exposure value (Ev)
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- Published on Monday, 06 December 2010 06:33
- Last Updated on Wednesday, 25 January 2012 11:56
- Written by Ric Morte
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you might want to read the previous article: light value
you might also be interested in these articles: film speed | shutter speed | aperture & stops | weston master v
This article should be read in conjunction with "Light Value", a term with which it is often confused.
Take any light-sensitive material and expose it to light and a photochemical reaction will take place. if you take a piece of photographic film out of the box and expose it in full daylight then all the light sensitive chemicals will be permanently changed.
When developed the film will be completely black - it will have been overexposed. Comparing this with a similar piece of film taken straight out of the box but not exposed to light, the film will be almost clear when developed.
The two situations represent the extremes of exposure: complete over-exposure to total under-exposure. Somewhere between these limits there must exist amounts of light that will provide us with shades of grey when the film is developed.
It is this "middle" amount of light in which we are interested because this will provide us with all the tones and nuances between extreme black and extreme white. The question is: how much light? The further question is: how should we refer to this "amount of light" and in what units can it be measured?
As photographers we already know that some light sensitive materials are more sensitive to light than others. The amount of light required to expose photographic film will depend on the sensitivity of the material itself to light: the greater the sensitivity the less light will be required to expose it correctly within the optimum range between over-exposure and under-exposure.
the factors affecting exposure
A piece of film in the open air can only have its exposure to light controlled by the amount of time for which it is exposed. Put the film inside a box and the amount of light can be controlled by the size of the hole through which light enters as well as the duration for which the hole is open.
So now we have the fundamental relation between the three factors:
- how sensitive the film is to light
- the size of the hole - specifically its area
- the duration the hole is open
exposure, film speed, aperture & shutter speed
In practice we do not talk about area of the hole (or lens), nor do we talk about the duration the hole (or lens) is open. For a camera these three factors translate to the more familiar terms:
- film speed (ISO)
- aperture
- shutter speed
When we compare the effect of each of these factors we must maintain a level playing field. There are in fact four variables, film speed, aperture, shutter speed and the amount of light in the scene - the latter corresponding to the amount of light falling on the scene and luminance of the surfaces being photographed. The analysis that follows maintains the scene luminance at a constant level; in other words there is the same amount of light available to the camera.
film speed
A film's sensitivity used to be referred to by its ASA [insert footnote: The ASA value (American Standards Association) was one of a number of scales for representing film speed. The others were the DIN system (Deutsches Institut für Normung) and the BSI system (British Standards Institute). All these systems have now been normalised into one system known by the acronym ISO (International Organization for Standardisation). 100 ASA is the same as ISO 100.] value (see the article on film speed for further details)
Figure 1: ASA film speed scale
Figure 1 shows the ASA values for film with each step representing a doubling of the film's sensitivity: 100 ASA is twice as sensitive to light as 50 ASA film. To maintain the same exposure 100 ASA film will require half the light of 50 ASA film. However our comparison requires us to maintain the scene luminance at a constant level. When we do this, doubling the film speed will double the exposure. Expressing this mathematically we can say:
Figure 2: proportional relation between exposure & film speed
aperture value
For a camera with a lens the light-collecting power is not just the area of the lens but depends on its focal length. Aperture is defined as the ratio of the focal length of the lens to its diameter, expressed as an f-number (see aperture and stops for further details). Each successive step on the scale of f-numbers doubles the aperture and so doubles the amount of light, effectively doubling the exposure:
Figure 3: aperture scale
We now have a scale of Apertures (Av, or Aperture Value) where each step on the scale represents a doubling or halving the amount of light. Expressing this mathematically we can say:
Figure 4: proportional relation between exposure & aperture
shutter speed
Doubling the shutter speed will make the shutter move faster so it is open for less time - this will halve the amount of light entering. This tells us there is an inverse relation between the two:
We can express shutter speed in seconds or, more practically, fractions of a second:
Figure 5: shutter speed scale
You will notice that the values are rounded and not exact doubles of the previous values (see shutter speed for further details). Ignoring this minor difference in notation we now have a scale of Shutter Speeds (Tv, or Timing Value) where each step on the scale represents a doubling or halving the amount of light.
