In my last post I whined in my typically British way about the weather and light levels here in the UK. It occurred to me after writing that post that perhaps many people reading it might not know what exposure value (E.V.) means. Modern digital cameras automate many photographic tasks for the photographer and so perhaps a lot of knowledge that was required in the days of film and manual exposures has been lost. Here is my explanation of this subject and its relevance for any photographer wanting more creative control over their images.
A bright sunny day with no shadows or clouds might generate an exposure value (EV) of around EV15. In program mode a modern camera will usually set the camera shutter to 1/500th of a second and the lens aperture to around f/8, but this is only one option that is available. An exposure value of 15 also allows camera settings of 1/60 at f/22 or 1/4000th at f/2.8. How these possible exposure combinations are employed by the photographer can have a great effect on the type of picture that results.
Definition of Exposure Value
Exposure value is therefore a shorthand method of describing a given lighting intensity that allows a variety of shutter and aperture combinations in order to achieve a correct exposure.
Exposure Value Table
|−6||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m||1024 m||2048 m||4096 m|
|−5||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m||1024 m||2048 m|
|−4||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m||1024 m|
|−3||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m|
|−2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m|
|−1||2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m|
|0||1||2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m|
|1||1/2||1||2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m|
|2||1/4||1/2||1||2||4||8||15||30||60||2 m||4 m||8 m||16 m|
|3||1/8||1/4||1/2||1||2||4||8||15||30||60||2 m||4 m||8 m|
|4||1/15||1/8||1/4||1/2||1||2||4||8||15||30||60||2 m||4 m|
This exposure value table is available from wikipedia, along with a somewhat more technical explanation than I provide here.
How to Use the Exposure Value Table
In our earlier example we stated that a sunny day might give us an EV of 15. If we read on the left hand side of the table down to EV15 we see that this line offers us a number of shutter speeds available at this EV (1/8000th to 1/8th of a second). If we then choose one of these shutter speeds we can look at the top or bottom of the table to ascertain the correct f-stop that corresponds with this shutter speed. Of course a modern automatic camera does this for you, but understanding the process behind the cameras choices gives you far greater control.
The use of shutter speed and aperture for creative control over your images is somewhat beyond the scope of this article, but will be the subject of a future post.
The EV table can also be used to measure the effect of increasing the ISO rating (ASA for film). The table is correct for 100 ISO, but doubling the ISO is equivalent to doubling the light level. Doubling the light level is conveniently the same as increasing the EV by one.
To illustrate this point, consider the following real world example:
We are photographing a stage production under tungsten stage lighting. Flash photography is forbidden. We measure the light and it comes out at EV7, which is quite a low level for most cameras. As we are unfortunately far back from the action, we are using a typical amateur telephoto lens with a maximum aperture of f/5.6. This light level and aperture combination gives us a shutter speed of only 1/4 of a second. While a tripod would give the necessary stability, it would not help to freeze the action on the stage, therefore we need to increase our ISO, but by how much?
Well, every time we double our ISO we double our shutter speed, if using the same aperture. The table is correct for 100 ISO. To get from a shutter speed of 1/4 second to a more reasonable 1/60th will require moving four rows on the table which is equivalent to 4 doublings of the ISO.
ie 100 -> 200 -> 400 -> 800 -> 1600 ISO
Historical Use of EV
Before the days of fully automatic point and shoot cameras, or even before TTL metering, photographers without access to an accurate light meter could use EV to guess a correct exposure.
How to guess exposure without a light meter.
On a bright sunny day, with clear shadows, a typical scene will register at around EV15.
Photographers could guess the correct exposure as being roughly equivalent to 1/ASA (ISO) at an aperture of f/16.
From the above table it can be seen that the “Sunny 16” rule does indeed give approximately this exposure (f/16 and 1/125th). Of course this rule of thumb requires other modifications for overcast conditions, haze, position of the sun relative to subject and so on, but it does give a starting point.