INTRODUCTION
The current digital single-lens reflex (DSLR) camera Figure 1 is still the benchmark standard for this genre of photography and its function will be described in detail. Other available systems will be mentioned separately.
CAMERA FUNCTION
Light from the subject passes through the aperture ring onto a mirror, reflected to and focused on a matte screen and then directed to the viewfinder/eye via a prism. When the shutter is operated, the mirror flips out of view, the shutter exposes the sensor for the appropriate period and the mirror returns to its original position, the cycle completing in a fraction of a second. Suffice to say in ‘viewfinder’ mode the aperture is fully open and closes electronically to the pre determined setting when the shutter is operated. As the shutter is placed directly in front of the sensor, it is known as a focal plane shutter. Control of exposure will be discussed fully later in the book.
Single-lens reflex cameras were initially designed to be held and used at eye level. With most current digital cameras, the rear screen can also be used as the viewfinder; however, dental systems can be both heavy and bulky, so for reasons of stability, eye level viewing is recommended.
The DSLR is based on the identical principle of the earlier cameras with a sensor replacing film. The major difference in practical terms is whilst the film is moved for sequential images, the sensor is fixed, and the ‘images’ are transferred electronically in milliseconds to a capture card, usually an SD card. These cards incorporate a slider to protect images from inadvertent deletion (Figure 2).
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The image on a digital camera is captured by light sensitive pixels (picture elements) on the sensor with upwards of 15 million being the norm. The process of producing an image is complex, (see Figure 3), and outside the remit of this text. Each pixel will represent one dot of the original subject and theoretically the more pixels on a sensor the higher the quality of the image, however there can be a tradeoff. Take the sensor (Figure 3b), to accommodate 20 million pixels, each one would be smaller than the same size sensor with 15 million pixels, the larger pixels can 7sometimes give a better result than the smaller, so it is not always a case of more is better.
Current DSLR cameras can be divided into two groups, those incorporating a full frame or 24 × 36 mm sensor and the most popular using a smaller ‘cropped’ sensor. Canon APS-C sensors are 22.4 × 14.8 mm and the Nikon slightly larger at 24 × 16 mm. All the current lenses have been developed from those designed for 35 mm film cameras and their specifications in terms of focal length, maximum aperture and magnification ratios on macro lenses still relate to that format. Most cameras used in dental photography today will have cropped sensors rendering the details on magnification ratios inaccurate and they will be addressed where appropriate (Figure 4).
COMPONENTS OF A DSLR SYSTEM
A DSLR camera body is the basis of a photographic system with the operator choosing the appropriate lens and other accessories for specific applications.
A basic camera system will consist of a camera body, a lens, and all but some top end ‘professional’ cameras, will incorporate a built-in flash unit.
A camera system for dental photography will require a body, appropriate macro lens, and usually a flash illumination system which are described in detail later. From the dental photography point of view ‘entry level’ camera bodies are totally suitable.
FOCUSING
Focusing can be achieved automatically or manually by adjusting the focus ring on the lens. Either way, using the camera at eye level the image will be seen on the focusing screen. Typically, this is ground glass with etched markings to facilitate alignment of the subject (Figure 5).
DIOPTRE WHEEL
If the operator looks into the viewfinder whilst aiming the camera at a white ceiling or similar, he/she can adjust the sharpness of the etchings by turning the ‘dioptre’ wheel situated near the viewfinder. This will adjust the viewfinder's optical system to the user's individual eyesight (Figure 6).
Figure 4: Direct comparison of Canon cropped and full frame sensors.(Courtesy: Canon sensors loaned courtesy of Mick Edwards FIXATION Ltd.
THE LENS
Focal length determines the angle of the field of view, the lower the focal length number, the wider the field of view (Figure 7).
When close-up photography is considered, the focal length is critical as it also controls the lens to subject distance (working distance) and perspective.
On a cropped sensor camera, a 35 mm lens covers what the eye sees and on a full frame system a 50 mm lens would be the standard.
Lenses can be divided into two groups, those with a fixed focal length and therefore a fixed field of view, are known as PRIME lenses. The other group and very popular for general photography have a variable field of view, e.g. 24–200 mm and are known as ZOOM lenses.
Specialist lenses for close-up photography, macro lenses, are a subgroup of PRIME lenses and are available in a variety of fixed focal lengths.
The advantage of a DSLR system is the ability to purchase the body individually and choose an appropriate lens for a given situation.
Apart from the optics, lenses contain one major physical component, the aperture or diaphragm. This is a variable-sized opening controlled electronically from the camera body (see Figure 10).
