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Photo Basics Series - Exposure

Exposure Triangle
Exposure Triangle
Mark Kelly

This is the first installment of Photo Basics, where the basic concepts are covered.

For any given scene there is an optimal exposure. An image exposure is made up from a combination of three elements, aperture, shutter speed and ISO.  To the left is a graphic representation, an exposure triangle. Changes in any one of the elements will effect choices or compromises needed for the others.

Aperture is the amount of light let into the camera through the camera lens. The size of the opening for light to go through is changed by a series of plates inside the lens. Think of it like the iris in your eye. The aperture is given a number, an f number, which is a mathematical ration between the focal length of the lens and the size of the opening. The elements of the image affected by aperture are focus, depth of field, and bokah. A fast lens has a larger opening and a larger front glass element (but a lower number representing the f-stop) with maximum apertures like 1.4 & 2.8
Shutter speed is the amount of time the film or sensor is exposed to light. Shutter speed is represented in seconds or fractions of seconds 1/2000, 1/2, 2 sec. The elements of the image affected by shutter speed are camera blur, subject blur and motion. A fast shutter speed is a very short duration 1/2000
ISO is the rating of film or the sensor for it’s ability to gather light. A higher ISO rating is able to expose with less light than a lower ISO. 100, to 400 are typical ratings, 1600, 3200 and above. ISO is said to be faster as the rating increases, slower as it decreases.

In later installments how each element affects the image will be explained


  • Patrick Rall-Detroit Autos Examiner 5 years ago

    I just found your column...very cool reading. What do you shoot with, Mark? I love your fireworks shots from a while back. I used to do fireworks shows for a living and since getting into photography Ive wondered what it would be like to try catching fireworks. I do all of my own photography for my automotive column and Ive gotten into equine photography. I look forward to reading and seeing more of your work!

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