Specifications

DEPTH OF FIELD, WHAT IS IT?
All lenses have a characteristic called Depth of Field. Depth of Field is a zone
in front of the lens that is in focus. It is measured as the minimum distance and
maximum distance from the lens where objects are in focus. All objects inside this
minimum and maximum bracket zone will be in focus. The further away you go from
this bracketed "in focus zone", either toward or away from the camera the more out
of focus the objects become.
THE PROBLEM WITH AUTO IRIS LENSES.
The Auto Iris lens has a movable aperture inside the lens that controls the
amount of light allowed to pass through it. This aperture also directly controls the
Depth of Field of the lens. So an Auto Iris lens will have a variable Depth of Field
depending on the amount of light entering the lens. This causes the minimum and
maximum bracket zone of focus to change when the lighting level changes.
During the day or in bright lighting conditions the Iris is closed down to a small
opening, and the Depth of Field is quite large. So called Pin Hole cameras exhibit
this effect where all objects in the scene are in focus. However with movable iris
cameras at night or in low lighting conditions the Iris is open wide, this causes the
Depth of Field to collapse down to a smaller minimum and maximum bracketed
zone of focus. An object that was inside the zone of focus during the day can be
outside the zone of focus at night.
THE SOLUTION.
One way to set up the camera to minimize the Depth of Field problem, is to
adjust cameras focus at a time when the light level is at its lowest. This may not be
convenient for your installers.
The best way is to use a SHADE 5 FILTER PLATE also called a #5 welders
glass filter. It is the filter plate that welders look through to protect their eyes during
welding operations. When you have full daylight or bright lighting conditions the filter
plate is placed over the cameras lens to simulate half lighting conditions.
This filter plate costs only a few dollars and should be carried by installation
personnel to every job site for use with Auto Iris lenses, especially those jobs using
outdoor cameras. By using the filter plate to adjust the Auto Iris, you will be centering
the control range of the Auto Iris system. This will give you optimum performance
from your cameras Iris control system.
CCTV TRAINING MANUAL PAGE 6 OF 28