Kirk Marshall Photography

Exposure is a measure of the amount of light reaching the image sensor during a single shutter cycle. Exposure can be thought of as water filling a cup; a properly exposed image fills the cup to the brim. An image that is under exposed hasn’t received enough light (i.e. the cup is only partially full) and an image that is over exposed has received too much light (i.e. the cup is overflowing). 

In my post of August 13, 2019 I introduced the concept of exposure and discussed how by default in auto mode the camera attempts to evaluate the light over the scene and adjust the settings to achieve middle grey, which is for most situations a properly exposed image (i.e. a full cup of water). That’s not always what we want so we can use the exposure compensation setting to override the default action to force proper exposure of lighter than average scenes (i.e. a snow field) or darker than average scenes. 

The problem with auto mode is that the camera choses settings to achieve proper exposure and we have little or no control over what settings are used. Taking the camera out of auto mode allows us to assume control of the exposure and chose specific settings to achieve our artistic view.

One of the settings that effects exposure is aperture. Opening the aperture (smaller f-number) allows more light into the camera and can brighten a dark scene to an acceptable level to properly expose the image. However, using a smaller aperture directly effects the depth of field and may not be compatible with our artistic vision. 

The key to understanding exposure is the exposure triad; which is comprised of aperture, shutter speed, and film speed (or ISO). 

I’ve already discussed aperture so next I’ll discuss shutter speed and then film speed.