Focal Length for Surveillance Cameras-A Selection Guide

As the security industry continues to evolve, the variety of surveillance camera models and features has expanded significantly. Cameras now come in a wide array of shapes, functionalities, and image qualities, making the selection process increasingly complex. This abundance of options often leaves many people feeling overwhelmed when trying to choose the right surveillance camera. One of the most common dilemmas faced during this decision-making process is how to select the appropriate focal length for a surveillance camera. In this article, we will delve into the intricacies of choosing the right focal length for your surveillance needs, providing you with the insights necessary to make an informed decision.

Quick guide

1. What is Focal Length?

2. How Do Different Focal Lengths Affect Surveillance?

3. How to Choose the Focal Length of a Surveillance Camera?

4. Important Reminder

5. Final Thoughts

Conclusion

1. What is Focal Length?

Focal length, in the context of optical systems, refers to the distance from the optical center of a lens to the point where parallel light rays converge to a focus. In surveillance cameras, it specifically denotes the distance between the optical center of the lens and the camera’s CCD (Charge-Coupled Device) or CMOS (Complementary Metal-Oxide-Semiconductor) image sensor. The focal length is a critical parameter that influences several key aspects of the captured image, including the size of the image, the field of view (FOV), and the depth of field (DOF).

Field of View (FOV)

The field of view is defined as the angular extent of the observable scene that can be captured through the lens. It is measured as the angle between the two extreme edges of the viewable area. A shorter focal length lens provides a wider field of view, allowing the camera to cover a larger area, whereas a longer focal length lens narrows the field of view, focusing on a smaller, more specific area.

Depth of Field (DOF)

Depth of field refers to the range of distance within which objects appear acceptably sharp and in focus in the captured image. In simpler terms, it is the distance range in which the camera can maintain a clear image of the subject. The depth of field is influenced by several factors, including the focal length of the lens, the aperture size (f-stop), and the distance to the subject. While focal length is a significant factor, it is important to note that the depth of field is also affected by the aperture setting and the distance between the camera and the subject.

Understanding these concepts is crucial for selecting the right focal length for your surveillance camera, as it directly impacts the camera’s ability to capture the desired scene effectively. Whether you need to monitor a broad area or focus on specific details, the focal length will play a pivotal role in achieving your surveillance objectives.

concept of depth of field

2. How Do Different Focal Lengths Affect Surveillance?

In the current market, most mainstream surveillance cameras are network cameras with typical focal lengths of 2.8mm, 3.6mm/4mm, 6mm, 8mm, 12mm, and 16mm. Some cameras also feature zoom capabilities, allowing for adjustable FLs. But what exactly are the differences between these various FLs?

In simple terms:

  • The larger the focal length, the farther the camera can see, but the narrower the field of view.
  • The smaller the focal length, the closer the camera can see, but the wider the field of view.

To illustrate these points, let’s refer to a comparison chart of focal length effects available online:

Comparison chart of focal length effects available online

Observations from the Comparison Chart:

  • At the same distance (e.g., the first column): As the focal length increases, the field of view becomes narrower, but the details within that view become clearer.
  • At the same focal length (e.g., the first row): As the distance increases, the field of view becomes wider, but the ability to identify specific details decreases.
  • For facial or human shape recognition: The farther the distance, the larger the focal length required to maintain clarity (diagonal observation).

Detailed Analysis:

  • Short Focal Lengths (e.g., 2.8mm, 3.6mm/4mm):
    • Field of View: Wide
    • Best Use: Ideal for monitoring broad areas such as parking lots, large rooms, or open spaces where capturing a wide scene is essential.
    • Detail Level: Lower at longer distances, making it less effective for identifying specific details such as faces or license plates from afar.
  • Medium Focal Lengths (e.g., 6mm, 8mm):
    • Field of View: Moderate
    • Best Use: Suitable for medium-range surveillance, such as hallways, entrances, or smaller outdoor areas where a balance between field of view and detail is needed.
    • Detail Level: Moderate, providing a good compromise between area coverage and the ability to discern details.
  • Long Focal Lengths (e.g., 12mm, 16mm):
    • Field of View: Narrow
    • Best Use: Perfect for long-range surveillance, such as monitoring specific points of interest like gates, driveways, or distant perimeters where detailed observation is crucial.
    • Detail Level: High, allowing for clear identification of specific features at a distance.

Practical Implications:

When selecting a surveillance camera, consider the specific requirements of the area you need to monitor. For instance, if you need to cover a wide area but don’t require high detail, a camera with a shorter focal length will be more suitable. Conversely, if you need to focus on a specific area and capture fine details, a camera with a longer lens will be more appropriate.

By understanding these focal length effects, you can make a more informed decision that aligns with your surveillance needs, ensuring optimal coverage and detail for your security setup.

3. How to Choose the Focal Length of a Surveillance Camera?

Previously, we discussed the effects of different FLs. So, how should we choose the FL of a surveillance camera in practical applications? We can consider the following dimensions:

1. Surveillance Range

The focal length of a surveillance camera determines the size of the surveillance image and the effective visible distance, which is the surveillance range.

  • Small Range Surveillance:
    For situations with a small surveillance range and a wide field of view, such as a living room or a small shop, you can choose a camera with a smaller focal length. For example, a camera with a 3.6mm or 4mm focal length can have a field of view of 75°-90°, an effective surveillance distance of over 5 meters, and can cover an area of 40㎡-80㎡.
  • Large Range Surveillance:
    For larger surveillance areas, such as spaces over 10m×10m, you need to choose a camera with a larger focal length. Due to this, the surveillance angle will be smaller. To achieve complete coverage, consider using a PTZ (Pan-Tilt-Zoom) camera or installing multiple cameras.
  • Fixed Long-Distance Surveillance:
    For fixed surveillance areas that are far away, such as a parking spot 30 meters away, you need to choose a camera with an even larger focal length, such as 12mm or more.

