APERTURE AND EXPOSURE TIME
As you make the image sharp through the focus adjustment, it will also be necessary to adjust the brightness of the image.
There are two controls available for this: the aperture/iris setting on the lens, and the exposure time setting of the camera.
•Aperture: opening the aperture allows more light to fall on the sensor. The aperture setting is also called the f-
number; f-numbers are usually indicated on the lens’s aperture ring and typically go from an open setting of F/1.4
or F/2.8 to a closed setting of F/22 or F/32. Using a bigger aperture (lower f-number) will make the image brighter.
However, it will also decrease the depth of field – the range over which the focus is sharp. Even for a flat specimen,
some depth of field is necessary because each camera is oblique to the plane of the specimen; also, poor depth of
field may make it difficult to achieve a wide range of calibration target positions (see following section).
At higher magnifications and with higher resolution cameras, care should be taken when using very small
apertures (high F-numbers). In some cases, this can limit resolution due to diffraction; for example, with a 5
megapixel camera and a 75mm lens, using apertures of F/16 or higher will result in very blurry images. In these
cases, it will be necessary to find the best balance of depth of field (requiring high F-numbers) and resolution
(requiring low F-numbers).
Note that the aperture may not be changed after the system is calibrated.
•Exposure time: this is the amount of time the camera sensor gathers light before reading out a new image. Longer
exposure times make the image brighter but can also create blur if significant motion happens during the exposure
times. For many tests, blur is not a concern for the specimen itself, but can be an issue when acquiring images of a
hand-held calibration grid. A rule of thumb is to keep the exposure time below 1/f, where f is the focal length of
the lens (in mm). So, for a 50mm lens, this would mean a limit of approximately 20ms.
In contrast to aperture, exposure time may be adjusted after the system is calibrated if lighting conditions change
or the specimen becomes brighter/darker.
Controls for focus and aperture differ by lens; two common C-mount styles are shown below.
This lens has a focus and aperture ring, each with
locking knob. The aperture ring is normally closest
to the camera. Loosen the locking knob (if
present); make any adjustments; and tighten the
lock before calibrating.
