Depending on the type of camera you are using, you have two choices when it comes to focusing on an image, manual or auto. The switch for changing between manual and auto focusing is found on the lens, not the camera body. Both have advantages and disadvantages. I will discuss both methods and describe how I actually use both.
Manually focusing on an image is the traditional method used by most dentists, especially if they have used a 35mm camera prior to switching to a digital style camera. Manual focusing involves the photographer turning and adjusting the focusing ring or rings on the lenses so that the image captured will be sharp and crisp. The focusing ring can be set to preset points such as 1:1 for a close up image and 1:10 for a portrait style image. Organizations such as the AACD have standard setting based on these ratios for the different photographic views required for accreditation. These ratio settings can be found on the barrel of the lens. As the focusing ring is turned the ratio setting can be noted. The advantage of using manual focusing is that a constant image can be achieved for each setting. This means that both the before and after image will have the same view. A 1:1, 1:2, etc. setting will give the photographer the same image each time it is used on a particular patient. While a constant image view is an advantage to using a manual setting it also puts the responsibility of having the correct distance from the subject to the lens, focusing distance, on the photographer as opposed to the camera. The photographer must position the camera lens the correct distance to the smile or arch for the image to be in focus. This can be accomplished by noting the position of the photographer including the position of the feet. Each time the particular view is taken, 1:1, 1:2, etc, the same stance is duplicated. This will insure the proper distance of the lens to the image each time.
Using the auto focus feature of the lens allows the camera to focus the lens instead of the photographer. This allows for a focused image regardless of the distance of the lens to the image. When the shutter button is pushed half way the camera will move the focusing ring inside the lens in and out until a focused image is obtained. The camera will then alert the photographer that the image is in focus, usually with a “beep” sound. After hearing the “beep” the photographer will then press then shutter button all the way down to capture a focused image. By using auto focus the photographer can pay more attention to the image and not worry about taking a blurry image. By using auto focus the photographer can take usually take images at a faster pace without worrying about taking an unfocused image. The disadvantage of using auto focus is that a second image of the same view may be slightly different if the distance of the lens to the image was not the same. This is because the image will be in focus regardless of the distance.
Confused yet and not sure whether to use manual or auto focus? Let’s put this in simpler terms. If a dentist takes a photo of a smile that views from the distal edge of the tooth number 7 to the distal edge of tooth number 10 using manual focus both images will show the same view if he uses the same ratio setting. This is because the dentist will have to be the exact same distance from the smile for both images to be in focus. If the dentist uses auto focus he does not have to be the exact same distance for both the before and after but the first image may show lateral to lateral and the second may show canine to canine and still be in focus. However, as long as the dentist keeps the same distance, it does not matter if he uses manual or auto focus, the images will be the same. So the issue becomes how big of a deal is it for both images to be exactly the same.
I use both. I keep my lens on auto focus. I move to the exact same position for each type of view I take. I manually focus if the camera is not immediately focused. But because I am set on auto focus if the patient or myself moves or if my eyes are tired the camera will make small corrections ensure proper focusing.