Exposure When Taking Photograph
Exposure means how much the light comes to fill the film to record an image. Exposure is measured in Exposure Value unit (EV). Exposure value defines the combination of shutter speed and aperture of the lens when taking photograph. Exposure Value is also used for indicating the interval on photographic exposure scale (commonly called “stop”).
Exposure value is defined by formula:
EV=log2(N2/t); N=sqrt(2EV * t); t=N2/2EV
N = the relative aperture
t = exposure time (shutter speed)
EV 0 is defined by shutter speed (t) set to 1 s, and relative aperture (N) of f/1.0.
Setting EV on a camera
On most of SLR camera, there’s no way to set the EV. EV can be set from how long the shutter speed and how wide the aperture value set. You can use sunny 16 rule as a reference to measure the exposure.
For example, you want to use aperture f/4 to obtain a shallow depth of field in sunny condition and the ISO film is set to 100. From sunny 16 rule, f/4 means 4 stops faster than f/16. If one stop means double the amount of light, then the shutter speed should be set to (1/100)/(24) = 1/1600 s.
In modern digital camera, you don’t need to do metering to balance the light manually. The manufactures have design the camera with auto exposure. Mostly, the shutter speed will change automatically depend on lighting condition. The darker it is, the lower the number will be. But to prevent from camera shake, the shutter speed has to be faster than the reciprocal of the lens focal length multiplied by 1.6. If the lens focal length is 55mm, 55×1.6 is 80, so the shutter speed should be 1/80 or faster. If shutter speed lower than this, you can increase the ISO or use the flash.