bright days
LV12 California bright overcast
LV11
LV10 Dark, dreary overcast day in Boston, London or Paris
LV 9
LV 8
LV 7 Typical indoors; light outdoors about 10 minutes after sunset
LV 6
LV 5
LV 4
LV 3 Brightly lit night street scenes
LV 2 Typical night street scenes
LV 1 Dark scenes outdoors at night
LV 0 LV Zero is defined as the light level that requires a 1 second exposure at f/1 with ISO/ASA100 speed film.
LV-1
LV-2
LV-3
LV-4
LV-5 Scene lit by the full moon
LV-15 Scene lit only by starlight. I have loaded sheet film in light this dark, so don't expect to photograph it or meter it.
Some light meters use a similar scale, but shifted by a constant amount. For instance, the Gossen Luna-Pro uses a scale that reads 5 units higher, or reads 20 in full sun. It's still the same concept, and even those meters calculate the same Exposure Values, or EV, once you set your film speed. That brings us to:
EV, or Exposure Value
Exposure Value, or EV, varies from LV, or Light Value, depending on your film speed.
EV = LV at ISO 100
With ISO/ASA 100 speed film you expose with an Exposure Value (EV) equal to the Light Value (LV).
Easy, eh? Your meter will do this for you, but you can do it in your head, too, if you forget your meter. Here's how:
If you shoot slower film you of course have to use more exposure (EV) for the same Light Value (LV), and vice-versa.
The EV is easy to calculate even if you forgot your meter, since each unit is one stop different than the next. For instance, with film a stop slower than ISO/ASA 100 (like 50 speed Velvia) you just subtract one from the LV to get the EV. This adds one stop of exposure.
For instance, if your subject is at LV14, expose at EV13 with ASA 50 film. EV13 gives one stop more exposure than EV14.
With ISO/ASA 400 speed film you add two to the LV to get the EV, which is the same as subtracting two stops of exposure. Therefore with an LV14 subject you expose at EV16.
Remember that the higher numbers refer to higher light values, and therefore less exposure. This is because the exposure values that correspond to those higher numbers give less exposure.
EVs are a great idea: by talking about an EV you are talking about any one of many different combinations of aperture and shutter speed that give the SAME exposure. Cameras started to use these numbers in the 1950s, but today only the Hasselblad retains them. With every other camera one needs to use the scales on light meters to determine the EV values. Some cameras can be adapted, as I did to my Plaubel Makina 67, which simplifies their use with spot meters.
Zero EV is defined as f/1.0 at one second. Therefore, EV0 is a pretty long exposure. This is the same exposure as f/1.4 at 2 seconds, f/2.0 at 4 seconds, f/2.8 at 8 seconds and so on. EV1 is one stop less: f/1.4 at 1 second. EV 2 is two stops less: f/2.0 at 1 second or equivalent EV is a camera setting. It was popular in the 1950s to couple camera controls together so that once one set an EV one could rotate locked f/stop and shutter speed rings to choose between different equivalent settings. Today only Hasselblad continues the tradition. It is much easier to remember typical light conditions as a single EV number than combinations of camera shutter and aperture settings.
So what's the correct exposure for Velvia (ASA/ISO 50) in side-lit daylight? That light is LV14. Since Velvia is one stop slower than 100 we need to give it one more stop exposure, or SUBTRACT one EV from the LV to get the EV. Therefore, LV14 - 1EV= EV13. EV13 is 1/125 at f/8 or 1/15 at f/22. Light meters that read in EV have scales on the side that show you all the equivalent camera settings for any EV.
What tricks does this tell us? Well, in nature nothing gets brighter than something lit by full sun, which is LV15. If you see LV17 in your meter you know that that must be a white object in daylight. Guess what: that's how evaluative and matrix meters know that, too!
Source : http://www.kenrockwell.com
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