# Light value

In photography and photometry, the light value LW ( exposure value , Ev ) denotes a group of equivalent time-aperture combinations.

## definition From the light value 12 to the grid lines you follow the diagonals to the crossing point and get the exposure settings, here z. B. f / 4 and 1/250 s.

A light value LW is a number on a logarithmic scale with a base of two, which designates a family of f-stops and exposure times that all lead to the same exposure for a given subject brightness:

${\ displaystyle {\ text {LW}} = \ log _ {2} {\ frac {k ^ {2}} {t}}}$ (1)

Stand by

• k for the f-number (= reciprocal of the relative aperture, i.e. large f-number = small aperture), and
• t for the exposure time in seconds.

A light value of zero is thus defined as the exposure that is mathematically equivalent to the f-number 1 and the exposure time of one second. With constant subject brightness, every increase in the light value by one corresponds to halving the exposure, and every decrease by one corresponds to doubling. Light values ​​can also become negative.

Light values ​​are also used to describe exposure differences or corrections, if it should be left open whether the difference or correction can actually be implemented by changing the aperture or changing the exposure time. A difference of a light value corresponds to a difference of an aperture level or a time level.

## Table for the light values ​​of time-aperture combinations Older analog light meter, which has a light value scale in addition to time and aperture scales, here set to EV 9. Medium format lens Zeiss Planar 1: 2.8 / 80 mm with central shutter and light value scale (reddish numbers below right, here LW 12). The associated time-aperture pairings can be read at the top right, here set to f / 2.8 and 1/500 s.
LW 4 s 2 s 1 s 1/2 s 1/4 s 1/8 s 1/15 s 1/30 s 1/60 s 1/125 s 1/250 s 1/500 s 1/1000 s 1/2000 s 1/4000 s
f / 32 8th 9 10 11 12 13 14th 15th 16 17th 18th 19th 20th 21st 22nd
f / 22 7th 8th 9 10 11 12 13 14th 15th 16 17th 18th 19th 20th 21st
f / 16 6th 7th 8th 9 10 11 12 13 14th 15th 16 17th 18th 19th 20th
f / 11 5 6th 7th 8th 9 10 11 12 13 14th 15th 16 17th 18th 19th
f / 8 4th 5 6th 7th 8th 9 10 11 12 13 14th 15th 16 17th 18th
f / 5.6 3 4th 5 6th 7th 8th 9 10 11 12 13 14th 15th 16 17th
f / 4 2 3 4th 5 6th 7th 8th 9 10 11 12 13 14th 15th 16
f / 2.8 1 2 3 4th 5 6th 7th 8th 9 10 11 12 13 14th 15th
f / 2 0 1 2 3 4th 5 6th 7th 8th 9 10 11 12 13 14th
f / 1.4 −1 0 1 2 3 4th 5 6th 7th 8th 9 10 11 12 13
f / 1 −2 −1 0 1 2 3 4th 5 6th 7th 8th 9 10 11 12

Table 1 : Light values ​​LW for various combinations of aperture and exposure time

## Exposure time and f-number

The light value is made up of Orifice LWK (Engl. Aperture value , Av - not to be confused with "f-number") and the Zeitleitwert LWt (Engl. Timevalue , Tv - not to be confused with "exposure time"). The equation (1) can be divided into two summands:

${\ displaystyle \ log _ {2} {\ frac {k ^ {2}} {t}} = \ log _ {2} {k ^ {2}} + \ log _ {2} {\ frac {1} {t}}}$ (2)

The aperture conductance is defined as the first of these two summands, which depends solely on the aperture number:

${\ displaystyle {\ text {LWk}} = \ log _ {2} {k ^ {2}}}$ (3)

You can imagine that the values ​​of the international aperture scale are simply numbered consecutively - see table 2. Every increase in the aperture value by one corresponds to a reduction of the aperture by a full aperture value. Apertures greater than f / 1 have negative conductance values. Intermediate values ​​are also possible; for example, the aperture value LWk 0.5 belongs to the aperture f / 1.2.

