A CCTV lens is an optical assembly that focuses light onto the camera’s image sensor. It generally includes:

• Front lens group: Determines angle of view (focal length).
• Diaphragm (Iris): Regulates light intensity.
• Rear lens group: Matches the light “spot” size to the sensor format.

The Iris (Diaphragm)

The iris is a mechanical assembly of overlapping leaves (or blades), forming a variable aperture. Common f-numbers range in sequence: 1:0.7 → 1:1.0 → 1:1.4 → 1:2.0 → 1:2.8 → 1:4 → 1:5.6 → 1:8 → 1:11 → 1:16 → 1:22 → 1:32 → 1:45 → 1:64.

• Wide open iris (e.g., f/1.2): Maximum light throughput, minimal depth of field, and potential for more optical aberrations.
• Closed-down iris (e.g., f/16): Greater depth of field, but increased diffraction may soften the image.

Lens Classifications

• Optical Construction
• Spherical vs. Aspherical: Aspherical lenses offer less aberration and can have lower f-numbers (e.g., f/0.8) for better low-light performance.
• Coatings (“multicoated” or “MC”) to reduce internal reflections and improve contrast, especially in low-light scenes.
• Focal Length Variation
• Fixed-focal (prime) lens: A single focal length (e.g., 2.8 mm, 4 mm, 6 mm).
• Varifocal lens: Allows manual adjustment of focal length within a range (e.g., 2.8–12 mm).
• Motorized Zoom (transfocator): Remotely controlled change of focal length.
• Iris Control
• No iris: Used in cameras that rely solely on electronic shutter to handle different light levels (common in small indoor fixed cameras).
• Manual iris: Suitable for constant lighting or combined with a camera’s auto shutter in semi-stable environments.
• Auto iris: Adjusts aperture in response to changing light. Typically has a cable to the camera for control signals.
• Video Drive (V.D.): The lens itself contains the control circuitry, analyzing the composite video signal. (Older style, becoming less common.)
• Direct Drive (D.D.): The camera’s internal circuitry controls iris motor with a DC signal.
• P-iris: A more advanced system ensuring the iris stays near the aperture sweet spot for maximum sharpness (balancing aberration vs. diffraction).

Miniature Lenses

Modern integrated “bullet” or “turret” cameras often incorporate mini-lenses with mounts like M12 or D14, which can be either fixed or varifocal. They function similarly to standard CS- or C-mount lenses, but are physically smaller.

KEY LENS SPECIFICATIONS

• Focal Length
• Determines the angle of view. Smaller focal lengths (e.g., 2.8 mm) yield wide angles; larger focal lengths (e.g., 16 mm or 50 mm) provide narrow, zoomed-in views.
• Angle of View
• Often specified horizontally (HFoV) and vertically (VFoV). You can use charts or software to relate focal length to coverage area, considering the camera’s sensor size (1/3 in, 1/2.8 in, etc.).
• Relative Aperture (F-number)
• Affects brightness and depth of field.
• Lower f-numbers (like f/1.2) pass more light (better in low light) but can reduce depth of field and increase optical aberrations.

Choosing the Right Focal Length

Proper selection aligns with the surveillance task: detection, recognition, or identification. For example:

• Monitoring a wide area (parking lot): a short focal length (2.8–4 mm) for broad coverage but less detail on distant objects.
• Identifying faces at a doorway: a longer focal length (6–12 mm) focusing on the entry point, ensuring facial features are large enough on the screen.

Many system designers use design software that simulates how a scene looks at different focal lengths and sensor resolutions, ensuring the final image meets the project’s requirements for coverage and detail.

CONCLUSION

Fiber optic technology plays a crucial role in modern video surveillance, especially for long-distance or interference-prone environments. SFP modules, patch panels, mechanical or fusion splices, and media converters help integrate fiber into IP camera systems.

When planning lenses for the cameras at each end of the fiber (or any other medium), carefully consider:

• Angle of View (focal length)
• Iris Type (manual, auto, P-iris)
• Sensor Format Compatibility (1/2.8 in, 1/3 in, M12 mount, etc.)
• Image Quality (aspherical elements, multi-coatings, aberration control)

Choosing the correct lens is just as vital as selecting the cabling and network infrastructure. Working with a U.S.-based integrator or supplier like safsale.com can help ensure you get the right components to achieve your surveillance goals, whether it’s wide-area coverage, specific identification points, or high-reliability fiber backhaul—across public, private, or industrial sectors in the United States.

RELATIVE APERTURE

Relative aperture is the ratio of the iris diaphragm’s diameter to the lens’s focal length. It is commonly expressed as 1:2.8 (pronounced “f/2.8” or “F2.8”).

On many (especially photographic) lenses, you will see a diaphragm scale with standard f-numbers: 2.8, 4, 5.6, 8, 11, 16, 22. Each adjacent marking on the scale increases or decreases light transmission by approximately 2× (one “stop”), which corresponds to changing the relative aperture by a factor of about 1.4. In practice, some older or specialized lens series might not follow this exact sequence at the lowest f-numbers.

F-NUMBER

The F-number (e.g., F/1.3) printed on a lens is simply the diaphragm number that specifies its largest possible aperture. A lens with F/1.3 transmits more light wide open than a lens with F/2.8, benefiting low-light performance but potentially reducing depth of field.

Key point:

• A higher F-number (e.g., F/2.8 vs. F/1.4) means a smaller maximum iris opening, hence letting in less light.
• The inverse of the F-number is the “relative aperture.”

