Supplementary Appendix (for KSSTA website):
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Part III: Photographic essentials
Principles of photography
Although the jump from the traditional SLR to digital SLR was large, the principles and fundamentals of photography have not changed since the very first photocamera. All cameras rely on the emission of reflected light off a given subject that travels through the lens, mirrors the image in an upside-down direction and projects it onto a light-sensitive medium, either film roll or a digital charge-coupled device (CCD) unit (Figure 5).
In the middle of the lens is the aperture, which expands and contracts, and therefore controls the amount of light projected on to the light-sensitive medium. The focal-plane shutter is located in front of the CCD. It functions in a similar way as a stage curtain and shields the sensor from light.In order to make a photo, the mirror swings up so that the reflected light travelling through the camera can hit the light sensor. Although this principle is similar for any (D)SLRs, it differs for point-and-shoot cameras.
Exposure
In photography, exposure is the total amount of light that is allowed to fall on the light-sensitive medium. In order to get an appropriate exposure, a balance between three parameters, the exposure triangle, consisting of International Organization for Standardization (ISO) speed, formerly expressed as American Standard Association (ASA) number, aperture, and shutter speed, has to be found.
ISO
In the days of traditional film photography, a film with a certain ASA number had to be chosen before usage. The ASA number, used to describe the light sensitivity, wherein a higher number represented a higher sensitivity to light, was a property of the film itself. Where an ASA number of 200 would be ideal for bright daylight situations, an ASA 1600 film roll would be favorable in low light conditions, in combination with a faster shutter speed (the amount of time the reflected light hits the photographic sensor), and therefore minimized the possibility of blur (motion in an image). [15]
Today’s DSLRs are more versatile. Different ISO speeds are controlled electronically and can be changed with every exposure for every lighting situation. Since the introduction of the DSLR, the term ISO speed is more commonly used to describe light sensitivity. Nowadays, basic entry-level DSLRs commonly range from ISO 100 up to 3200. Point-and-shoot cameras often even have a smaller light sensor and are more susceptible to noise (a random optical texture on photographs). It should be noted however that digital film sensors are constantly improving, and so is their ability to suppress noise.
DSLRs are very similar to film roll and therefore, just as in film roll, a higher ISO speed will dramatically increase the noise in the final image (Figure 6). Specialized noise reduction software, which reduces noise in post-production might overcome these issues, but has its limitations.
Aperture
Aperture refers to the diaphragm inside a lens. This diaphragm is composed of multiple movable blades that together form a rough circle. The amount of light that travels through the lens is controlled by the aperture. A large aperture lets in more light, while a smaller lets in less light. Aperture size is described in f-stop numbers, wherein, paradoxically, a larger f-stop number corresponds to a narrower lens opening. A large aperture opening would correspond to a lower f-stop number, like f/2.8, while a small aperture opening would correspond to a higher number, like f/16. Therefore, a good low-light lens would have a low f-stop number.
Traditional SLR lenses have the f-stop range on the lens, while newer lenses don’t, because the aperture in DSLRs is controlled electronically.
Changing aperture will result in a different zone of shallowness, which is the area in a photo that is sharp. In photographic terms this zone of sharpness is called depth of field (DOF). Typically, a larger aperture, corresponding with a lower f-stop number, will create a narrow field of focus on your subject. A smaller aperture, corresponding with a high f-stop number, will create an extended field of focus. The field that is “out of focus”, and therefore not sharp, is called blur (Figure 7A-C). For photography in a well-lit operating room, one can often reside to a higher f-stop number, being more than f/5.6. In clinical situations, it is advised to set the f-stop lower .
Shutter speed
The time reflected light from the subject shines on the CCD is called shutter speed. Shutter speed is controlled by an electronically controlled curtain that shields the CCD sensor from light.
The longer the shutter stays open, the brighter the photo will be, and vice versa.
Typically, a fast shutter speed is used to freeze motion in, for example, action sports. A longer shutter speed can be used to photograph situations where less light is available (e.g. darker operating rooms). Using a longer shutter speed to capture movement will result in blurred images. A shutter speed of 1/125 of a second, or quicker, is ideal for medical applications.
Exposure triangle
Although every DSLR has an automatic exposure function, it is important to know that a proper exposure is the combination of certain ISO speeds, f-stop numbers, and shutter speeds. These three parameters can be varied in a logical way (Figure 8). Theoretically, a photo with a proper exposure and doubling of the ISO speed, in manual mode, without any other alterations, would result in an over-exposed photo. However, in combination with a higher f-stop number, it would be properly exposed and it would have a deeper depth of field. A combination of a higher ISO speed, a lower f-stop number, and a higher shutter speed would also result in a proper exposure. Pre-determined exposures that are manually set will result in consistently lit photos.
For example, a properly exposed photo during on a bright day might have an aperture setting of f/16, with a shutter speed of 1/200 of a second and ISO 200. With an aperture setting of f/11, the photo will be 1 stop under-exposed, because the difference between f/11 and f/16 is 1 stop. When adjusting the shutter speed to 1/100 of a second, the photo will be properly exposed again. A virtual demonstration can be found on the Kamerasimulator website. [11]
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Lenses
The properties of lenses are twofold: they provide magnification and control brightness.
