Controlling the brightness of images

In my last column I discussed the pros and cons of the different types of cameras - the single lens reflex (SLR) and the Point and Shoot.

In my last column I discussed the pros and cons of the different types of cameras - the single lens reflex (SLR) and the Point and Shoot. This month I want to discuss four features of digital cameras that can affect the brightness of an image. In other words, what settings affect the amount of light that is reflected off an image and recorded by the camera. I will be talking about the features found on an SLR camera. Some, or all of these features, may also be found on Point and Shoot cameras. I am assuming that the camera can already take an image of sufficient quality and size. The four features I will discuss are the flash, aperture, shutter, and ISO.

Flash systems come in several types for SLR digital cameras. The two that are used most for dentistry are ring flashes and twin flashes. With the ring flash, which is the most common, the lights surround the lens, and completely fill the image with light. Since the lights are mounted close to the lens, the flash is directed to a concentrated area. This is useful when photographing inside the mouth. Although about half the price of twin flash systems, ring flashes do have a downside. Since they are so effective at filling the image with light, they remove all shadows. Thus, the texture and subtle anatomy of anterior teeth may be washed out, if the system is not used correctly. The twin flash design has two lights mounted opposite each other. They can be fixed or movable. Twin flash systems have the advantage of allowing some subtle shadows to be present, yet still illuminate the image so that anterior anatomy will show up better. But the twin flash design may not work as well with intraoral photos as the ring flash does. I will talk more about flash systems in future columns.

The aperture is the opening on the end of the lens farthest from the camera body. Think of the aperture as a window. It can be wide open, or almost closed, or every increment in between. The more open the window (aperture) is, the more light comes in. The more closed the window (aperture), the less light comes in. The amount the aperture is open/closed is expressed as the f-stop number. The number is a fraction such as 1/2.8, 1/8, 1/11, 1/32. While they are fractions, f-stops are commonly referred to only by their denominators. Thus, f-stop numbers are referred to as 2.8, 8, 11, etc. Lower f-stop numbers refer to a lens that is more open. Higher numbers refer to a lens that is more closed. This means that if a lens is set to an f-stop of 2.8, it will let in more light than if it is set at 32. The lower f-stop number also gives an image less depth of field. A retracted smile taken at f-stop 2.8 will be very bright, but the molars will not be in focus. The same retracted smile taken at f-stop 32 will have all the teeth - including the molars - in focus, but the image will be darker.

The shutter is found just behind where the lens attaches to the camera body. Think of the shutter as a door. Remember as a child when your mother kept telling you to shut the door? Well, that’s how the camera shutter works. It is either open or closed. The longer it stays open, the more light gets to the computer chip to record the image. The aperture is a function of the distance the lens is open. The shutter is a function of how long the lens stays open. The shutter setting is usually referred to in fractions of a second, according to how long the lens remains open, such as 1/90, 1/200, 1/350, etc. Just like the aperture, the shutter setting is commonly expressed by only the denominator such as 90, 200, 350, etc. This means that the lower the shutter number, the brighter the image will be because more light reaches the computer chip. The higher the shutter number, the darker the image will be because less light reaches the computer chip. A retracted smile taken at a shutter speed of 90 will be brighter than one taken at a shutter speed of 300.

The ISO setting is the sensitivity to light of the computer chip that captures the image. It serves a similar function to the speed rating of print film. ISO numbers generally range from 100-200-400-800-1600-3200. The settings are a trade-off. The chip is less sensitive to light at the lower numbers so the image will be darker. The higher the ISO number, the more sensitive the chip is to light, so the image will be brighter. The lower the ISO number, the smoother the image will be. The higher the ISO number, the grainier the image will be. This means a smile taken at an ISO setting of 100 will be darker, yet smoother, than a smile taken at ISO 800, which will be brighter but grainy.

In future articles, I will discuss how to find the balance of all these settings so that your images are perfect each time you take one. After all, time is money, and you don’t want to waste time with retakes.

Dr. Tony Soileau is a general dentist from Lafayette, La. He has taught digital photography at the Pacific Aesthetic Continuum in San Francisco, the Institute of Oral Art & Design in Tampa, Fla., and the Esthetic Epitome in Charlotte, N.C. He is currently a co-director for the genR8TNext digital photography program. He lectures on the use of digital photography, digital radiography and computerized case presentations, and high-tech marketing. He can be reached at (337) 234-3551 or by e-mail at tony@tonysoileau.com.

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