Difference between revisions of "Light and Exposure Values (LV & EV)"

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(Corrections, clean up, remove duplication. Exposure scales are not logarithmic, they are exponential)
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'''Light Value''' ('''LV''') and '''Exposure Value''' ('''EV''') are numerical scales for measuring the amount of light, and the exposure, respectively. These are often confused - not least by the labeling on some cameras; the difference is that LV measures how much light there is, and EV measures how much of the light is allowed into the camera. The scale is logarithmic- that is, each step up on the LV scale is twice the amount of light, and a step up on the EV scale correspondingly halves the exposure.
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'''Light Value''' ('''LV''') and '''Exposure Value''' ('''EV''') are scales for measuring the light value and exposure value, respectively. These are often confused, not least by the markings on some cameras. LV is absolute light intensity, while EV allows for film sensitivity. At ISO100 / DIN21 they are equivalent. Each step up or down the scale halves or doubles the light.
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Typical values of LV in daylight vary between about 16 (extremely bright sun), down to around 10 (dark clouds) and of course, lower in very bad weather, at night or indoors.  For most purposes, a range of (say) 2 to 18 covers most conditions where the photographer can still see their subject. Many [[light meter]]s are not sensitive enough to reach even this range, although for some special purposes (such as astronomy) values beyond this range may be required.
 
  
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LV is often used as a scale on a [[light meter]]; the LV must then have a factor added for the [[film speed]] to get the required EV.  The film speed factor for 100ASA is zero, so EV and LV numbers are the same at this speed; slower speeds require a negative factor - so 25ASA film would mean that the EV is LV-2 (i.e. two stops slower than for 100ASA).
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Typical values of LV in daylight vary between about 16 (bright sun), down to around 10 (dark clouds) and of course, lower in very bad weather, at night or indoors. For most purposes, a range of 2 to 18 covers most conditions where the photographer can still see the subject. Many [[light meter]]s are not sensitive enough to reach even this range, although for some special purposes (such as astronomy) values beyond this range may be required.
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The single-number EV is then converted into a set of combinations of aperture and shutter settings - for example, EV 14 may be 1/250s at f/8, 1/125s at f/11 or 1/60s at f/16 - and others, limited by the shutter speeds and apertures available.
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Exposure meters often display EV which can be directly transferred to the exposure setting ring of some cameras, otherwise the value must be converted to corresponding aperture and shutter speed settings. These are usually shown on a calculator dial or scale on the meter itself.
 
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|image_source= http://www.flickr.com/photos/24225011@N04/2579469995/in/pool-camerapedia
 
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|image_text= EV scale and locked aperture/shutter rings<br>on a [[Kodak Retinette]]
 
|image_text= EV scale and locked aperture/shutter rings<br>on a [[Kodak Retinette]]
 
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The [[Light-value system|'''Light Value System''']] is a method of using the LV/EV on a camera, where the exposure is set by dialling the LV or EV; the camera may then have a fixed aperture/shutter combination, or possibly give the user an adjustment which moves the aperture and shutter settings together to maintain the same EV.
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The [[Light-value system|'''Light Value System''']] is a method of using EV on a camera, where the exposure is set by dialling EV, the camera then has a fixed aperture/shutter combination set by the photographer.
  
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In the photo (left) the EV is set on the red scale by turning the lower ring, which locks the aperture and speed rings together; the speed/aperture can then be adjusted in sync using the upper ring. Some cameras only have the EV scale (e.g. the [[Kodak_Pony_828/135#Pony_II|Kodak Pony II]] or the [[Kodak Auto Colorsnap 35 ]]), and do not have separate speed or aperture markings.
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In the photo (left) EV is set on the red scale by turning the lower ring, which locks the aperture and speed rings together, the speed/aperture can then be adjusted together using the upper ring. Some cameras only have an EV scale (e.g. the [[Kodak_Pony_828/135#Pony_II|Kodak Pony II]] or the [[Kodak Auto Colorsnap 35 ]]) and do not have separate speed or aperture markings.
 
