Intra-oral photography is essentially macrophotography, which is a photo-graphic technique that allows you to take magnified images of the teeth. Clas-sically, macrophotography is where the image on the sensor or film is the same size as the object itself, i.e. a reproduction ratio of 1 : 1. For intra-oral photography it is not just a question of being able to focus really close to your patient’s teeth, as this can be uncomfortable or even claustrophobic for the patient, but of being able to choose an appropriate working distance. Your
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Fig. 4.2 A digital single-lens reflex (SLR) camera with a high quality macro lens and ring flash.
Fig. 4.3 A 105 mm Nikon macro lens.
working distance must also allow sufficient light from your flash to get to the teeth that you are photographing without the patient’s cheek getting in the way. The importance of this is most obvious if you want to take a photograph of a single tooth towards the back of the mouth. The main challenges of macro-photography in dentistry are that it tends to limit the depth of the photo-graphic field and makes it difficult to illuminate objects to get the correct exposure.
Reproduction Ratio
Different reproduction ratios are used in dental photography; however, intra-oral photography needs somewhere between 1 : 1 and 1 : 3 (Fig. 4.4). Lenses capable of producing these reproduction ratios are known as macro lenses. The different ratios for different intra-oral photographs will be discussed later;
however, it is important to be consistent with the ratio that is used for the
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different views that you take. When you take a photograph it is sensible to make a note of the reproduction ratio as part of the information that you record about the photographs, as this will enable you to use the same settings for any subse-quent photographs. This consistency allows ‘before’ and ‘after’ photographs to be comparable without adding to your workflow by requiring resizing or crop-ping of the images.
Depth of Field
The depth of field of a photograph is the distance in front and behind an object that the camera has been focused on (focal plane) that appears to be in focus.
In general, approximately 50% of the depth of field is in front of the focal plane and 50% behind the focal plane. The depth of field is dependent upon the size of the aperture or metal diaphragm inside the lens, which is usually referred to as an f-stop (Fig. 4.5A–D). A good depth of field is determined by a large f-stop (Fig. 4.6A,B). The problem with a small aperture, however, is that this limits the amount of light getting to the film or sensor in your camera and therefore affects the exposure of the object.
Fig. 4.4 Reproduction ratios used in intra-oral photography on a 105 mm macro lens.
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Fig. 4.5A–D The relationship between f-number and aperture opening.
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C D
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P A T I E N T S ’ F A Q S
Q. What are you going to use the photograph for?
A. You might explain that photographs before treatment commences are important to help show the patient those areas you think need addressing and as a pre-treatment record of their mouth. If you want to use them for other areas such as teaching or for publication then your consent process must cover this, although patients are often happy to oblige.
Q. What if I decide that I don’t want photographs to be used for teaching or publication after consent?
A. You should reassure the patient that they can view images at any time and withdraw consent for use. It is important, however, to discuss clearly with the patient if you are planning to use the pictures on a website as they must understand the irretrievability of photographs put online.
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Exposure
Exposure relates to whether the image is too light (overexposed, Fig. 4.7) or too dark (underexposed, Fig. 4.8). If a tooth is overexposed then you will lose high-light details and the teeth will look ‘washed-out’ or all white. In general, expo-sure can be controlled by the aperture or shutter speed of the camera and the output of the flash. For dental photography, the aperture is determined by the depth of field that we want, and the shutter speed has to be synchronized with the flash unit (usually between a shutter speed of 1/60 and 1/250 seconds).
This means we need to have a camera which either allows aperture priority
Fig. 4.6A,B The differences in depth of field between large and small f-numbers can be seen with all the teeth looking in focus in (A) but only the second premolar in focus in (B).
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Fig. 4.7 Overexposed teeth showing a lack of details and ‘washed-out’ appearance.
Fig. 4.8 Underexposed photograph of the upper right anterior teeth.
(control of the aperture) or a manual mode (control over both aperture and shutter speed).
The exposure of teeth in the mouth can be altered by several different methods.
In addition to adjusting f-stop, as described above, the exposure can be control-led by the flash unit using TTL exposure metering. The TTL exposure metering
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works by averaging the amount of light from the object, equating it to a mid-grey and choosing the correct settings to get this mid-mid-grey in the picture.
Alternatively, if the object is still underexposed because the maximum light output of the flash is insufficient to illuminate the object then it is possible to make the sensor more sensitive. This involves changing the ISO value from 100 to 200. It is also possible to reduce the f-number as a means of increasing the light entering the camera but this has to be balanced against the loss of depth of field.
Finally, it is possible to modify the TTL exposure metering using exposure com-pensation (+/− EV) settings (Fig. 4.9). You may need to do this when you are taking very close-up pictures of teeth that will be brighter than mid-grey and so TTL exposure results in the teeth being underexposed. This will require a positive exposure compensation. Experimentation will show how much this positive compensation needs to be, e.g. +1 EV. You might also need to increase the exposure compensation by +1 to +2 EV when taking a mirror shot for an occlusal photograph. This will compensate for the lack of illumination provided by the flash (as a result of loss of light through reflection and increased distance between flash and object).
Fig. 4.9 An example of the exposure compensation setting button and scale.
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Illumination
Intra-oral photographs should be carried out using a ring flash (Fig. 4.10) or twin flash that gives an even illumination of the teeth. The unit should allow modification of its output by TTL metering. The main reported disadvantage with ring flashes is that they can produce multiple reflections from the surface of the tooth with a flattening effect as they eliminate shadow (Fig. 4.11A,B).17,19
Fig. 4.10 A Canon Macro Ring Flash MR-14EX showing a two-tube arrangement that allows some variation in the balance of illumination from the left or the right side.
Fig. 4.11A,B A comparison of the illumination from a ring flash showing a flattened surface (A) compared to illumination from a two-tube ring flash with illumination predominantly from the right side (B).
A B
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If light can be directed from the side, then some slight shadowing can be pro-duced in order to give texture and depth to the teeth.16,19 Some ring flashes contain two tubes which allow the light to come predominantly from one side to produce an image with more apparent contours (Fig. 4.11A,B).
K E Y C L I N I C A L T E C H N I Q U E S
Before you begin
• Ensure that the teeth are clean of plaque, lipstick and any other debris and suction excessive saliva.
• Warm up occlusal mirrors in a hot water bath to avoid fogging.
• Moisten cheek retractors before use.
Taking a photograph
• Set the reproduction ratio on the lens.
• Position the patient in a reclined position and move their head to make taking the view as easy as possible.
• Move in and out towards the subject to focus.
• Try to keep the centre line of the face in the midline of the photograph.
• Compose the photograph and think about the balance and symmetry and then take the photograph.
A dual flash system is an alternative to a ring flash that provides less flattened images, with better visualization of translucency and line angles. The dual flashes can be adjusted and thus the direction of our illumination can be con-trolled. Figure 4.12 shows the characteristics of photos taken with a dual flash system using direct frontal light, direct lateral light and indirect lateral light. As Figure 4.13A–D shows, photos taken with direct lighting appear much more flat and static, versus the volume and dimensionality seen with indirect lighting.
Note that a diffuser, or ‘bouncer’, can be attached to each dual flash to provide softer, indirect light.
Other Photographic Equipment
Cheek retractors
Cheek retractors are important for keeping the soft tissues away from the teeth and allowing easy camera viewing of the dentition. Generally, these come in plastic or stainless steel, although plastic retractors are more comfortable for patients (Fig. 4.14). It is important to ensure that these are autoclavable.
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Fig. 4.13A–D Photographs taken with direct lighting versus indirect lighting.
C A