140 PI-MAX/PI-MAX2 System Manual Version 5.F
sharp image on the photocathode of the intensifier. The ability of the lens to do this well
depends on a number of factors, as follows.
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/).
Resolution
This is a measure of the sharpness of the lens, that is, of its ability to resolve two closely
spaced lines. Resolution is commonly expressed as the Modulation Transfer Function
(MTF), which specifies the number of line pairs per mm that can be resolved for a given
valley depth between the two lines of each pair. In comparing the MTF of two systems, it
is important that the specified valley depth for both be the same. For any real lens,
resolution is a function of f/#, with maximum sharpness most often occurring at some
mid-range value. Thus, a lens which offers f/# settings from f/ 2.8 to f/22 will probably
be sharpest at f/8 or f/11. For this reason, even though focusing may be more sensitive at
maximum aperture, actual data acquisition should be done with a mid-aperture setting.
When working with an intensified camera though, keep in mind that the resolution limit
will ordinarily be determined by the intensifier, not the lens, and light gathering power
becomes the principle concern for the lens.
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 (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.
Baseline Signal
With the detector completely blocked, the CCD will collect a dark charge pattern,
dependent on the exposure time, detector temperature, and intensifier gain setting. The
longer the exposure time and the warmer the detector the larger and less uniform this
background will appear.
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