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|image_text= Exposure calculator<br>on a Kodak Bantam Colorsnap
 
|image_text= Exposure calculator<br>on a Kodak Bantam Colorsnap
 
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The photo on the right shows an LV exposure calculator; the dial is rotated to set the weather on the left scale, and the EV read off on the right, against the film speed - which is shown in [[ASA|DIN]] and two example film types. The EV is then set on a scale on the lens.
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The photo on the right shows an LV exposure calculator, the dial is rotated to set the weather on the left scale and EV read off on the right against the film speed, which is shown in [[ASA|DIN]] with two example film types. The EV is then set on a scale on the lens.
  
 
The centre of the dial is the [[red window]] for frame counting.
 
The centre of the dial is the [[red window]] for frame counting.
  
 
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The EV/LV system became popular during the 1960s, when the growth of colour photography made accurate exposure more important, but it died out again into the 1970s as cheaper, more compact electronics made in-camera light measurements possible and coupling of light meters to exposure controls made manually transferring values redundant.
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The EV/LV system became popular during the 1960s, when the growth of colour photography made accurate exposure more important, but it died out again into the 1970s as cheaper, more compact, electronics made in-camera light meters possible and coupling to exposure controls made manually transferring values redundant.
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In [[photography]] the EV or '''Exposure Value''' is a scale used for light measurements. [[Light meter]]s are often scaled in EVs, and many cameras feature an EV scale for setting the exposure.
 
  
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The EV can be calculated from the aperture and shutter speed thus:
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EV can be calculated from the aperture and shutter speed thus:
  
 
   EV = log<sub>2</sub> (N<sup>2</sup>/t)  &nbsp; &nbsp; &nbsp; &nbsp; (or, since log<sub>10</sub>(2) = ~3.322, EV = 3.322 log<sub>10</sub>(N<sup>2</sup>/t) )
 
   EV = log<sub>2</sub> (N<sup>2</sup>/t)  &nbsp; &nbsp; &nbsp; &nbsp; (or, since log<sub>10</sub>(2) = ~3.322, EV = 3.322 log<sub>10</sub>(N<sup>2</sup>/t) )

Revision as of 20:38, 15 May 2012

Glossary Terms

Light Value (LV) and Exposure Value (EV) are scales for measuring the light value and exposure value, respectively. These are often confused, not least by the markings on some cameras. LV is absolute light intensity, while EV allows for film sensitivity. At ISO100 / DIN21 they are equivalent. Each step up or down the scale halves or doubles the light.

Typical values of LV in daylight vary between about 16 (bright sun), down to around 10 (dark clouds) and of course, lower in very bad weather, at night or indoors. For most purposes, a range of 2 to 18 covers most conditions where the photographer can still see the subject. Many light meters are not sensitive enough to reach even this range, although for some special purposes (such as astronomy) values beyond this range may be required.

Exposure meters often display EV which can be directly transferred to the exposure setting ring of some cameras, otherwise the value must be converted to corresponding aperture and shutter speed settings. These are usually shown on a calculator dial or scale on the meter itself.

2579469995_f0b2f91418_o.jpg
EV scale and locked aperture/shutter rings
on a Kodak Retinette

The Light Value System is a method of using EV on a camera, where the exposure is set by dialling EV, the camera then has a fixed aperture/shutter combination set by the photographer.

In the photo (left) EV is set on the red scale by turning the lower ring, which locks the aperture and speed rings together, the speed/aperture can then be adjusted together using the upper ring. Some cameras only have an EV scale (e.g. the Kodak Pony II or the Kodak Auto Colorsnap 35 ) and do not have separate speed or aperture markings.

2579228666_864af649f3.jpg
Exposure calculator
on a Kodak Bantam Colorsnap

The photo on the right shows an LV exposure calculator, the dial is rotated to set the weather on the left scale and EV read off on the right against the film speed, which is shown in DIN with two example film types. The EV is then set on a scale on the lens.

The centre of the dial is the red window for frame counting.


The EV/LV system became popular during the 1960s, when the growth of colour photography made accurate exposure more important, but it died out again into the 1970s as cheaper, more compact, electronics made in-camera light meters possible and coupling to exposure controls made manually transferring values redundant.

EV can be calculated from the aperture and shutter speed thus: 

  EV = log2 (N2/t)          (or, since log10(2) = ~3.322, EV = 3.322 log10(N2/t) )

where

  • and t is the exposure time (shutter speed).

so, for example, an exposure of 1/250s at f8 is an EV of log2(8*8*250) = 14 (to 2 figures, actually nearer 13.96578...)

Link

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