The exposure value is a measurement of light intensity.

The "Exposure Value" of old (often abbreviated as 'EV') is a rather arcane way to measure the light density of an area. The definition is based upon an exposure value of 10 being 10 candles per square foot. Increasing this to 11 involves doubling of the candles. Going from a value of 10 to 9 is half as many candles.

The modern definition that I have found refers to a more modern system that defines EV 0 as the theoretical correct exposure of 1 second shutter speed at f/1.0. Each stop change in aperture or shutter speed darker increases the exposure value by 1. Thus f/1.4 at 1 second is EV 1 as is f/1.0 at 1/2 second.

Low light exposure values:

1. Security Lit Building
2. Bonfire or skyline after dark
3. Floodlit building
4. Fireworks and lit fountains
5. Christmas Trees and candles
6. School Auditoriums, Home Interiors, Traffic
7. Bright Interiors, Parks, Rides, Fairs
8. Bright Indoors, Indoor sports, stage shows
9. Neon Signs, Fires, Outdoor night sports
10. Sunrise and Sunset

The above table indicates that if you are taking a photograph of a Christmas tree, we are looking at an approximate exposure value of 5. This would indicate that at f/1.4 the shutter speed would be about 1/15th of a second. Likewise, at f/4 this would mean a shutter speed of 1/2 second.

EV = log2(f2/T)
"f" is the f-stop, and "T" is the time in seconds.

This newer form is used with SLR light metering that claim to be able to meter correctly over a certain range. For example: "EV 1-20 at ISO 100 and f/1.4". This statement means that with a f/1.4 lens and a film speed of 100, the camera will meter correctly for the level of light between EV 1 to EV 20. Using a lens three stops slower (f/4) will mean that the camera will meter correctly between EV 4 and EV 23.

## Why is this important?

The camera meter tries to produce a photograph that has an 'average' level of color. Most often, this average is defined as 18% gray (standard card used for metering). If the photograph averages to this value, then all is well and the photograph is metered correctly. However, if the photograph has a disproportionate amount of light or dark colors, the metering is off. If there is a significant amount of light colors in the photograph, the metering will darken the entire photograph to compensate. This may result in an underexposed photograph. Likewise, a large amount of dark colors will result in an overexposed photograph.

Increase the exposure value on the camera when photographing light subjects or subjects that are back lit. Decrease the exposure value when the subject is dominated by shadows or is dark in color.

http://www.chem.helsinki.fi/~toomas/photo/ev.html

In photographical terms, EV is probably the single most important number you will have to understand, to understand the theory behind the art of photography. This goes from your tiniest, least significant compact camera, to your cock-on-the-table style medium format camera wit a digital back.

Non-technical

Let us imagine a value called TCE. This TCE (The Correct Exposure) does not exist, because you might for a variety of reasons want a different exposure than the TCE. But for the sake of argument, let's assume TCE exists, and this is what you will want when you take a certain picture.

To get a correct exposure, you will want to have EXACTLY the right amount of light to capture your image. Not too much, and not too little.

So, what is it that might affect how much light comes to the film or imaging chip?

• Shutter speed - Imagine a mug with a lid containing a mysterious source of light, and the room you stand in is covered in darkness. Shutter speed would be how long you open the lid.
• Aperture - Same cup, same concept, but this time, how far you open the lid (if you open it a little - small aperture, i.e high aperture numbers (for example f/22). If you open it all the way - large aperture - i.e low aperture numbers (for example f/2.8)
• These are the two basic ones. The last factor that comes into play is your film speed, or the light sensitivity of your surroundings while holding the cup if you will.

That's all there is to it - these three factors combined allow you to manipulate the light in all kinds of ways (big depth of field through small apertures, freezing motion through fast shutter times, etc).

So, to get TCE, you will want to combine these three factors into JUST the correct way. Now, if you replace TCE with TCEV (The Correct Exposure Value), you understand what I have been on about.

EV is a number describing an exposure - any exposure - regardless of its "correctness".

Technical

The definition of EV=0 is an exposure of 1 second at f/1 using ISO 100 film, or any equivalent thereof (2 seconds f/1.4, 4 seconds f/2.0 etc)

The technical definition of EV is 2EV = LS/C.

• EV = the exposure value - explained above
• L = field (or zone) luminance -
• C = Exposure Constant - This is a constant that depends on what unit you are using to express the luminance (L)If you use candelas/ft2, it is 1.3. If you are using candelas/m2*, it is 12.5*. If you use apostilb, it is 3,98.
• S = film speed following the ISO standard

*) some of you might know cd/m2 as lux or lumens/m2,

This also means that 2ev = A2/T

• A = the f-stop number of the aperture
• T = shutter time in seconds

Combining these two; EV = log2(A2/T) = log2(LS/C) - which is the only formula you are likely to need, if you want to understand the basics of mathematics behind photography.

So what is the EV number used for?

Ah.. Well, the EV number is used internally in cameras - an EV number of 10, for example, would refer to all the combinations of shutter times and apertures that would give a given exposure using ISO 100 film. This is useful, because a camera only has to add one thing to this equation; A light measurement. A camera with a lookup table or an algorithm to calculate the correct EV is all set for using all the different combinations that are able to give you the exposure you want.

But why would I care, if the camera handles everything?

Because the camera doesn't always get things right. You may also want to use alternative exposures for artistic reasons.

Most cameras have an EV compensation wheel/dial, allowing you to choose how much you want to over/underexpose an image. This is usually measured in +/- 2EV, 1/3 steps. This means that you can over- or underexpose an image by two whole EV steps (which, incidentally, would mean the same as two full f-stops either way), in steps of 1/3 EV.

I hope that made things a little clearer. Any corrections or questions are welcome!

also see: Auto exposure bracketing, photography metanode.

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