In part one of this article (January 2009) we took a broad look at digital single-lens reflex photography touching on the many features, functions and benefits of its use. This month, we’ll dive a little deeper into the bottomless pit that is DSLR theory, and how to put that theory into practice.
To recap, a DSLR camera’s mode dial enables the user to switch between many picture settings, the most important (and functional) being the Av (the user sets the aperture, or lens opening, and the camera determines the shutter speed), Tv (the user sets the shutter speed, the camera sets the Av) and manual (the user sets both Av and Tv). While we could solely rely on these settings to achieve good pictures, understanding their functions will result in great pictures.
With DSLR photography, there are three exposure variables that will ultimately determine the output of the image: ISO, shutter speed and aperture value. In terms of the importance of these variables, the sum of the parts is truly greater than the whole.
The first exposure variable and the basis of the overall image quality is the ISO setting. In short, ISO refers to the camera sensor’s sensitivity to light. Typically measured in values of 100, 200, 400, 800, 1600 and 3200, an ISO specification in a DSLR camera is no different from the ISO assigned to single-lens reflex (SLR) films. Instead of light hitting film like in the old-fashioned days of camera and film, however, today’s light source hits the DSLR’s sensor instead of the film.
A low ISO setting (between 100 and 400) sets the camera sensor for typical daytime lighting conditions. A low ISO setting will result in a clean picture with fine detail and minimal noise, but will slightly increase the time needed to snap the picture.
In evening/night lighting conditions, a low ISO setting will not work as well; the camera’s sensor cannot properly process the lighting information because it’s just too dark. You may find the same poor lighting conditions indoors, especially if you are trying to document your Level 5 drywall finish. In this case, adjust the ISO setting to a higher-end number (between 800 and 3200) to compensate for the lack of light. The camera will now successfully snap the shot—but at the cost of image noise, a phenomenon that occurs when the camera attempts to compensate for the poor lighting.
As a general rule, keep those ISO settings as low as possible and utilize the camera’s other exposure variables (shutter speed and aperture) to increase the exposure of your image in order to maintain the highest quality possible.
A camera’s shutter speed (or Tv) is measured in seconds or fractions of a second, and determines the length of time light will hit the camera’s sensor. A shutter speed of 1/1000 for example would mean that the exposure time would be 1/1000th of a second, or a millisecond. On the other end, a shutter speed of 15” would mean an exposure time of 15 seconds.
On a bright day without the aid of sunglasses, keeping your eyes open for a long period can prove to be somewhat uncomfortable. To combat that problem, you blink faster to relieve the strain of the bright light. A camera’s shutter acts in the same way. The longer the shutter remains open, the more the image is exposed to the light–and if the shutter speed is set too high for a picture snapped during the day, it will become overexposed. In daylight conditions, a small shutter speed will often suffice since there is a strong presence of light allowing the sensor to capture the image quickly.
Indoor lighting conditions as well as late evening/night photography will require a longer exposure time since there is less natural like to aid our camera’s sensor. A general rule of thumb is that a shutter speed of 1/60 or longer will require the aid of a tripod. This is because a longer exposure time increases the chance of blur caused by say, a photographer’s shaking hands. Of course you could always increase the ISO setting to compensate, but your picture quality would suffer.
As a general rule, the greater the source of light, the lower the shutter speed.
The final main component of our DSLR journey is the Aperture value, or Av, which serves as a filter for letting light through. A camera’s aperture acts similar to the pupil of a human eye in that it opens wider as light decreases to let in more available light, and gets smaller when light increases to reduce the amount of light entering the eye.
Aperture value is measured in f-stops and is displayed as f/x, where x equals a numeric value. What often confuses new DSLR users is that a lower f-stop value means a camera’s lens opening is larger than that of a higher f-stop value. Just think of it like steel framing gauges, where the higher the gauge number is, the lighter the steel is per square foot.
Aside from determining the amount of light permitted to pass through, the f-stop also plays a key role in determining the depth of field, or DOF.
By utilizing a larger aperture, we can create a nice sense of depth with our image focusing the viewer’s eye on the content in the foreground—a useful approach to capture the focus of a detailed plastering job for example.
While this shallow depth of field works well up close, if you were to take some shots of an exterior building from a distance, you would want to utilize a small aperture (a high value) to ensure that all of the details remain intact—useful for when you want to capture the entire building.
To surmise, at large aperture settings (small f/x values), objects relatively close and far away from the lens will appear sharp—a narrow depth of field. At smaller apertures (large f/x values), the depth of focus is widened, and everything within range will maintain a similar degree of detail.
A Double Edged Sword
It would be sweet if all three exposure variables could be adjusted without affecting things such as the quality of the photo, but nothing comes without a price.
• Although it will produce a dramatic shot resulting from the narrow DOF, a large aperture (small f/ratio number) is subject to image distortion such as chromatic aberration and lack of sharpness. This problem can usually be solved by mounting the camera on a tripod or other fixed surface.
• Small apertures (large f/ratio numbers) can cause unsharpness in the photo as well, due to diffraction of light. However, small apertures can be necessary especially when capturing multiple objects or a landscape shot.
• Long exposure times require a tripod, and will usually blur the photo if you photograph moving subjects. Long exposure times can also suffer from pixel substitution, which occurs when the camera simply can assign a color to a certain area of an image. Short exposure times however are not always possible because the light may be too low.
• Low film sensitivities (low ISO number) require longer exposure, which is not always possible, and high film sensitivities suffer from thermal noise.
Bottom line: Of the amount of light available, which of the three exposure variables will be most important to you? Deciding exposure settings requires some practice and experience. Several combinations of settings will produce the same exposure, but with a different result in quality.
With any new tool, software or hardware, it’s going to take time to feel it out and become accustomed to it. While it can be (and will be) frustrating, once a certain degree of comfort has been obtained, taking images with your DSLR will pay off dividends. No longer will there be a need to hire professional photographers only to find out that they failed miserably in trying to obtain your vision.
The DSLR camera enables contractors to document their hard work either for personal recordkeeping or to proudly show off to potential clients. In the end, anyone with knowledge of the fundamentals of digital photography can make a picture’s worth greater than a thousand words.
Craig Wood is the designer, graphics & art for AWCI’s Construction Dimensions as well as an expert in visual and audio based technology.