Hours of Idleness-A Photographer's Journey in St. Louis

Exposure

Posted in photography, technique by Jason Gray on June 27, 2010

The number one most important photographic process affecting the eventual outcome of your images is exposure. Extremes of exposure, overexposure (too much light enters the camera) or underexposure (not enough light enters the camera), have consequences resulting in pictures that are either too bright or too dark. Here are a couple of examples from the web:

Overexposure Example:

Underexposure Example

So how do you control exposure in order to avoid these types of images? The two main controls on a camera that work in coordination with each other to regulate exposure are:

1. Shutter Speed refers to the length of time that the camera’s shutter curtain (opens and closes to let light into the camera) will remain open. Here are the relating adjustments made to shutter speed and their effects:

A slow Shutter Speed- lets in a larger amount of light. Unfortunately for most people, shutter speeds below 1/30th of a second are difficult to hand-hold with good results, meaning that their images result in a degree of blur (how a photograph registers movement, either of the camera or in front of the camera, during an exposure). Also, at slower shutter speeds, if you have something in frame that happens to be moving, it will show a trail of blur behind it. The “BULB” setting enables you to keep the shutter open for an indefinite period of time (variable by camera) that you control; you will have to press the shutter button once to open the shutter curtain and once again to close it.

A fast Shutter Speed- lets in less light to the camera. Unfortunately for the photographer, fast shutter speeds require more light for a proper exposure to occur than slower shutter speeds do. However, if you have the light, shutter speeds of 1/60th to 1/125th of a second can freeze most human locomotion, and 1/250th of a second can pause a human at full sprint. Note, a number with ” behind it denotes whatever number of actual seconds that the shutter will remain open; just a plain number, 500, for instance, represents that the shutter will be open for that fraction of a second, as in 1/500th.

2. Aperture is, literally, the size of the lens opening. This opening is measured in terms of f-stops or f-numbers. Each f-stop expresses a ratio, or a fraction, of the focal length being currently used. For instance, on a 200mm lens think of f/4 as 1:4, or 1/4, of the lens’ focal length; so at f/4, the lens’ aperture is open 50mm in diameter. Common f-stops that you will come across include, f/2, f/2.8, f/4, f/5.6, f/8, f/11, and f/16. These numbers are chosen because they either represent a doubling of the light of the f-stop preceding or an increase of half as much light as the f-stop following. Looking at the sequence of numbers above, this becomes obvious (ie. f/4 is double the light of f/2, while f/2.8 is half the light increase of f/4 from f/2). For certain prime lenses, you may find an f-stop range of between f/1.4 to f/32. Remember that a larger aperture (converse to f-number) admits more light. Aperture size is controlled internally to the lens by a device called a “diaphragm”. The diaphragm is a circular sheet of overlapping metal blades that expand or contract depending upon the f-stop setting. The diaphragm is visible through the front element of the lens, and the blades can be either straight or rounded (rounded diaphragm blades produce an out of focus area that is thought to be more pleasing). Lens manufacturers and lens types vary as to the number of blades used on the diaphragm, but the more blades that you have (8 or 9 is commonly ideal, for example) the less angular the edges of the aperture will be. Changes to the size of the aperture affect the depth of field, area within focus, of the picture (see myFocusing write-up for more on Depth of Field vs. Aperture).

Note: If you adjust either the shutter speed or the aperture size, you must also adjust the other. For example, a fast shutter speed stops fast action, but it also reduces the amount of light reaching the light sensitive media; so, a wide aperture setting (aka. a low f-stop) may be required to offset some of the reduction in light caused by using a faster shutter speed. However, doing so shrinks your depth of field, so there are costs and trade-offs to consider, before you establish your composition.

In terms of Setting your Exposure, there are certain inescapable factors to consider beforehand. These are:

1. Lighting- by now, I’m sure that you understand that the amount of light in a scene affects both the shutter speed and the aperture; for instance, a dark, overcast day might require a reduction in shutter speed and/or a smaller f/stop. There are a few tricks though, for getting at least close to a proper exposure under certain settings, and they are as follows:

Sunny 16 Rule: First, set your shutter speed to match your ISO setting. Then,
-If your lighting is sunny causing hard, well-defined shadows, set your aperture to f/16.
-If your lighting is slightly overcast causing distinct shadows with soft edges, set your aperture to f/11.
-If your lighting is overcast causing very soft, diffused shadows, set your aperture to f/8.
-If your lighting is dark clouds/heavy overcast causing no shadows, set your aperture to f/5.6.
-If your lighting is sunrise/sunset, set your aperture to f/4

Proper Flash Exposure Equations:
-To determine correct Aperture-> GN/D=A
-To determine feasible Distance-> A/GN=D
-To determine sufficient Guide Number-> AxD=GN
Note: a Guide Number is a number supplied by the flash manufacturer for use in Flash Exposure Equations; a higher Guide Number indicates a more powerful flash. Also, by using the above equation for distance, most cameras’ built-in speedlight generates enough power to properly expose a subject at about 10′ away.

During a slow shutter speed exposure, a flashlight, or a speedlight fired multiple times, can be used to “paint” a subject with light.

The Inverse Square Law- states that an object 5′ from the speedlight, or other light source, receives twice as much light as an object 10′ from the source of light.

Using the Exposure Compensation button as a good practice, if your main subject is darker than the background (like a person in front of a window), then it might be advantageous to notch up the positive compensation meter a few stops.

When using a speedlight mounted on top of your camera, you can over-ride the flash’s default “fill-flash” setting quickly by switching your camera’s meter over to “spot”

Rules for using the “Bounced Flash Technique”- refers to angling the flash-head of a speedlight so that its light reflects off a ceiling or a wall in order to scatter and diffuse the light reaching the subject.
-Angle of Incidence equals the Angle of Reflection: In other words, if you are standing five feet from your subject, and the ceiling or wall is five feet away from you, then you can set your flash-head angle at 45 degrees and be spot on in terms of your lighting (just remember the Inverse Square Law, though).
-Bouncing light off of a colored surface will cast some of that color back off onto your subject. Likewise, bouncing light off of a lighter colored surface will reflect more light than onto a darker colored surface.

2. Subject- understanding your subject and its anticipated activity will help you set your exposure parameters accordingly. For instance, if your subject is a race car, barreling down the track, then understanding that using a shutter speed of 1/2500th of a second will freeze its motion might be advantageous. On the other hand, if you want to emphasize the speed and nature of the fast moving vehicle, you could use a slower shutter speed and pan your camera along with the vehicle as the exposure is made to throw only the background out of focus.

3. Depth of Field- in terms of exposure, it is important to understand that if you want a wide depth of field you’ll have to shoot at a slower shutter speed. The majority of the sweeping landscape shots that you are probably familiar with, that have a very large portion of the frame in focus, were shot on tripods for exactly that reason.

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  1. […] Exposure- the total amount of light that reaches the film or digital sensor in a camera Possibly related […]

  2. […] of Photography writings, but it is basically a box of varying size, with an opening called an aperture at one end, and some sort of light sensitive media (film, sensor, etc.) on the other. In terms of […]

  3. […] the photographer aims the camera at something and takes a picture, he or she is making an exposure.  An exposure is the net result of a combination of several mechanical, chemical or electronic […]

  4. […] the photographer aims the camera at something and takes a picture, he or she is making an exposure.  An exposure is the net result of a combination of several mechanical, chemical or electronic […]


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