EXPOSURE
Ambient light exposure
Exposure is determined by the intensity of light hitting the sensor, and the duration. Three factors control this: The size of the lens aperture, designated by ‘f’ numbers, which control the amount of light hitting the sensor; the shutter speed, which controls the duration of exposure; and the sensitivity of the sensor to light, which is determined by the ISO setting. They are interrelated; a change in one factor will require a change in one or both of the others (Figure 8).
Consider Figure 9, the ISO has been set at 800. Note that the exposure times differ by one half from left to right and get longer, and that the sizes of holes get smaller from left to right. Although the aperture numbers designated ‘f’ appear unrelated, each hole from left to right is exactly twice the area of its predecessor. Thus, in every combination above, the amount of light passing through each aperture and reaching the sensor, 9is the same. Should the sensitivity be halved, i.e. ISO 400, correct exposure would be achieved by moving the shutter speeds one place to the left and the exposure time at f22 would be 1/7.5 seconds (i.e. doubling the exposure) (Figure 9).
The aperture size is of prime importance in close-up/dental photography as it controls ‘depth of field’ (DoF) which will be discussed in detail in a later chapter. Figure 11 taken with apertures of f2.8 and f22 demonstrates the significance.
APERTURE SIZES
A lens is defined by two factors, its focal length and maximum aperture, e.g. 100 mm (focal length) f2 (maximum aperture).
‘The ‘f’ number of an optical system is the ratio of the system's focal length to the diameter of the entrance pupil’ (Wikipedia) (Figure 12). Simply this is the maximum diameter of the usable part of the lens. For example, a 100 mm f2 lens will have a workable lens diameter of 100/2 = 50 mm. A 100 mm f4 lens will have a workable diameter of 100/4 = 25 mm. The latter lens will obviously be smaller and therefore cheaper to manufacture.
Wide aperture lenses such as f2 were more necessary in film days, but current sensors with their low light sensitivity renders this less necessary today as ISO changes can be made at any time.10
Early lenses incorporated mechanical diaphragms with click stops separating apertures and a fixed ratio (1:2) between the resultant areas. Current lenses have electronically controlled diaphragms which are infinitely variable, and although ‘f’ values are shown in the display, the relationship from one to another is not known.
The appropriate lens and flash systems for dental photography will be covered in depth in subsequent chapters but the flash function itself will be covered below.
With reference to the aperture and its relation to depth of field (DoF), for close-up clinical work an aperture of f22 will be advocated. This is a very small diameter opening and in combination with a low ISO setting, a very long shutter speed would be required rendering a hand-held camera system impractical. In view of this, an appropriate flash system is recommended.
From Figure 13, it is obvious that the typical flash unit integral to most cameras will not illuminate a subject close to the lens and a flash system fitting on or around the lens is recommended.
FLASH SYSTEM FUNCTION
External flash systems incorporate a power pack which fires the flash tube when the camera shutter is operated. Prior to the 1980s most units were discharged fully on each cycle and the correct exposure was calculated either via a simple formula, or the operator purchased a flash meter to measure the output. Every flash unit has a guide number related to an ISO value and referenced to either feet or metres. A typical guide number being 22 in metres for an ISO of 200. For a subject 2 metres away, one divided 2 into 22 = 11 and the operator would select an aperture of f11 on the lens. Some systems had variable outputs, e.g. ½, ¼, 1/8, etc. but the calculations still had to be made.
AUTOMATIC FLASH EXPOSURE
Virtually all current flash systems can be automatically controlled, which, in the author's opinion greatly facilitates and simplifies consistent results. Older flash systems for film cameras will not function automatically with digital cameras as their operating system is entirely different.
It is assumed that the ISO has been pre-set. When the shutter is operated a pilot flash (blue) (see Figure 14) leaves the flash gun and a return signal (yellow) enters the camera through the pre-chosen aperture.11
Figure 15: Diagram representing small aperture as opposed to large aperture in Figure 14a.
The return signal is measured by the camera's computer which then calculates the intensity/duration of the exposing flash (red) to ensure a correctly exposed image. This cycle takes approximately 20/1,000,000 seconds.
Figure 14 shows a large aperture; if the operator chooses a smaller aperture, the flash duration would increase appropriately (Figure 15).
The major advantage of this system is the power unit is not fully discharged for every exposure and therefore allows rapid subsequent 12photographs, however, if an attempt is made to take a picture when there is insufficient power, the result can be underexposure, or a system lock up. In the latter instance the answer is to switch the camera off and on again and for the former, allow the flash to re-charge and check the ‘ready’ light is illuminated.
FILM SENSITIVITY (ISO)
ISO numbers refer to the sensitivity of the sensor and have been directly transposed from film.
Films vary in their sensitivity to light, as expressed by their ISO film speed. The higher “the ISO number, the ‘faster’ the film and the less light you need to take a picture. Fast films of ISO 400, 800 and higher are recommended for dim lighting or fast action such as sports photography.”