To maintain the same exposure we can increase the aperture (Av) by one step and compensate by shortening the timing value (Tv) by one step. We now have an exactly defined reciprocal relation between the two. Expressing this mathematically we can say:
Figure 6: inverse relation between exposure & aperture
combining the four factors
We are now in a position to combine all four factors:
Figure 7: relation between exposure, film speed, aperture & shutter speed
We can express this as an equality by introducing a constant into the equation:
Figure 8: relation between exposure, film speed, aperture & shutter speed for an arbitrary constant, k
defining exposure value
The question remains: how much light is required to expose the film correctly? We have established that this depends on the sensitivity of the film. If our scale of sensitivity is based on a logarithmic scale (strictly a base-2 scale) then:
- doubling the sensitivity will require half the exposure - obtained either by halving the aperture or doubling the shutter speed
- halving the sensitivity will require doubling the exposure - obtained either by doubling the aperture or by halving the shutter speed
[awaiting equations: exposure value]
To achieve this we can use our scale of Light Values - but only if we specify the film speed for which the relation holds true. It is as if we have two sheets of paper, both of which have a scale: a scale of light values and a scale of exposure values. The question is - where should we put the drawing pin to lock both scales together? The question is better phrased as "How much light out there in the scene produces a specific exposure for a particular film speed inside the camera?" The Ev scale is a base-2 logarithm scale. Like Lv it is open ended. The Exposure Value is defined as:
[awaiting equations: exposure value]
exposure value in relation to aperture and shutter speed
Given that the Exposure Values can be made to match the Light Values for film speed ISO 100 we can draw up a table to show all possible combinations of shutter speed and aperture that will give the same exposure:
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Pop-up version of the table (reduced size) |
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Full PDF version of the original. (Opens in new window or tab)
The values shown on the chart are normalised for ISO100 film speed. |
The colours used in the table are arbitrary and have no connection with spectral colours nor colour temperature.
Having established the relation between aperture, shutter speed and film speed for ISO 100 we can build a more complex set of tables that show the relation between aperture and shutter speeds for a range of ISO film speeds. These combinations are shown in the next two sections.
shutter speed in relation to exposure value and aperture
The table in this section shows shutter speeds for different combinations of film speed and aperture:
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Pop-up version of the table (reduced size) |
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Full PDF version of the original. (Opens in new window or tab)
The values shown on the chart are for a range of ISO film speeds between 100 and 3200. |
Note that the Ev scale can be extended to cover negative values - corresponding to extremely low-light conditions. For simplicity these lower values have been omitted from the table.
aperture in relation to exposure value and shutter speed
The table in this section shows apertures for different combinations of film speed and shutter speed:
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Pop-up version of the table (reduced size) |
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Full PDF version of the original. (Opens in new window or tab)
The values shown on the chart are for a range of ISO film speeds between 100 and 3200. |
As with the previous table and for the same reason, the lower Ev scale values have been omitted.
linking light value (LV) to exposure value (EV)
0 for a shutter speed of 1 sec with an aperture of f/1 using film speed of ISO 100. If we define the brightness of a scene to be 0 to produce this exposure then our scales will match with Lv=Ev=0 at ISO 100.
By pinning the LV scale to the Ev scale for ISO 100 at the point where Lv=Ev=0 we end up with two matched scales where, for example, a Light Value of 5 in the scene will produce and Exposure Value of 5 in the camera.
The Ev scale is not restricted to just 1 aperture setting, 1 shutter speed setting and 1 fiilm speed. We have already established there is a reciprocal relation between aperture and film speed. Film speeds are also defined in terms of a doubling or halving the exposure. We can therefore establish the rerlationship between all the parameters:
