Exposure – Speed, Aperture, and ISO Simplified

Exposure deals with controlling the amount of light allowed to strike film or a photo sensor, and the result produced by that light. There are three ways to increase or reduce exposure and how we use them impacts three other things – 1) how the photograph depicts motion, 2) whether or not all, part, or none of the picture is in sharp focus, and 3) how faithfully the image accurately depicts the relative brightness and color of what was photographed. The controls we use are shutter speed, aperture size, and ISO, a measure of sensitivity to light. Each interacts with the other two to influence our results. We’ll cover them one at a time.

Shutter Speed
The shutter is a precision mechanism between the lens and the light-sensitive material that can be opened and closed for predetermined lengths of time. Just as opening window shutters allows light to pass into the room, opening the camera shutter allows light to pass into the camera. The length of time it takes for the shutter to open and close is called its speed. Shutter speed can have a large impact on how motion is depicted in a photo. It’s not an issue for creating a still life image, but action shots are another matter. A short shutter time (higher speed) tends to “freeze” motion; Lower shutter speeds (longer exposure times) can produce blur that suggests motion and speed. Choosing a shutter speed to get the effect you want might require a trade-off with one or both of the other 2 controls.

“Aperture” is the name given to the opening in the lens that allows light to pass into the camera. A few specialty lenses have a fixed aperture, but in most it can be adjusted over a wide range, with their largest opening allowing 500 to 1000 times more light to enter the camera than their smallest opening. Another thing affected by aperture size is depth of field, the range of distance in front of the camera where objects appear to be focused sharply. Portrait and fine art photographers sometimes want to keep the primary subjects photographs sharply focused with other areas and objects being out of focus and indistinct. This is accomplished with larger apertures. Landscape photographers use smaller apertures when they want everything to be in sharp focus.

Photosensitive materials have time-light reciprocity, a fancy way of saying that twice as much light for half the time produces the same result as half as much light for twice the time. This table shows equivalent exposures.

Time - Light Reciprocity

This third component of exposure control has to do with the sensitivity of the film or photo sensor to light. Higher ISO settings mean more sensitivity to light. We already said that shutter speed and aperture setting control the total amount of light that gets into the camera, and that time-light reciprocity exists. Reciprocity also applies to sensitivity and total light exposure. An ISO setting of 50 takes twice as much exposure (total light) to produce a given result as an ISO setting of 100 does. The next table shows equivalent exposures for commonly used ranges of shutter speed, aperture setting and sensitivity.

ISO-Total Light Reciprocity

Increasing ISO settings will increase “noise” in digital images, randomly placed areas with incorrect brightness and/or color. This is because increasing the ISO setting is similar to turning up the volume of AM radio because of static. What you want to hear (signal) is louder, but so is the noise (static). Film rated at high ISO values shows the increased noise as graininess. Today’s materials usually do very well up to ISO 400; experiment with higher settings so you can learn what level of noise is acceptable for your purposes. If your images are noisier than you like, you can open the aperture, slow the shutter speed, or both. But sometimes we have to accept more noise than we want in order to get a picture at all. Noise in a photo can be reduced after the fact with image editing software, a subject for a future post.

You are now on the threshold of a new realm of photographic creativity as you break the chains of automatic exposure. It might be uncomfortable at first, but just be sure you know how to adjust aperture, shutter speed, and ISO, and start practicing. Tomorrow’s post will suggest some things to try to help get more at ease with leaving auto exposure behind.

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Follow this link for more about aperture settings.

© 2011, TheDigitalPhotoCoach.com. All rights reserved.

© 2011, TheDigitalPhotoCoach.com. All rights reserved.

One Response to “Exposure – Speed, Aperture, and ISO Simplified”

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  1. [...] I recommend that you make a point to learn how to adjust your camera’s aperture settings. If all else fails, look in your manual — the information is there. There is information about how to manage aperture, shutter speed, and ISO rating in this discussion: Speed, Aperture. and ISO Simplified [...]

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