Teledyne Princeton Instruments PI-MAX 3, System Manual

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188 PI-MAX
®
3 System Manual Issue 2
15.5 Lens Performance
Imaging applications require that a lens be mounted to the detector. Because the lens
characteristics affect system performance, it may be helpful to review some basic lens
concepts.
Basically, light from an object enters the front of the lens and is focused to a sharp
image on the photocathode of the intensifier. The ability of the lens to do this well
depends on a number of factors which are discussed in the following sections.
15.5.1 Throughput
The throughput of a lens is determined by its aperture which can ordinarily be set to a
number of different values, or f/ stops. The higher the number after the slash, the
smaller the aperture and the lower the throughput.
Depth of field considerations make the focus adjustment most sensitive at maximum
aperture (smallest f/).
15.5.2 Depth of Field
Depth of field is a measure of how the sharpness of a lens varies with respect to the
distance of an object from the lens. For any given aperture, there is a depth of field,
usually marked on the barrel of the lens. Objects within the zone will be sharply imaged.
Objects closer or further than the depth of field will not be as sharp. The further an
object is from the point of sharpest focus, the less sharp its image on the CCD will be.
The point of maximum sharpness is located 1/3 of the way into the depth of field zone.
For example, if the indicated depth of field for the selected aperture extended from 3 ft
to 6 ft, the point of maximum sharpness will be at 4 ft.
For good focusing sensitivity, the depth of field should be small (i.e., large aperture.) If
the aperture is small, the depth of field will be deep, making it difficult to establish the
point of sharpest focus. For example, with a 50 mm lens, at f/4 the depth of field will
extend from 8 ft to infinity. By focusing at full aperture, the depth of field will be as
shallow as possible. As a result, the effects of even very small focusing adjustments will
be readily observed, allowing the focus to be set with precision. Once the optimum
focus setting has been achieved, the aperture can be reduced to the point of maximum
sharpness. In some experiments, you may wish to adjust the aperture for optimum
signal level. However, the experiment setup parameters established with the
applications software can also be used to adjust the signal level, allowing the lens
aperture and focus to be optimized.