Stranica 70 [70]
In most real world situations there is no such thing as an ideal or "perfect" exposure. There is
simply a combination of settings that places the tonal values found in the scene most
appropriately within the capability range of the camera sensor, i.e. the mid-tones found in the
image fall roughly half way between the darkest and the brightest values.
ISO Speed
The ISO speed determines how sensitive the camera is to incoming light and correlates 1:1
with how much the exposure increases or decreases. Common ISO speeds include 100, 200,
400 and 800, although many cameras also permit lower or higher values. With DSLR
cameras, an ISO speed in the range of 50 - 800 (or higher) generally produces acceptably
low image noise. A lower ISO speed is almost always desirable, since higher ISO speeds
dramatically increase image noise. As a result, ISO speed is usually only increased from its
minimum value if the desired aperture and shutter speed aren't otherwise obtainable.
An ISO speed of 100 or lowest setting available is recommended.
Aperture (F-stop)
This parameter determines the aperture size and depth of field. An important consideration
given the dimensionality of the object being imaged, e.g.:
— Lower F-stop/wide aperture/shallow depth of field: fine for flat or 2D objects.
— Higher F-stop/narrow aperture/larger depth of field: may be more appropriate for 3D
objects, e.g. sculpture or reliefs.
In general, in order to maximise depth of field, it is useful to consider the closest and furthest
parts of the object which must be in focus. The plane of focus of the lens is then seta third of
the way from the closest point. If possible, calibration targets should be positioned on this
plane to aid focussing.
In addition, the sensitivity of the sensor to the wavelength of the incoming radiation will be a
determining factor in choosing an aperture setting, e.g. the low sensitivity of camera sensors
to UV radiation will usually require the use of large apertures (low f-stop number).
Shutter speed (exposure time)
This parameter determines the duration of the exposure or how long the camera shutter will
be open or closed to incoming light from the camera lens. The influence of shutter speed on
exposure correlates exactly 1:1 with the amount of light entering the camera. For example,
when the exposure time doubles the amount of light entering the camera doubles. A faster
shutter speed equates to shorter exposure time.
Specific settings will be dependent on the sensitivity of the sensor to the wavelength of the
incoming radiation, the aperture setting chosen and the type of radiation source (i.e.
continuous vs. pulsed).
Version No. 1.0 62 Date : 14/10/2013