In my last column I discussed the different types of cameras, SLR and Point and Shoot, and the pros and cons. 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 how much light is reflected off the image and recorded by the camera. I will be talking about the features found on a SLR type camera. Some or all may be found on Point and Shoot cameras as well. 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 but the two that are used for dentistry the most are Ring Flashes and Twin Flashes. The Ring Flash is the most common. The lights surround the lens and completely fill the image with light. Because the lights are mounted close to the lens the flash is directed to a concentrated area which is useful when photographing inside the mouth. Ring Flash systems are about half the price of Twin Flash systems. But they do have a down side. Because they are so effective at filling the image with light they remove all shadows. That means the texture and subtle anatomy of anterior teeth may be washed out if not used correctly. The Twin Flash design has two lights mounted opposite each other. They can be fixed or movable. They have the advantage of allowing some subtle shadows to be present yet still illuminating the image so anterior anatomy will show up better. But they are twice as expensive as Ring Flash systems and my not work well with intraoral photos as the ring flash does. I will talk more about Flash systems in future columns, particularly the advantages of E-TTL systems which many don’t understand.
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 and every amount in between. The more open the window (aperture) is the more light comes in. The more closed the window (aperture) the less light that 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. But we do not pronounce the number as a fraction but only goes by the bottom number. So F-stop numbers are referred to as 2.8, 8, 11, etc. The lower numbers refer to a lens that is more open. The higher numbers refer to a lens that is more closed. This means if a lens is set to an F-stop of 2.8 it will let in more light than if it was set to 32. The lower number also gives the image less depth of field. A retracted smile taken at 2.8 will be very bright but the molars will not be in focus. The same retracted smile taken at 32 will have all the teeth including the molars in focus but the image will be darker.
The shutter is found on the camera body just behind where the lens attaches. Think of the shutter as a door. Remember when you were a kid and you mother kept telling you to shut the door? Well, that’s how the 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 distance the lens is open and the shutter is a function of time of how long it stays open. The shutter setting is usually referred to in fractions of a second according to how long it stays open such as 1/90, 1/200, 1/350, etc. But just like the aperture, it is expressed as only the bottom number such as 90, 200, 250, etc. This means that the lower the number the brighter the image will be because more light reaches the computer chip and the higher the 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 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. The numbers range from 100-200-400-800-1600-3200 (on some cameras). 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 number the more sensitive the chip is to light so the image will be brighter. The lower the number the smoother the image will be. The higher the 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 your images are perfect each time you take. After all, time is money and you don’t want to waste time retaking images.