In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light (that is, when the camera's shutter is open) when taking a photograph. The amount of light that reaches the film or image sensor is proportional to the exposure time.

What is exposure time used for?

The exposure time is how long the camera will be exposed to the light (photons) emitted from your sample. The longer the exposure time, the more photons the detector will receive, resulting in increased pixel intensity and a “brighter” image.

How do you calculate exposure time?

Count the number of increased stops. If it was two stops, for example (ISO 100 to 400) then you just add those two stops to the shutter speed (30 seconds to 2 minutes) after resetting the ISO back to 100 and the exposure mode to Bulb. These are reciprocal exposures (30 seconds and 400 ISO equals 2 minutes and 100 ISO).