As mentioned before, image brightness corresponds to the lens’ aperture combined with a certain shutter speed and ISO speed. Magnification, which is an outcome of the lens’ focal length, is described in millimeters (mm). Focal length corresponds to the focal distance, which is the distance from the point where the image converges in the lens to the point where it projects on the film sensor. A longer focal distance will result in more magnification and a lower angle of view, where a shorter focal distance will result in less magnification and a higher angle of view.
Lenses with a 50 to 60 mm focal distance are closest to the human viewing angle (Figure 9A-C). Lenses with a shorter focal distance are called wide-angle lenses (or extreme wide-angle lenses under 21 mm), because they provide a wider angle of view, and hence less magnification. These types of lenses are often used in a view of an entire operating room set up. Lenses with a higher focal distance are called telephoto lenses; these provide magnification and a narrower field of view. Telephoto lenses are often used in close-ups of surgical photography or sports photography. Macro or micro lenses are used to photograph small subjects with a high level of detail. The ratio between the actual size of the subject and the size on the film sensor is called reproduction ratio; for example, if the image on film is the same size of the subject, the reproduction ratio is 1:1. Macro lenses are often used for product or nature photography and are essential for medical application. Zoom lenses allow the photographer to vary the focal length within a pre-defined range. While zoom lenses have an advantage in dynamic subject situations, they are often less sharp and have fewer light-gathering abilities as fixed focal length lenses, also called prime lenses. For medical application, prime lenses are advised (Table 5).
Sensor sizes
The size of the built-in light sensor has a considerable effect on the outcome of the image (Figure 10). The use of lenses on a small-format sensor will result in magnification of the subject, compared to the same subject, on the same distance, with the same lens, on a full-frame sensor. This phenomenon is referred to as the crop factor and differs per camera model and brand. The most common crop factors are 1.3x, 1.5x, and 1.6x. Multiplying this number with the focal distance of the lens will result in the full-frame focal length lens subject framing equivalent.
Exposure and histograms
Although many professional photographers still use hand-held light meters, modern day DSLRs are equipped with built-in light meters. Digital SLRs offer light metering modes, with different algorithms for different situations. When center-weighted metering is selected, the center of the viewfinder is used to meter lighting conditions. Multi-zone metering evaluates the amount of light that the CCD receives across the entire frame and then combines that information in order to achieve proper exposure. Spot metering uses a smaller area in the viewfinder to calculate exposure. Both multi-zone and center-weighted metering are excellent in locations where the light is more or less evenly distributed. With spot zone metering only a small area of the scene will be measured. This is very helpful in an operating room setting, where dedicated OR lamps generally overexpose the ambient light by a certain amount of stops.
Reviewing photos on the on-camera LCD screen can be deceiving, because of the small size and the brightness of the LCD screen. A better way to evaluate the photo is by looking at its histogram. Different camera models have different ways to access the histogram (Figure 11).
The histogram is a representation of the tonal distribution of a particular photo in a graph. The distribution of blacks is shown in the far left, the grays are shown in the middle and pure whites are shown in the right. Over-exposed photos will have a bias to the right end of the graph. Under-exposed photos will have a bias to the left end of the graph. Although often hard to achieve, a balanced photo would have their brightness representations across the graph and tapering off on both ends. [8]
Lighting
Correct lighting is essential to all photography. It is important to understand that light has many characteristics. As well as being light or dark, lighting can also be soft or hard. Typically a large light source close to a small object will create a soft light with soft shadows. A small bright light on a far away subject will result in a hard light with hard and well-defined shadows.
An efficient way to light a subject in the operating theatre is by aiming two dedicated OR lamps on low light output in a 45 degrees downward angle in the direction of the subject. Not only will this provide enough light to use a high aperture, which will result in higher detail, it will also eliminate shadows. For medical application try to minimize shadow effects, by positioning light in a downward angle in the same direction as the camera.
In surgical situations it is esstential to be able to identify shape and texture. The use of on-camera flashes during operations will result in hard shadows on the subject. A diffused light source, for example a softbox, can be used to minimize shadows. A ring flash will produce shadowless images as long as the subject is small enough to be covered within the circular flash tube.
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[END OF APPENDIX]
Figure legend for appendix
[FIGURE 5]
Schematic image of light travelling through a camera before the mirror swings up.
[FIGURE 6]
ISO 12800 /
ISO 25600
The use of a higher ISO value will result in more noise in the photo.
Nikon D700, Nikon 50 mm 1.4 AF D lens at f/5.6.
[FIGURE 7A-C]
Example of shallow depth of field. Focus in these four photos lie on the patella reamer collet in the middle of the basket. Same camera and same lens have been used for these photos. Nikon D700, Nikon 50 mm f.1.4 AF D lens, ISO 1600 f/2.0 1/500 sec.
ISO 1600 f/3.5 1/160 sec
The depth of field increases dramatically when a higher aperture is used.
ISO 1600 f/8.0 1/30 sec
[FIGURE 8]
Illustration of the interaction between aperture and shutter speed and the effect that has on both depth of field and motion blur, in a properly exposed photograph.
[FIGURE 9A-C]
Three photos taken from the same location using a wide-angle, a standard lens and telelens.
Nikon D700, Sigma 12-24 mm f/4.5-4.6 EX DG Aspherical HSM at 12 mm.
Nikon D700, Nikon 50 mm f/1.4 AF D lens.
Nikon D700, Nikon 105 mm f/2.5 AI-s.
[FIGURE 10]
The difference in magnification between full-frame and smaller size sensors.