Lens Backfocus and Adjustment Procedures
TECH TIP
VICON TECHNICAL SERVICES GROUP
Subject: Lens Adjustments
Product: All Lenses
Number: 1400-0001-04-00
Date: 07/03/01
Lens Backfocus & Adjustment Procedures
1. Backfocus
The distance from the lens mounting surface on the front of the CCTV camera and the position of
the pickup device (CCD chip or tube) is called the flange focal distance, or backfocus distance.
The surface that the lens mounts up against is called the flange. On “C” style cameras, this
distance is 17.5mm. On a “CS” style camera, the distance is 12.5mm. This distance is adjustable
over a small range on virtually all CCTV cameras. “C” mount lenses may be installed directly on
“C” style cameras, or, by using a 5mm C-CS adapter ring, on “CS” style cameras. “CS” lenses
cannot be mounted on “C” style cameras. See Figure 1.
17.5mm
Mounting Flange
C-Mount Lens
Imager
Tube or CCD
CS-Mount Lens
Mounting Flange
12.5mm
Figure 1: “C” vs. “CS” Mounting Difference
Theoretically, it should be possible to just thread in a lens and have the image in focus, but in
reality, manufacturing tolerances on both lenses and cameras combine to make it necessary to
backfocus whenever a lens is installed on a camera.
If a lens is not correctly backfocused, it is highly likely that the camera/lens assembly
cannot deliver a sharply focused picture either all the time or under conditions of reduced
light levels.
General
Backfocus adjustment can be performed on the bench; it does not have to be performed with the
camera in place in its final location. Doing the procedure indoors greatly simplifies the process of
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reducing ambient light levels. This procedure should always be done with the lens iris at its
maximum open position.
Fixed Focal Length Manual Iris Lenses
1. Thread the lens into the camera.
2. Adjust the focus ring on the lens to the maximum infinity setting. In most cases, the focusing
ring can be recognized by the presence of distance numbers, usually marked in both meters
and feet.
3. Open the lens iris to its maximum. If the picture on the monitor is too bright, reduce the
ambient lighting (if you are indoors) until the monitor image is optimum.
4. While viewing a good, high-contrast target at a distance of at least 30 feet (10 meters) from
the camera, move the camera’s backfocus adjustment until the monitor displays the sharpest
possible picture.
5. Lock the backfocus adjustment in place, if the camera has a backfocus lock. Not all cameras
have a separate backfocus lock.
Fixed Focal Length Autoiris Lenses
1. Thread the lens into the camera and make any wiring connections.
2. Adjust the focus ring on the lens to the maximum infinity setting. In most cases, the focusing
ring can be recognized by the presence of distance numbers, usually marked in both meters
and feet.
3. Reduce the ambient lighting (if you are indoors) until the lens is opened to its maximum. The
simplest way to determine this is to look into the lens. If the location does not allow the
reduction of ambient light, a neutral density filter may be used. Neutral density filters are
described at the end of this Tech Tip.
4. While viewing a good, high-contrast target at a distance of at least 30 feet (10 meters) from
the camera, move the camera’s backfocus adjustment until the monitor displays the sharpest
possible picture.
5. Lock the backfocus adjustment in place, if the camera has a backfocus lock. Not all cameras
have a separate backfocus lock.
Motorized Zoom Lenses
1. Thread the lens into the camera and connect the lens to a lens controller. If the lens has an
autoiris, connect the lens iris cable to the camera.
2. Using the lens control, zoom in on a good, high-contrast target at a distance of at least 75
feet (25 meters) from the camera. Use the maximum telephoto setting.
3. Using the lens controller Focus Far control, obtain the best-focused image of the selected
target.
4. Non-Autoiris Zoom Lenses: Reduce the ambient lighting (if you are indoors) until the lens is
opened to its maximum. Use the lens control to open the iris. If the location does not allow
the reduction of ambient light, a neutral density filter may be used.
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Fax 631-951-2288
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5. Autoiris Lenses: Reduce the ambient lighting (if you are indoors) until the lens is opened to
its maximum. The simplest way to determine this is to look into the lens. If the location does
not allow the reduction of ambient light, a neutral density filter may be used.
6. While viewing a good, high-contrast target at a distance of at least 75 feet (25 meters), move
the backfocus adjustment until the monitor displays the sharpest possible picture.
7. Zoom the lens to its maximum wide-angle setting, and repeat Step 6 to obtain the sharpest
image.
8. Zoom the lens to the maximum telephoto setting to see if the image is still sharp. If it goes out
of focus, readjust the lens controller Focus control (do not readjust the backfocus
adjustment). Repeat the zoom in – zoom out cycles to obtain optimum focus.
9. Lock the backfocus adjustment in place, if the camera has a backfocus lock. Not all cameras
have a separate backfocus lock.
Neutral Density Filter
This is a piece of gray tinted glass. They are widely available at photographic supply stores and
come in different densities and diameters. To use it, simply hold it against the front of the lens
while making adjustments. It is not necessary to fasten the filter to the lens.
Other means of reducing ambient light are the glass lenses used in brazing masks (a number 2 or
3). Do not use a film or glass darker than ND 3.0; doing so may degrade picture quality and make
focusing difficult.
2. AutoIris Adjustments
Most autoiris lenses in use today fall in to one of two categories. Adjustments will vary between
styles.
Video Drive lenses:
These lenses have internal circuitry to process the video signal and can be identified by the ALC
and/or Level adjustments located on the lens barrel.
1. Level: This is the main adjustment for the video level that the lens will maintain under varying
lighting conditions. Too high will open the iris and cause a bright picture; too low and the
picture will be dark. After setting the level in daylight, place a dark filter over the lens to
simulate night conditions. The iris should open to compensate for the reduced light level.
2. ALC: This will adjust the sensitivity of the iris. Adjusting towards Peak will allow the iris to
react more quickly to changing light levels.
DC Drive Lenses: These lenses have no internal electronic amplifier circuits or external electrical
adjustments.
1. If the camera has an “EE/AI” switch, place the switch in the “AI” position. Adjust the level
control on the camera for the correct video level setting, as above under Video Drive Lenses.
Vicon Industries Technical Services Group
800-348-4266
Fax 631-951-2288
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