The lens is the tool which captures the light and is the component that has the greatest impact on the quality and accuracy of the clinical photograph. Lenses are described by their focal length and speed. Other features which describe a lens include their macro capability, wide angle, telephoto, and zoom capabilities.
Focal length is the distance between the lens and the image sensor when the subject is in focus and typically stated in millimeters (e.g. 60mm, 105mm). A longer focal length lens increases the working distance from the subject at the same reproduction ratio. For example, if shooting the distance with the 60mm macro is 4 inches at a specific reproduction ration, the shooting distance with the 105mm macro will be about 8 inches. This longer focal length lens is useful for taking intraoperative photographs, oral cavity or vaginal examination photographs, or photographs of children that are agitated when approached.
The focal length will determine the “angle of view” of the lens.
Lens speed is a lens feature that is defined by how much light the largest aperture available on a lens (e.g. f2.8) will allow to reach the sensor. A “fast” lens allows more light to reach sensor at its largest aperture than a slower lens at its largest aperture. The smaller the number, the larger the aperture, the faster the lens. An f1.4 lens lets in twice as much light as f2.8.
Reproduction Ratio is the lens’ ability to capture an image size on the sensor compared to the actual object size. Most images captured in medical photography are at a reproduction ratio from 1:1 to 1:8, making a lens with macro or 1:1 reproduction (life size) a desired feature. A 1:1 reproduction capability is demonstrated on the barrel of the lens. An aperture range from 2.8 to 32 is desirable because of the improved depth of field a smaller apertures. Typical macro lenses have a focal length of 60mm or 105mm. A 60mm macro lens is ideal to capture most clinical images and cramped examination rooms. It can also be used as a standard lens for everyday use. A 105mm lens can be added as you build your camera system. A benefit to the 105mm lens is the greater distance from the subject allows for more even distribution of light when taking photographs at 1:1 reproduction ratio.
Digital Single Lens Reflex (DSLR) Camera Body
The camera body is simply the box which holds the sensor. Desired features include single-lens-reflex, interchangeable lenses, Auto-ISO, through-the-lens flash metering, autofocus, automatic exposure, full frame sized sensor format.
The Single Lens Reflex is named for the “reflexing” mirror that allows the photographer to view and frame the photograph just prior to capturing the image. Light captured by the lens strikes a mirror and is reflected into an optical pentaprism and then the view finder. When the shutter is released, the mirror rotates upward exposing the image-capturing sensor. When the mirror is up, light is not directed to the view finder and the image is temporarily out of view.
Exposure is defined as the amount of light (number of photons) which strike the sensor. And is determined by the lens aperture, shutter speed, ambient light, and ISO or sensitivity of the photo sensor to the light. If the aperture and shutter speed settings inadequately expose the sensor at a set ISO, the image will be too dark. At the same aperture and shutter speed, if the camera is set on Auto-ISO the camera will automatically increase the ISO to ensure the image will be appropriately exposed. This feature is useful in medical photography when the aperture is extremely small to obtain the best depth of field and the flash is unable provide an adequate amount of light to properly expose the subject.
Through-the-Lens (TTL) flash metering gives flash control to the camera and its operator. When the shutter is released and flash fires, the light hits the subject and is reflected back to the lens and then the photo sensor. The light then bounces off the film sensor to a sensor which reads the light and shuts off the flash when appropriate exposure, as sensed by the camera is achieved. Prior to TTL metering, camera to subject distance and aperture had to be manually measured and adjusted to provide appropriate exposure for a subject. With TTL, one no longer has to calculate the distance and correct aperture using flash.
The camera’s format describes the size of image sensor. An FX format (full frame) sensor is 36mm x 24mm and approximates the size of 35mm film. A DX format sensor is 24mm x 16mm. Using an FX lenses with a DX sensor will render a cropped image by a factor of 1.5x (not magnified image). This is important in medical photography when using reproduction ratios marked on the barrel of the lens. If using a DX format camera with an FX lens, the reproduction ratio reflected on the barrel of the lens is not accurate. An FX camera is preferred over DX for medical photography.