Difference between revisions of "Light and Exposure Values (LV & EV)"
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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 doubles or halves the light respectively. | '''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 doubles or halves the light respectively. | ||
| − | 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 | + | 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 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 range LV 0 (semi darkness) to LV 16 (bright sunlight) corresponds to a light increase of 65,500 fold (2<sup>16</sup>), easily handled by our eyes but impossible for any film or light sensor to encompass. | ||
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. | 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. | ||
Revision as of 22:19, 17 May 2012
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 doubles or halves the light respectively.
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 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.
The range LV 0 (semi darkness) to LV 16 (bright sunlight) corresponds to a light increase of 65,500 fold (216), easily handled by our eyes but impossible for any film or light sensor to encompass.
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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| 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.
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| 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
- N is the relative aperture (f-stop)
- 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...)