2. Usage Scenarios

Often, we choose a surveillance camera based on specific scenario requirements. If you don’t want to delve into complex focal length parameters, you can roughly judge the choice of camera focal length based on the following usage scenarios:

  • Small Spaces (e.g., garage, storage room):
    It is recommended to choose a 2.8mm focal length, with an optimal surveillance distance within 3 meters.
  • Indoor Home or Small Shops:
    It is recommended to choose a 4mm focal length, with an optimal surveillance distance of 3-5 meters.
  • Home Courtyard, Balcony, Entrance:
    It is recommended to choose a 6mm focal length, with an optimal surveillance distance of 5-10 meters.
  • Outdoor Roads, Alleys:
    It is recommended to choose an 8mm focal length, with an optimal surveillance distance of 10-20 meters.
  • Fixed Long-Distance Surveillance:
    It is recommended to choose a 12mm focal length, with an optimal surveillance distance of 20-30 meters.
  • Varifocal Lens:
    Some camera lenses are varifocal and can adjust the focal length based on the surveillance scene range.

3. Practical Needs

The previous methods of choosing the focal length of a surveillance camera share a common premise: being able to clearly see people or objects. However, in practice, our focus might differ, such as:

  • Wide Range Surveillance:
    Sometimes we may not care about clearly seeing people or objects; we just need a wide surveillance range. In this case, you can choose a camera with a smaller focal length, such as 2.8mm. There are also fisheye cameras with a surveillance angle that can reach up to 180°, covering everything in front of the lens.
  • Long-Distance Small Area Surveillance:
    Sometimes we might focus more on a small fixed area at a long distance, such as a parking spot 50 meters away. In this case, you can choose a camera with a focal length of 16mm or more, focusing on a small area rather than the overall range.
  • Narrow Long Area Surveillance:
    Sometimes we need to monitor a narrow long area, such as a 5m×30m range. A camera with a small focal length might not see clearly at a distance, while a camera with a large focal length might not see nearby. We can consider cameras with dual or even triple varifocal lenses, which can monitor both near and far areas simultaneously.

By considering these dimensions, you can choose the most suitable surveillance camera focal length based on your practical needs, ensuring the best surveillance effect.

4. Important Reminder

The article includes some charts and mentions many specific data points. However, it’s important to note the following:

Different manufacturers may use different image sensors and lens calculation methods. As a result, the field of view and visible distance parameters can vary slightly between cameras with the same focal length from different manufacturers. Always refer to the actual specifications of the equipment.

5. Final Thoughts

While understanding focal length is crucial, choosing a surveillance camera also involves other factors to find the most suitable model for your needs.

1. Resolution

The resolution of a camera determines the clarity of the captured images. Higher resolution cameras provide more detailed images, which can be crucial for identifying faces or license plates. Common resolutions include 720p, 1080p, 4MP, and 4K.

2. Night Vision

Night vision capabilities are essential for 24/7 surveillance, especially in low-light conditions. Cameras with infrared LEDs or advanced low-light sensors can provide clear images even in complete darkness. Look for specifications like IR range and low-light performance.

3. Power Supply and Network Connectivity

Consider how the camera will be powered and connected to your network. Options include:

  • Wired Power and Ethernet (PoE): Provides reliable power and data transmission over a single cable.
  • Wireless: Offers flexible installation but requires a stable Wi-Fi connection and a power source.
  • Battery-Powered: Ideal for locations without easy access to power outlets but may require frequent battery changes or recharging.

4. Smart Features

Modern surveillance cameras often come with smart features that enhance functionality:

  • Motion Detection: Alerts you when movement is detected in the camera’s field of view.
  • Facial Recognition: Identifies known individuals and alerts you to unknown faces.
  • Two-Way Audio: Allows you to communicate with people near the camera.
  • Cloud Storage: Stores footage in the cloud for easy access and backup.

5. Environmental Considerations

Ensure the camera is suitable for the environment where it will be installed. Look for weatherproof and vandal-proof ratings if the camera will be used outdoors. Specifications like IP66 or IP67 indicate good weather resistance.

6. Installation and Maintenance

Consider the ease of installation and maintenance. Some cameras come with user-friendly installation guides and mounting kits. Regular maintenance, such as cleaning the lens and updating firmware, is also important for optimal performance.

Our product usage scenario diagram

Conclusion

Choosing the right surveillance camera involves careful consideration of various factors, with the focal length being critical. It directly impacts the camera’s field of view and effective surveillance distance. For small spaces, a shorter focal length provides a wide field of view, while larger areas or long-distance surveillance require longer focal lengths.

However, the focal length is just one piece of the puzzle. It’s crucial to also consider other important features such as resolution, night vision capabilities, power supply and network connectivity, smart features, and environmental suitability. Each of these elements plays a significant role in ensuring that the camera meets your specific surveillance needs and provides reliable and clear footage.

Different manufacturers may have slight variations in their lens and sensor technologies, so always refer to the actual specifications of the equipment you are considering. By taking a holistic approach and evaluating all these factors, you can make an informed decision and select the most appropriate surveillance camera for your requirements, ensuring comprehensive and effective security coverage.


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