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Exposure Values (EVs) and Light Values (LVs)
In order to understand how EV settings work, you must first be familiar with the concept of f-stop and shutter speed settings.
Briefly the smaller the f-stop number, the larger the aperture size it represents. Each aperture setting is either half or twice the size of its neighbour.
- f/8 - for example - is half the size of f/5.6, and
- f/5.6 is twice the size of f/8.
Similarly, shutter speed steps are also either half or twice as fast as there neighbour.
- 1/30th - for example - is twice as long as 1/60th, and
- 1/60th is half as long as 1/30th.
Thusly, a number of different combinations of aperture size and shutter speed all produce the same degree of exposure. For example, f/5.6 at 1/60th provides the same level of exposure as f/8 at 1/30th (where the amount of light halves, and the duration of exposure doubles).
Of course, there are even more permutations;
- f/8 at 1/30th is also the same as f/11 at 1/15th (half the light for twice as long), or
- f/4 at 1/125th (four times the light for a quarter of the time), and
- f/2.8 at 1/250th (eight times the light for an eighth of the time) etc.
It follows that, a one step alteration to any one component of combinations of settings will half or double the level of exposure, this is known as a
one stop change.
The Exposure Value (EV) is a numerical scale that represents a combination of a camera shutter speeds and f-numbers that yield the same level of exposure.
The EV scale starts at 0, which represents a 1 second exposure at f/1.0. Lenses with an aperture that big are as rare as unicorns, but it's the same as a 2 second exposure at f/1.4, or a 4 second exposure at f/2, etc.
EV 15 equates to full sunlight with distinct shadows, while EV -4 would be a scene lit by a full moon. An EV is therefore a convenient "system" for describing the quality of light.
A table of descriptions of light quality and their EV rating can be found here.
The EV scale steps align with intervals on the photographic exposure scale. In other words, an increment of one step on the EV scale represents a one stop decrease in exposure, and conversely a one step EV decrease corresponds to a one stop increase in exposure.
For example, if EV 9 corresponds to f/4 and 1/30th of a second, EV 8 is f/4 at 1/15th of a second, and EV 10 is f/4 at 1/60th of a second (plus any other combination of settings that produce the EV scale value).
EVs and 1950s camera design
To simplify the process of setting alternative aperture and shutter speed combinations, a German camera shutter manufacturer - called Friedrich Deckel - first developed the Exposure Value (EV) concept in the 1950s. The likely impetus for this was the rise in popularity of colour film, which required greater exposure accuracy than black and white photography (modern 35mm colour film started to become available in the mid 1930s).
From 1954 onwards, numerous camera (and shutter) manufacturers adopted Deckel's Exposure Value Scale (EVS); including Hasselblad, Kodak, Konica, Olympus, Ricoh, Seikosha and Voigtländer, to name but a few.
They introduced lenses with coupled shutter and aperture dials, and an EV scale, such that, after setting an exposure value, adjusting either the shutter speed or aperture made a corresponding change to the other, in order to maintain an EV setting, e.g. say the camera was set to EV9 with a combination of f/4 and 1/30th - opening up to f/2.8 would automatically and mechanically increase the shutter speed to 1/60th.
When camera models with built-in light meters started to emerge, some also metered against an EV scale (as opposed to an aperture or shutter speed scale), and correct exposure was accomplished by transferring the meter's EV reading to the lens, through the adjustment of lens apertures, shutter speed settings, or an EV setting which automatically set the combination of speed and aperture. The EV scale also went under the name of
Light Value (or LV) once incorporated into metering systems.
The EV fell from favour as a means of setting a camera exposure towards the end of the 1960s, when meter coupling became more common, and removed the need to manually transfer a meter reading to lens settings, or even set anything at all (with the camera doing this automatically). Nevertheless, many makes and models retained exposure compensation facilities that are graded as EV steps (e.g. +/- 2 EV).
The EV scale can still be used as a rough guide to exposure setting in the absence of a light meter, e.g. EV 14 is hazy sunlight with soft shadows, EV 13 is cloudy bright with no shadows, 12 is overcast, and so on (for a 100 ISO film, etc.
(*) EVs and TTL metering SLRs
* I have supplemented the text of this article (15th March 2019) in response to a naive edit to the Fujica ST601 page on Camera-Wiki (subsequently removed). This said that the camera's metering system waspeculiarbecause it locks access to slow shutter speed selection for EV levels outside of the meter's lowest level of sensitivity. The truth is, be the camera a Fujica, Canon, Nikon or Pentax, from the 60s, 70s, or 80s, all will have limits to the metering sensitivity, most likely at the lower light levels, which is exceeded by the extreme capabilities of the physical exposure control parameters.
EVs remain relevant to TTL metering SLRs, since the scale is used to specify meter sensitivity levels.
The meters of these cameras have an EV range; that is to say, they have limits to their sensitivity and cannot measure very low or high levels of light. These limitations can manifest when extremely fast films are used in bright light, or more significantly, slow films are used in low light. In the instance of the former, the problem is simply that the camera's top shutter speed is too slow. What usually happens in the case of the latter is that the camera can have a sufficiently low shutter speed, BUT, the EV is not within the meter's capabilities.
Many TTL metering SLR user manuals have details of their meter's limitations shown in a table. For example, here's the details for a 1971 Miranda Auto Sensorex EE.
Make no mistake, this is not a peculiarity of the Sensorex; all TTL SLR cameras have limitations to metering sensitivity levels. Many cameras also have an in-built device that locks the shutter speed selection dial when the limits of metering sensitivity are reached - to prevent under exposure (although the Sensorex happens to also have an override switch, allowing selection of locked-off speeds).
Upper sensitivity limits became insignificant as metering technology improved, but here's the same table for a 1979 Pentax ME Super. The sensitivity of meters was generally improved, but so too was the shutter speed range ... so there are still lighting conditions for which the camera blocks the slowest shutter speeds.
An EV can be calculated from the aperture and shutter speed using the formula:
EV = log2 (N2/t)
N is the aperture (f-stop)
t is the exposure time (the shutter speed denominator).
For example, with an exposure of 1/125th at f/5.6
log2 (5.6 x 5.6 x 125)
= log2 (3,920)
≈ EV 12
A log base 2 calculator can be found here.
Other pages in this series at Camera Portraits
Analog/Analogue/Argentic? ... or we could just call it film photography!
Apertures and f-stops
Film versus mega-pixels
Flash photography with an electronic flash gun