With film cameras one needed to buy the correct film for a given situation, now with digital sensors, the sensitivity can be changed within the camera.
The term ISO is often erroneously equated to ‘International Standards Organisation.’ This ‘body’ per se, does not exist! ISO standards were set by the ‘International Organization for Standardization’ which would have different acronyms in different languages (IOS in English and OIN in French for Organisation Internationale de Normalisation.) In view of this, the organisation decided to give it the short form ISO. ISO is derived from the Greek isos, meaning equal.
This body, however, did set the standards for film/sensor settings.
The lower the ISO, the higher the quality of the image, and for dental photography a setting of 200 can be considered the norm, ISO sensitivity normally starting at 100.
FOCAL PLANE SHUTTER
The shutter keeping light away from the sensor is not a simple open/closed structure but consists of metal, previously cloth, curtains moving in sequence across the focal plane. In practice, particularly at higher shutter speeds, the total surface area of the sensor is not completely exposed at any given time. However brief the shutter setting, e.g. 1/1,000 of a second, it is slow in comparison to electronic flash of 1/20,000 sec. or faster. Therefore, it is essential when using flash, a shutter speed is selected when the whole surface area of the sensor is exposed to the brief duration of the flash (Figure 16). This is known as the flash synchronisation speed and fully discussed in Chapter 4.
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FLASH SYNCHRONISATION SPEED
All manufacturers have a recommended shutter speed when electronic flash is used, and they differ from make to make. The function and importance will be discussed where appropriate.
WHITE BALANCE
The white balance (WB) setting control allows the user to pre-set the sensor to render colours accurately according to the source of illumination. Whilst the human eye/brain will generally determine the correct colour of an object however it is illuminated, the camera sensor is not that accurate, and the WB should be pre-set to the light source as appropriate (Figure 17).
In the clinical situation virtually all the photography will involve the use of electronic flash and the appropriate WB should be pre-set to Flash.
One can of course choose ‘AUTO’ (AWB) when the camera will generally determine an appropriate setting.
CAMERA MODES
All modern DSLR cameras offer Mode settings. These are features which the operator can choose according to the shooting situation. They will be introduced here and expanded upon where necessary.
There is some variation in terminology between manufacturers and variations in options between models from the same manufacturer. Generally according to the mode selected, the operator will pre-set some information, such as ISO and WB and the camera will select the remainder of the operating cycle.
Nikon
Mode selected | Shutter speed | Aperture |
|---|---|---|
P (programmed auto) | Selected by camera | Selected by camera |
S (shutter-priority auto) | Selected by photographer | Selected by camera |
A (aperturepriority auto) | Selected by camera | Selected by photographer |
M (manual) | Selected by photographer | Selected by photographer |
Mode selected | Shutter speed | Aperture |
|---|---|---|
P (programmed auto) | Selected by camera | Selected by camera |
In this mode, the operator will initially select the ISO and WB and the camera will select the appropriate combination of shutter speed and aperture. Usually aiming for a shutter speed that will lessen the chance of camera shake. The operator will also have a choice of selecting manual or auto focus.
Mode selected | Shutter speed | Aperture |
|---|---|---|
S (shutter-priority auto) | Selected by photographer | Selected by camera |
In situations where the shutter speed is of optimum importance, e.g. nature and sports photography, the operator will pre-select the ISO and WB and choose an appropriate shutter speed.14
Figure 18: Mode choices: (a) Aperture priority set on Canon camera, and (b) Aperture priority mode set on Nikon camera.
The camera will automatically select the appropriate aperture. The operator will also have a choice of selecting manual or auto focus.
Mode selected | Shutter speed | Aperture |
|---|---|---|
A (aperture-priority auto) | Selected by camera | Selected by photographer |
The aperture controls ‘depth of field’. The operator pre-selects ISO and WB, chooses and sets the aperture and the camera will select the appropriate shutter speed. The operator will also have a choice of selecting manual or auto focus. This is the mode recommended by the author.
Mode selected | Shutter speed | Aperture |
|---|---|---|
M (manual) | Selected by photographer | Selected by photographer |
In this situation, the operator will select everything, ISO, WB, shutter speed, and aperture. The operator will also have a choice of selecting manual or auto focus.
Canon
The Canon system offers more options and variations in some designations.
- M&P are the same as for Nikon and equal in function
- AV is Aperture priority
- TV is Shutter priority
- With the mode set to the green icons “AUTO” Nikon and “A” Canon, (Figure 18) the camera will select everything apart from auto or manual focus, and the operator has no other control!
Aperture priority mode ‘A’ or ‘AV’
As DoF is of prime importance in clinical photography the author's recommendation is that this mode is selected when setting up. “A” for Nikon and “AV” for Canon. The significant difference between the two makes is the setting of the flash synchronisation speed which will be covered in depth later.