CCTV lens and focal length: how to choose the right view for every camera

Choosing the wrong lens is one of the most common reasons a CCTV system disappoints. The camera works, the picture is sharp, and yet when the customer needs to identify someone, the person is a few pixels tall at the edge of a wide shot. Nothing is broken. The camera was simply never able to do the job.

Focal length decides how wide or narrow a camera sees, and therefore how much detail it captures at a given distance. Getting it right at the survey stage costs nothing. Fixing it later means a return visit, a new camera and an awkward conversation.

This guide explains the basics in plain terms and gives you practical rules for choosing a lens on site.

What focal length means

Focal length is measured in millimetres and printed on the camera's specification sheet, for example 2.8 mm, 4 mm, 6 mm or a varifocal range such as 2.8–12 mm.

• A short focal length (2.8 mm or less) gives a wide view. You see more of the scene, but each object is smaller.
• A long focal length (8 mm, 12 mm or more) gives a narrow view. You see less of the scene, but distant objects appear larger.

The exact field of view also depends on the size of the image sensor. Two cameras with a 4 mm lens can show slightly different angles if their sensors differ. Always check the horizontal field of view in degrees on the data sheet rather than relying on the millimetre figure alone.

Field of view in practice

As a rough guide, on common 1/2.8" or 1/2.7" sensors:

These are approximations. Manufacturers vary, so use the actual figures for the model you plan to install.

Why resolution alone is not enough

Customers often believe that more megapixels solve everything. A higher-resolution camera does capture more detail across the whole image, but a wide lens spreads that detail over a large area. An 8 MP camera with a 2.8 mm lens may give less detail on a face at 10 metres than a 4 MP camera with an 8 mm lens.

The useful measure is pixel density: how many pixels cover each metre of the scene at the distance you care about. It depends on three things together:

1. The camera's horizontal resolution in pixels.
2. The width of the scene at the target distance, which depends on the lens.
3. The distance itself.

Pixel density targets

Industry guidance commonly uses targets similar to these, measured in pixels per metre at the target:

These numbers come from long-standing operational requirement methods used by security professionals. Treat them as starting points, not guarantees, because lighting, motion and compression all affect the result.

Working it out on site

You do not need complicated software for most jobs. A simple calculation is enough.

1. Find the horizontal field of view angle for the lens.
2. Work out the scene width at the target distance: width ≈ 2 × distance × tan(angle ÷ 2).
3. Divide the camera's horizontal pixels by that width.

For example, a 4 MP camera is 2,560 pixels wide. With a 90° lens, at 5 metres the scene is about 10 metres wide, giving roughly 256 pixels per metre. That is enough to identify someone at 5 metres. At 10 metres, the scene is 20 metres wide, and density falls to about 128 pixels per metre. That is only enough to recognise them.

Many manufacturers also offer free lens calculators or mobile apps. They are useful at the survey, especially when the customer is watching and wants to see why you suggest a particular camera.

Fixed lenses or varifocal?

Fixed lenses are cheaper, smaller and have fewer parts to fail. They suit most indoor positions and many outdoor positions where the distance is short and predictable.

Varifocal lenses let you adjust the focal length during installation. Motorised versions can be adjusted remotely through the recorder or a web browser. They cost more, but they solve a lot of problems:

• You can fine-tune the view on site without carrying several cameras.
• You can tighten the view later if the customer's needs change.
• You can correct a slightly wrong estimate from the survey.

For gates, driveways, car parks and perimeter positions, a motorised varifocal camera is often worth the extra cost. For indoor overview cameras, a fixed lens is usually fine.

Choosing a lens for common positions

Front door or reception

The aim is usually to identify people as they enter. Place the camera so that faces are captured at a short, predictable distance, typically 2 to 4 metres. A 2.8 mm or 4 mm lens often works well, mounted at a modest height so the angle is not too steep.

Driveways and gates

Aim for a narrower view focused on the point everyone passes through, such as the gate itself. A 6 mm or varifocal lens is common. Add a second wide camera if the customer also wants an overview of the area.

Shop floors

Overview cameras with 2.8 mm or 4 mm lenses cover aisles and displays. Add tighter views on the till and the entrance where identification matters. Our guide to CCTV for retail shops covers layouts in more detail.

Corridors

A corridor is long and narrow, so a wide lens wastes most of the picture on walls. Use a longer lens, or consider corridor mode, where the camera image is rotated to portrait. Many IP cameras support this.

Car parks and perimeters

Distances are long, and lighting is often poor. Use longer lenses, check infrared range, and avoid trying to make one camera do everything. A wide camera for detection and a narrow one for identification at the entry point usually works better.

Number plates

Reading plates reliably is a specialist task. Plates are small, reflective and often moving. You usually need a narrow lens, a fast shutter, the right angle and dedicated settings. Many manufacturers sell specific plate-reading cameras. Set customer expectations carefully.

Lens and lighting

A longer lens collects less light, which can make the picture darker at night, especially on cheaper cameras. Check the aperture value (the f-number) on the data sheet. A lower number, such as f/1.0 or f/1.6, means the lens lets in more light.

Infrared illumination must also match the view. A camera with a 12 mm lens looking 30 metres away needs infrared that actually reaches 30 metres. Otherwise the night image will show a bright foreground and a black target area. Our guide to CCTV night vision explains this in more depth.

Common mistakes

• Using 2.8 mm cameras everywhere. It is the default lens on many kits, but it is wrong for long views.
• Promising identification at any distance. Be specific about what each camera can and cannot do.
• Ignoring mounting height. A camera mounted very high looks down on the tops of heads, regardless of the lens.
• Forgetting backlight. A camera facing a bright doorway may show only silhouettes. Choose a camera with good wide dynamic range, or change the position.
• Not checking at night. A view that looks perfect in daylight may be useless after dark.

Record your lens choices

Write down the lens, target distance and purpose for each camera in your survey notes. It helps the technician who installs the system, and it gives you a clear record if the customer later complains about image quality. A short line such as "Cam 3, gate, 6 mm, identify at 6 m" is enough.

If you use CamVerion, these notes can sit on the site visit and flow into the quote, so the installer sees exactly what was promised.

Explaining lens choices to customers

Customers do not need the maths, but they appreciate a simple explanation. Try showing two sample images, one wide and one narrow, from the same position. Point out that the wide image covers more but the narrow one shows a usable face. Most customers then understand why you suggest a mix of cameras.

Make the purpose of each camera clear in your quote. "Camera 4 is set to identify people at the gate" is easier to understand than "4 MP bullet, 6 mm". Our site survey checklist includes prompts for this.

A quick lens checklist

1. Agree the purpose of each camera: detect, observe, recognise or identify.
2. Measure the distance to the target area.
3. Choose a focal length that gives the right pixel density at that distance.
4. Check the field of view in degrees on the actual data sheet.
5. Check the infrared range and low-light performance.
6. Consider varifocal for long or uncertain distances.
7. Record the lens, distance and purpose for each camera.
8. Test the view in daylight and at night before handover.

A camera is only as good as the view it is given. Spend a few extra minutes on lens choice at the survey, and you will save return visits, protect your reputation and give customers footage they can actually use.