cover f / 0.5 f / 0.7 f / 1 f / 1.4 f / 2 f / 2.8 f / 4 f / 5.6 f / 8 f / 11 f / 16 f / 22 f / 32 f / 45 f / 64 f / 90 f / 128
LWk (Av) −2 −1 0 1 2 3 4th 5 6th 7th 8th 9 10 11 12 13 14th

Table 2 : Orifice conductance LWk for different orifices

The time conductance is defined accordingly as the second summand from equation (2), which depends solely on the exposure time:

${\ displaystyle {\ text {LWk}} = \ log _ {2} {\ frac {1} {t}}}$ (4)

Each increase in the time conductance by one corresponds to halving the exposure time - see Table 3. Exposure times longer than 1 s have negative conductance values. Intermediate values ​​are also possible; for example, the exposure time 1/90 s includes the time conductance LWt 6.5.

time 2 s 1 s 1/2 s 1/4 s 1/8 s 1/15 s [1/16] 1/30 s [1/32] 1/60 s [1/64] 1/125 s [1/128] 1/250 s [1/256] 1/500 s [1/512] 1/1000 s [1/1024] 1/2000 s [1/2048] 1/4000 s [1/4096] 1/8000 s [1/8192]
LWt (Tv) −1 0 1 2 3 4th 5 6th 7th 8th 9 10 11 12 13

Table 3 : Time conductance values ​​LWt for different exposure times (the exact numerical values ​​in square brackets)

Using equations (1), (3) and (4), equation (2) can be formulated as:

${\ displaystyle {\ text {LW}} = {\ text {LWk}} + {\ text {LWt}}}$ (5)

That is, by adding the diaphragm conductance and the time conductance, the light value of the relevant time-diaphragm combination results.

## Brightness and film speed Table on the back of the Gossen Profisix light meter for provisional determination of illuminance levels in lux and fc from light values at ISO 50/18 °

Light values ​​describe time-aperture combinations, not brightness. A relationship between light value and brightness can only be established with reference to a film sensitivity or an exposure index ( EI , given in ASA or linear ISO values). The exposure index represents an amount of light, and the exposure index that leads to a correctly exposed image for a specific photographic medium is the sensitivity of that medium.

Measuring reflected light with an exposure meter solves the following equation:

${\ displaystyle {\ frac {k ^ {2}} {t}} = {\ frac {L \ cdot S} {K}}}$ (6)

Stand by

• k for the f-number,
• t for the exposure time in seconds,
• L for the luminance of the motif in candela per square meter,
• S for the exposure index in ASA and
• K for the object measurement calibration constant of the light meter in candelaseconds per square meter.

Correspondingly, measuring incident light with a light meter solves the following equation:

${\ displaystyle {\ frac {k ^ {2}} {t}} = {\ frac {E_ {v} \ cdot S} {C}}}$ (7)

Stand by

• k for the f-number,
• t for the exposure time in seconds,
• E v for the illuminance of the light in lux,
• S for the exposure index in ASA and
• C for the light metering calibration constant in lux seconds.

This means that the exposure meter translates the measured luminance L (when measuring objects) or illuminance E v (when measuring light) into a family of time-aperture combinations k and t (a light value), which correspond to the selected exposure index S (the sensitivity) lead to a correctly exposed image. In particular, a change in the exposure index with the subject brightness remaining the same will lead to the determination of a different time-aperture combination and thus a different light value. Without reference to an exposure index, it is not possible to assign a time / aperture combination or a light value to a subject brightness.

In order to convert the light value determined for a given exposure index S 0 for a given subject brightness into the light value for another exposure index S, the number of doublings between S 0 and S (if S is greater than S 0 ) is added or subtracted Number of halves (if S is less than S 0 ). For example, the light value for ISO 1600/33 ° (EI 1600) is three greater and that for ISO 50/18 ° (EI 50) is two less than that for ISO 200/24 ​​° (EI 200). So be LW S 0 of Exposure Index S 0 light value determined. If the subject brightness remains the same, the light value LW S for exposure index S is calculated as:

${\ displaystyle {\ text {LW}} _ {S} = {\ text {LW}} _ {S_ {0}} + \ log _ {2} {\ frac {S} {S_ {0}}}}$ (8th)

This means nothing else than that with constant subject brightness, for each doubling of the sensitivity, you have to stop down by one f-stop or the exposure time has to be halved.

