Exposure

 

 

Exposure is the amount light of per unit area striking an electronic image sensor and is determined by the camera shutter speed, lens aperture, and scene luminance. The camera’s ISO setting reflects the electronic image sensor’s sensitivity to available light also affects the final image result. Variations of each of these components has a major impact on the photograph. Aperture affects the depth-of-field or the sharpness in front of and behind the plane of focus. The smaller the aperture, the greater the depth of field. Shutter speed affects the blurriness or apparent motion of the photograph. The slower the shutter speed, the greater the blur of a moving subject. ISO affects the graininess or noise of the final image. The higher the ISO, the more grain or “noise” will be seen in the photograph. The exposure triangle attempts to demonstrate the effect of these variables.

 

Likely the best table explaining the effects of aperture (top row) shutter speed (middle row) and ISO sensitivity (bottom row) has on the final outcome of a photograph. Study and understand this and you will understand exposure.

Exposure Value (EV) is a number that represents a combination of a camera’s shutter speed and aperture, such that all combinations that yield the same exposure have the same EV. For example, the combination of 1/125 and f8 gives the same EV as 1/500 and f4 gives the same EV as 1/15 and f22. (see figures). However, you will notice that each of these images, though have the same EV, give a different photograph. The photograph with f4

Equivalent exposure combinations of shutter speed and aperture. For example
Equivalent exposures, yet with different results.

A “stop” in photography exposure is a doubling or halving of the light. Changing the exposure by a “stop” can be accomplished by adjusting the ISO, aperture, or shutter speed. For example, changing the ISO from 100 to 200 increases the sensitivity of the photographic image sensor by a factor of 2 or one “stop.” Changing the shutter speed from 1/125 to 1/250 decreases the amount of light striking the sensor by a factor of 2. Changing the aperture from 5.6 to 8 decreases the amount of light striking the sensor by a factor of 2.

f-numbers in 1 stop increments.

Proper exposure occurs when detail can be seen in the highlights (bright areas) and shadows (dark areas) of a photograph.

 

An image is Underexposed is defined when detail of an image is not seen in the shadows of a photograph. The overall image “appears” darker than expected.

 

An image is Overexposed is defined when detail of in image is not seen in the highlights, or bright areas of a photograph. The overall image “appears” brighter than expected.

Examples of variation of exposure by one stop increments either lighter or darker.

 

Gray World Assumption is a white balance method that assumes your scene, on average, is a neutral gray. It holds true if there is a good color distribution within a scene. A camera’s meter uses neutral gray (18% reflectance) as a measure of how to adjust a camera’s settings (aperture, shutter speed, IS0) if placed in an automated mode. This is important to know when you see a photograph that appears darker or lighter than expected. For example, a dark-skinned patient against a dark background will appear lighter than expected in the final image because the camera is rendering the image toward an 18% gray. Since the total reflectance is less than 18% gray the camera thinks “lighten the image to 18% reflectance.”

 
Difficult exposure situations. The dark-skinned person on the left is over exposed due to camera’s desire to expose to a neutral gray. The light-skinned person on the right is under exposed due to the camera’s desire to expose to a neutral gray.

White Balance is a camera adjustment feature which allows the camera operator to determine the accurate appearance of white, and thus other colors, in a photograph. Temperature of the ambient light affects the colors of the seen in the photograph. White balance adjustments are worked out according to color temperature settings from amber through white to blue. Fluorescent lights, for example, have a temperature of around 4000 degrees Kelvin which gives a green tint to uncorrected white balanced photographs. See the color temperature scale from digitalcameraworld.com. Tint is another white balance adjustment feature that adds a green-magenta shift.

Image results with corresponding white balance camera settings.

Exposure compensation is a camera feature that allows the photographer to make rapid adjustments in the photograph by adding or subtracting

When your subject doesn’t fit the typical exposure paradigm, you will have to make adjustments to obtain the most accurate image. This may be adding or subtracting light from your subject. This can be done in full or partial increments, depending on your camera settings.