LENS RESOLUTION

Lens resolution refers to how many fine details (in lines per millimeter, or lp/mm) the lens can render distinctly. Higher resolution lenses can better reproduce small features without blending them together. In practice, especially in CCTV, manufacturers typically group lenses into:

• Standard Lenses: Suitable for older or lower-resolution CCTV (up to ~960H analog).
• Megapixel Lenses: For IP or HD cameras with resolutions from about 1.3 MP to 5 MP (commonly labeled “1.3 MP,” “2 MP,” “3 MP,” or “5 MP” rated lenses).

Because many manufacturers do not provide detailed lp/mm (lines per millimeter) in their specs, a lens might simply be marketed as “2 MP” or “3 MP.” In an ideal world, we would see MTF (Modulation Transfer Function) curves, which show how the lens preserves contrast (modulation) across spatial frequencies.

MTF (Modulation Transfer Function)

MTF testing shows how the lens impacts contrast at various spatial frequencies. An MTF value near 1.0 (or 100 %) indicates near-perfect reproduction of the test pattern’s contrast. As frequency increases (finer details), lens aberrations typically reduce contrast. Once the MTF drops to about 10 % or even 30 %, the details in the image become severely diminished.

In professional optics:

• 70–100% MTF: Very good lens performance.
• 30% MTF: Acceptable/mediocre.
• Below 30%: Performance is considered poor.

Implication for Security: If a “2 MP” lens only achieves 30% MTF at a resolution equivalent to ~900 TV lines, it may not fully support a 2 MP (1920×1080) camera’s potential. The resulting image might look softer or less detailed than the camera sensor could otherwise produce.

LENS MOUNT TYPES

Common CCTV lens mounts:

• C-Mount
• CS-Mount

These differ by the flange focal distance:

• C-mount: 17.526 mm
• CS-mount: 12.5 mm

A CS-mount camera can often accommodate a C-mount lens if you add a 5 mm C–CS adapter ring (see illustration), but not vice versa. Many cameras also have an adjustable back-focus mechanism that can help fine-tune focus at the sensor plane.

Important: The lens format (1/3 in, 1/2.8 in, etc.) should be equal to or larger than the camera sensor format to avoid vignetting (dark edges).

ADHERING TO LENS/IMAGE SENSOR FORMAT

• If the camera sensor is larger than the lens coverage circle, you’ll get black edges or vignetting.
• If the lens is designed for a larger sensor than the camera’s, the lens effectively provides a narrower field of view (FOV) than its nominal specification on the smaller sensor.

For the FOV to match calculations, the lens format should match or exceed the sensor format.

LENS SETTINGS & ADJUSTMENTS

You can separate lens adjustments into two main categories:

• Settings for proper exposure given the lighting conditions (back-focus, iris controls such as “ALC” or “Level”).
• Settings for depth of field and focus (selecting aperture, focus distances, etc.).

1. Back-Focus Adjustment

Some cameras have a mechanical back-focus mechanism. Ensuring correct back-focus is crucial — especially for varifocal or zoom (transfocator) lenses. If the lens is incorrectly positioned relative to the sensor:

• The image might look acceptable in bright daytime (when the iris is partially closed, thus depth of field is large).
• In low-light (iris wide open, minimal depth of field), focus might degrade or be lost entirely.

Procedure for a prime lens:

• Open the lens iris fully (use a neutral density filter to avoid overexposure if necessary).
• Set the lens focus ring to “∞” (infinity).
• Place a focus target at the approximate “infinity” distance for that focal length. (e.g., for a 4 mm lens, your “infinity” test might be around 40 ft / 12 m away. Exact distances can be derived from lens charts.)
• Adjust the camera’s back-focus screw or ring so the target is sharply focused.
• Tighten or lock the back-focus once satisfied.

Varifocal or motorized zoom lenses require more iteration:

• Adjust at the longest focal length for a sharp image at “∞.”
• Switch to the widest focal length, check focus again.
• If focus has changed, readjust.
• Alternate until focus is consistent across the full zoom range.

Not all varifocal lenses allow perfect “parfocal” operation (holding focus through the entire zoom range). Some designs might sacrifice easy back-focus adjustments.

2. ALC (Automatic Level Control)

Some auto-iris lenses, especially video-drive types, feature manual knobs:

• ALC (Average vs. Peak): Typically labeled “A” (average) ↔ “P” (peak).
• Level: Overall brightness calibration.

When the scene is high-contrast (e.g., a bright background with a dark foreground subject), you can tweak “ALC” toward “P” to give priority to bright highlights, or “A” for overall average. Then use Level to shift the exposure. This helps mitigate backlight situations.

3. “Level” Adjustment

The Level knob sets the lens’s baseline exposure offset. Often factory preset, but you may fine-tune if:

• Turn Level to see if the on-screen brightness changes.
• If it’s too bright (overexposed), turn it down until the image is normal.
• Then rotate an additional small increment (~1/4 turn).

Now the lens/camera combination should keep the sensor near its ideal exposure in typical lighting fluctuations.

4. Selecting Depth of Field and Focus Range

Depth of field (DOF) is the range of distances within which objects appear sharp. It depends on:

• Aperture (iris setting).
• Focal length.
• Focus distance.

With wide-angle lenses (e.g., 2.8 mm–6 mm), DOF is generally large enough that focus issues rarely arise. For longer focal lengths (8 mm, 16 mm, 20+ mm), DOF becomes narrower, especially at night when the iris is wide open.

Using a lens with a manual iris or a camera that supports P-iris (automatically optimizing aperture) can help maximize sharpness throughout the DOF range. Software calculators or a “CCTV Designer” tool can show how DOF changes with distance, f-number, and sensor size.