User manual

8.0 Lenses
8.1 Overview
The Cohu 7100 Series camera accepts a wide range of lenses. Select the appropriate lens for
given applications. This chapter details the imaging chip and lens specifications that must be
considered when selecting a lens for the camera.
8.2 Lens Selection
To select a proper lens, imaging chip size, desired focal length and lens-mount must be taken into
account.
Beyond these specifications, consider the depth of field (F-stop values), and the level of control
needed over the lens iris (aperture or opening). Additionally, determine zoom capability is
needed.
8.3 Imaging Chip Size (format)
The imaging chip on cameras has a specific format, measured in inches, that represents the
surface area of the device with which the camera captures light. The Cohu 7100 camera uses an
imager that is “slightly” over ½ inch. When selecting a lens, choose a lens that can project an
image that will cover the entire surface of the imaging chip. Thus, choose a lens with an image
format equal to or larger than the size of the imaging chip.
Smaller format lenses cannot cover the entire surface of the imaging chip. If a lens is used that
has a smaller format than the imaging chip, the corners of image will appear to be darkened.
Larger format lenses, though more expensive, will work on smaller sized imaging chips.
However, since larger lenses cover more area than the size of the imaging chip, excess light will
enter the camera outside of the active image area. The excess light can reflect inside the lens
mount and may cause a slight fog in the image.
Ideally, choose a lens that has the exact format of the imaging chip. Typical lenses below 6mm
may be ½” or smaller in format. These lenses may cause some undesired clipping.
8.4 Focal Length
The focal length of a lens is the distance, in millimeters, from the optical center of the lens to the
camera s imaging chip. This distance determines the amount that objects within a camera’s image
are magnified.
Lenses with high focal lengths produce larger, more magnified objects. Such lenses are called
telephoto lenses. Conversely, smaller focal length or wide-angle lenses show objects at less
magnification than they appear to the human eye from the same distance. The normal value for a
given focal length is the distance at which the camera’s image is the same as human’s eyes.
Certain lenses have variable focal length, which gives users the ability to adjust the magnification
of objects in their image.
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