CCTV Lens Calculator: Focal Length from Width, Distance or DORI

How to use the CCTV lens calculator

Choose what you know about the scene: Cover a width when you know how wide the view must be, Reach a DORI level when faces or plates must be readable, or Angle to focal length when a datasheet or a plan gives you the field-of-view angle. Then pick the camera's sensor format and resolution and describe the scene.

The big number is the focal length the scene needs. Lenses come in standard steps, so the cards below it show the fixed lens and the varifocal range to buy, and the stats show what that real lens covers at your distance: the angles, the width and height, the pixel density and the DORI class. Copy the link to share the exact scene.

Choosing a focal length for a distance

Say a camera must cover a 10 m wide scene at 20 m with a 4MP 1/2.8" sensor. The focal length is the sensor width times the distance, divided by the scene width: f = sensor width × distance ÷ scene width. Here that is 10.4 mm.

No lens is made at exactly 10.4 mm, so the calculator picks the longest standard fixed lens at or below it, which never shows less than you asked for: the 8 mm lens covers 12.9 m at 20 m, at 208 px/m. A 2.8–12 mm varifocal set to 10.4 mm covers the 10 m exactly. These are the numbers the calculator shows when it opens.

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Fixed or varifocal lens?

Fixed lenses come in standard steps; the calculator uses 2, 2.8, 3.6, 4, 6, 8, 12, 16, 25 mm. To cover a width it picks the longest fixed lens at or below the focal length you need, so the view is never narrower than the scene. To reach a DORI level it picks the shortest fixed lens at or above it, so the pixel density never drops below the level.

A varifocal lens zooms over a range, here 2.8–12, 2.7–13.5, 5–50 mm, and is set on site to the exact focal length. It costs more, but it suits a scene you have not measured yet or a focal length that falls between two fixed lenses. Use the lens type option to show only fixed or only varifocal lenses.

Lenses for face identification and licence plates

DORI levels set how many pixels land on each metre of the scene. To identify a person you need the Identify level, 250 px/m; Recognise needs 125 px/m. At 10 m a 4MP 1/2.8" camera needs 4.8 mm for Identify, so the 6 mm fixed lens, which gives 311 px/m. A sharper 8MP camera reaches the same level with 3.4 mm, so the 3.6 mm lens.

Reading a number plate by eye starts from the same Identify level. Dedicated ANPR and LPR cameras set their own pixel and shutter requirements, so check the camera maker's guidance before you choose a lens for plate recognition.

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Converting a FOV angle to a focal length

If a datasheet or a design gives you the horizontal field-of-view angle, Angle mode turns it into a focal length: the sensor width divided by twice the tangent of half the angle. On a 1/2.8" sensor, a 60° view needs 4.5 mm.

The calculator then shows the nearest standard lens on each side: the 4 mm lens gives 65.8°, a little wider than asked, and the 6 mm lens gives 46.7°, a little narrower. Above 110° the image bends noticeably, and the calculator warns you.

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Lens calculator FAQ