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.
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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 entrylevel 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.
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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 fstop 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
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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]
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.
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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 fullframe 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.
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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]
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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 oncamera 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]
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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.
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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.
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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
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[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.
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[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.
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Nikon D700, Nikon 105 mm f/2.5 AI-s.
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[FIGURE 10]
The difference in magnification between full-frame and smaller size sensors.
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[FIGURE 11]
An over-exposed, properly exposed and over-exposed image and their according histograms.
Dark distribution is shown on the left side of the histograms, light distribution on the right side.
Exposure as in the middle image is desired, because of it’s wide distribution of colors.
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