## Light value and APEX system

The light value as well as its two guide values ​​from which it is composed are part of the more comprehensive APEX system . This also defines guide values ​​for brightness and sensitivity. Orifice Av (. Engl aperture value ), Zeitleitwert Tv (. Engl time value ) and their sum, the exposure value Ev (. English exposure value , not to be confused with illuminance E v ) we already know (all symbols as above):

${\ displaystyle {\ text {Ev}} = \ log _ {2} {\ frac {k ^ {2}} {t}}}$ (1')
${\ displaystyle {\ text {Av}} = \ log _ {2} {k ^ {2}}}$ (3 ')
${\ displaystyle {\ text {Tv}} = \ log _ {2} {\ frac {1} {t}}}$ (4 ')

The other sizes of the APEX system are the Helligkeitsleitwert Bv (engl. Brightnessvalue ) and Empfindlichkeitsleitwert Sv (engl. Speed value ). They are defined as follows:

${\ displaystyle {\ text {Bv}} = \ log _ {2} {\ frac {3,125 \ cdot L} {K}} = \ log _ {2} {\ frac {3,125 \ cdot E_ {v}} { C}}}$ (9)
${\ displaystyle {\ text {Sv}} = \ log _ {2} {\ frac {S} {3,125}}}$ (10)

The factor 3.125 in equations (9) and (10) is used to shift the zero point of the Sv scale to the exposure index ISO 3/6 ° (exact value: ASA 3.125 = 6 DIN) - see Table 4. Without this factor The zero point of the Sv scale would be ISO 1 / 1.07 ° = ASA 1.00 = 1.07 DIN and would thus result in "crooked" numbers for the main ISO values.

This allows the relationships between aperture, exposure time, light value, sensitivity and brightness to be formulated very simply and elegantly. Equation (2) or (5) becomes in APEX notation:

${\ displaystyle {\ text {Ev}} = {\ text {Av}} + {\ text {Tv}}}$ (11)

The equations (6) and (7) are in APEX notation:

${\ displaystyle {\ text {Ev}} = {\ text {Bv}} + {\ text {Sv}}}$ (12)

And from equations (11) and (12) it follows immediately:

${\ displaystyle {\ text {Bv}} = {\ text {Av}} + {\ text {Tv}} - {\ text {Sv}} = {\ text {Ev}} - {\ text {Sv}} }$ (13)

The brightness conductance Bv is therefore identical to the light value at ISO 3/6 °. But instead of referring to this unusual (because it is extremely low) sensitivity, the specification of brightnesses as "light value at ISO 100/21" (sometimes abbreviated as EV 100 ) has found a certain spread among photographers. Also for specifying z. B. of work areas of exposure meters or autofocus systems, this derived unit is common. There is nothing wrong with that, as long as the reference to a sensitivity is not omitted and EV and EV 100 are not confused with one another.

The notation of the guiding values ​​( values ) of the APEX system is inconsistent in the literature. They can also be found with a capital V (EV, AV, TV etc.) or with a small V as a subscript.

ISO lin. (ASA) 3 [3.125] 6 [6.25] 12 [12.5] 25th 50 100 200 400 800 1600 3200 6400 12,500 [12,800] 25,000 [25,600] 50,000 [51,200] 100,000 [102,400]
ISO log. (DIN) 6 ° 9 ° 12 ° 15 ° 18 ° 21 ° 24 ° 27 ° 30 ° 33 ° 36 ° 39 ° 42 ° 45 ° 48 ° 51 °
Sv 0 1 2 3 4th 5 6th 7th 8th 9 10 11 12 13 14th 15th

Table 4 : Sensitivity guide values ​​Sv for various exposure indices according to the linear and logarithmic ISO scale (the exact numerical values ​​in square brackets)