Image Sensors

Historically, 35 mm film has remained the photographic standard in medical photography. Digital cameras, however, do not use 35 mm film, but rather capture images using a light sensitive image sensor. There are two types of image sensors currently available: charged coupled devices (CCDs) and complementary metal–oxide–semiconductors (CMOS). A full explanation of the working mechanics of image sensors, as well as the specific advantages and disadvantages of CCD versus CMOS sensors, is beyond the scope of this chapter. However, in simplified terms, image sensors function by converting light into an electrical pattern that is converted to and stored as a digital image. When the shutter of the camera opens, it allows the passage of light onto the sensor, which is comprised of a silicon grid containing millions of capacitors. The incident light is converted to an electrical charge (via the photoelectric effect) proportional to the light’s intensity, with each capacitor accounting for one “pixel” of the image. This electronic charge pattern is then digitized and converted into an image. ² Pixel density is directly related to resolution, but is only one factor in image quality.

Sensors can vary widely in size, but are typically smaller than conventional 35-mm film frame. As a result, when taken in identical photographic environments, the digital image sensor only captures the central portion of an image as compared to a full 35-mm film frame, thus producing a cropped image ( Fig. 9.1 ). In effect, this mimics the properties of a lens with a longer focal length and smaller field of view. The effective lengthening of focal length is termed the focal length multiplier (FLM) and is used to correct the size of the sensor in relationship to the traditional 35-mm window. The focal length multiplier can be calculated by dividing the diagonal of a 35-mm film frame by the diagonal of the sensor. For instance, a dSLR camera with a FLM of 1.5 and a lens with a 60-mm focal length produces an image similar to a 35-mm film SLR with a focal length of 90-mm (60-mm × 1.5). ³ , ⁴

To address these issues, full-frame sensors have recently become commercially available. Cameras with full-frame sensors contain image sensors that are the same size as a 35-mm film frame. While more costly, this has the advantage eliminating the discrepancies (i.e., cropped images and effective lengthening of focal length) that exist between conventional 35-mm SLR photography and dSLR photography. In all likelihood, most camera manufacturers will move to full-frame sensors as production costs decrease, eliminating the need for FLM calculations.

Lens and Focal Point

The goal of photography in facial plastic surgery is to obtain images of high quality and clarity that emphasize the fine details of the face, producing accurate photographs with minimal distortion. A high-quality lens is one of the most important foundations for achieving this goal. In simple terms, a camera lens functions by directing the path of captured light onto on either photographic film or on a digital image sensor, creating a rendition or reproduction of the captured image. ³

One of the many advantages of using an SLR camera is the ability to interchange lenses between camera bodies, depending on the type of photograph needed. Each lens is designed with a set focal length, which is defined as the distance in millimeters from the optical center of the lens to the focal point (which is located on either the digital image sensor or film). For the purposes of facial plastic surgery, the ideal focal length is between 90 and 105 mm. These types of lenses are known as “portrait” or “macro” lenses, and are designed for near focusing. ⁵ Lenses with a shorter focal length of 50 to 60 mm have a wider angle of view, and are commonly utilized in photodocumentation for aesthetic procedures outside of the face and neck. However, these lenses produce a centralbulging or “fish eye” appearance when used to photograph the face, and as such must be avoided in facial plastic surgery ( Fig. 9.2 ). ⁴

Aperture and Shutter Speed

Aperture and shutter speed are two related settings that directly affect each other, and can influence a number of photographic characteristics such as the level of exposure. The aperture of a photographic lens is comprised of an adjustable diaphragm (analogous to the iris of a human eye) with a central opening (analogous to the pupil) which allows for the controlled passage of light through the lens. Aperture size is measured in f-stops, which is calculated based on the ratio of the focal length of the lens to the aperture diameter. Therefore, smaller f-stop values correspond to a larger aperture size and vice versa.

The camera shutter controls the amount of time that the camera’s photographic film or image sensor is exposed to the light captured through the lens and aperture. This can be set within a very wide range of values, but is most commonly adjusted between 1/1,000th and 1/60th of a second. Therefore, the combination of aperture size and shutter speed can be finetuned to control the amount of light exposure allowed on the photographic sensor or film. ⁴ , ⁶

Depth of Field

In photography, the depth of field (DOF) refers to the distance range in which all included portions of a photograph appear in focus. For the purposes of facial plastic surgery, this should include the entire face with the nose being the focal point—that is, the point in the photograph with the greatest definition and clarity. With dSLR cameras, the DOF can be manipulated by altering one of three camera settings: the focal length of the lens, the distance of the photographer to the subject, and aperture size. ⁴

The DOF varies inversely with the focal length. Therefore, lenses with a longer focal length such as those used in portrait photography have a shallower DOF and vice versa. As previously mentioned, the ideal focal length for use in facial plastic surgery is between 90 and 105 mm, since this setting produces accurate portrait images with minimal distortion. Subsequently, the lens focal length cannot be easily utilized to adjust to the desire DOF. Similarly, the photographer-to-subject distance may also be difficult to alter, based on the space restrictions of the photographic studio being used.

As such, the aperture setting is the easiest and most effective way to vary the DOF. Smaller aperture sizes decrease the amount of light that enters the camera, but increase the DOF ( Fig. 9.3 ). This can produce the desired DOF, but may potentially lead to an underexposed photograph. To counterbalance the decrease in aperture size, the shutter speed can also be decreased (keeping the shutter open longer) to admit more light into the camera. Therefore, if an increased DOF is achieved by decreasing aperture size (increasing the f-stop), then an appropriate decrease in shutter speed is critical to ensure adequate exposure. ⁶

These settings can be adjusted manually on a dSLR camera. However, fine-tuning these settings to obtain the ideal DOF while maintaining adequate exposure can be cumbersome to the novice photographer. Most dSLR cameras come with automated options that allow the camera to adjust some or all of these settings based upon the desires of the photographer. The aperture priority mode allows the photographer to manually adjust the aperture size, while the camera adjusts the remaining settings (e.g., shutter speed) to ensure adequate exposure. In the senior author’s experience, proper DOF is best obtained by using the aperture priority mode to set the appropriate aperture and allowing the camera to adjust the remaining settings. ⁴

Lighting

The lighting arrangement in a photographic studio is a critical component in obtaining high quality images. Proper lighting can highlight skin coloration, facial contour, and fine anatomical detail, and is especially critical in the evaluation for rhinoplasty. Lighting arrangements can vary widely from a single light source produced by a cameramounted flash, to more sophisticated studio arrangements using multiple light sources. ⁷ Use of different lighting arrangements can have a dramatic effect on the captured image, each with specific advantages and disadvantages.