Shutter Speed

The purpose of the shutter is to block the light from exposing the imaging surface until the moment of exposure.  With film, any light will cause a chemical reaction that changes the characteristics of the film to produce an image.  With modern digicams, light on the imaging sensor has no adverse effects.  As such, consumer digicams often use an electronic shutter rather than a physical one.

DSLRs usually have a physical shutter sometimes coupled with an electronic shutter for faster shutter speeds.



The speed of the shutter determines the length of time the imaging surface is exposed to the light.  The longer the shutter speed, the longer the imaging surface is able to collect photons of light.  Shutter speeds are correlated to stops much the same way apertures are.  Values, however, reflect timing rather than diameter.  As such, they are stated in the seconds, or fraction thereof.

For example, a shutter speed of 1/125" will allow twice as much light to collect on the imaging surface as a speed of 1/250".  It is, therefore, 1 stop faster.

While the aperture directly affects the DOF, the shutter speed directly controls the recording of moving objects.

A shutter speed of 1.6" at f/29, ISO 200:  Notice everything here is fuzzy.  The corals are blurry and the shrimp looks like a smear.  Yet, notice the rock is sharp.  The reason for this is the rock was not moving for during the exposure.  The corals were swaying in the current, and the shrimp was self-propelled.

Increasing the shutter speed to 1/25" at f/4,5 ISO 200: Everything is pretty sharp in this photograph.  The corals and shrimp have well-defined edges.  The faster shutter speed left the imaging surface open to light for a shorter period of time.  As such, the moving objects had less chance to move and, therefore, appear to be frozen in that splice of time.  We still see a bit of motion blur in the tips of the antennae, though.

On this particular lense, f/4,5 is the maximum aperture opening.  Opening it up further to increase the amount of light gathered by the lense is simply not possible.  To increase the shutter speed further while still obtaining the correct exposure, we need to increase the sensitivity of the imaging surface itself.

In this example, we have doubled the ISO rating twice:  200 -> 400 -> 800; it is two stops more sensitive.  This allows us to use a shutter speed 2 stops faster, increasing the shutter speed from 1/25" -> 1/50" -> 1/100".  The actual shutter speed used was 1/80" as it produced a more accurately exposed image.

At ISO 800 we get 1/80" at f/4,5.  We managed to freeze the motion in the tips of the antennae at the expense of increased noise (grain) in the image.