Surveillance Camera Focal Length

Focal length is the lens property that decides how wide or narrow a camera's view is, and surveillance cameras are no exception. A lens with a shorter focal length gives a wider view, covering more of an area with less detail per part of it. A lens with a longer focal length gives a narrower view, covering less of an area with more detail for the part it covers. This post explains that idea for surveillance cameras specifically, where the view must serve observation over time and across an area.

The post states no focal length figure for any lens, because figures belong to a model specification, and no model specification is published on this site. Its purpose is to give surveillance buyers the vocabulary to describe the view they need, so that an inquiry can be answered properly, and so that a surveillance installation can be planned camera by camera rather than guessed as a whole.

Focal length and the surveillance view

A surveillance view is a view of an area over time, and focal length shapes that view in two ways at once: how much area one camera covers, and how much detail it shows within that area. Those two effects pull in opposite directions, and every surveillance lens choice is a balance between them. A wider view covers more area but shows less detail; a narrower view shows more detail but covers less area.

The balance depends on what the surveillance picture must do. A picture that must show general movement across a yard needs a wide view, because the event may happen anywhere in the yard. A picture that must show a recognisable view of a person or vehicle at a gate needs a narrower view, because the detail at that point is the point of the camera. A surveillance installation usually needs both kinds of view, from different cameras, and the focal length choice should be made camera by camera.

The balance also depends on the area itself. The same width of view covers a small room completely and a large yard only partly, so the area and the camera position must be described alongside any discussion of focal length. A buyer who discusses focal length without the area is discussing an abstraction, not a view, and the resulting lens choice may not fit the real installation.

Planning surveillance views by focal length

Planning a surveillance installation by focal length means walking the area, real or on a plan, and deciding, for each camera, what it must show. For each camera, ask three questions: what area or point must it cover, what must the picture show of that area or point, and what may it ignore? The answers to those questions describe the view needed, and the focal length follows from the view, not the other way round.

Neighbouring cameras should be checked against each other, because surveillance coverage without gaps is the goal. A camera whose view leaves a gap, or whose view overlaps wastefully with its neighbour, has been planned alone rather than as part of an installation. The focal length of each camera should be chosen so that the cameras together cover the area without gaps and without needless overlap.

Light, weather and seasons should be considered alongside focal length, because a view that works in daylight may fail at night, and a view that covers a yard in one season may be partly blocked in another. Those conditions do not change focal length, but they change whether the view focal length gives is actually useful, and they should be described in the inquiry so that the lens offered is judged against the real scene.

Adjustable focal length in surveillance

Where the right view is not known until the camera is up, an adjustable lens, such as a varifocal or zoom lens, lets the view be set after installation. That flexibility suits surveillance installations where the area is hard to survey in advance, where one camera model must serve several positions, or where the point of interest may move. The varifocal and zoom pages on this site cover those cases, and they should be read alongside this post by buyers whose views are uncertain.

Where the view is known and fixed, an adjustable lens may still be useful at installation, to fine-tune a planned view, but it should not be assumed to be necessary. A fixed view, set correctly at installation, is simpler and cannot drift, and for a surveillance camera that must give the same view every day, that stability is itself a virtue. The varifocal versus fixed guide explains the trade-off, and it applies to surveillance cameras as much as to any other camera.

Where the view must change during use, such as a camera that watches an area in general and examines points within it at different times, a motorized adjustable lens may be needed. That case should be described explicitly in the inquiry, since it changes the lens, the camera system and the terms, and a lens offered without that description may not support the change of view the job requires.

What to confirm before ordering

Confirm the lens category and, where one is offered, the model that fits the buyer's surveillance camera and the area it must cover. Confirm the specifications, including the focal length behaviour of the exact lens offered, in writing, together with the minimum order quantity, lead time and price for the quantity required, and confirm compatibility with the camera and mounting arrangement.

Confirm which documents are supplied with the order. On this site the certificate position is: Certificates available upon request. No certificate name, number or issuing body is stated here. Confirm the view for each camera position separately, rather than asking for one lens type for a whole installation, since different positions need different views, and a single lens type will serve some positions badly however well it serves others.

Frequently asked questions

Does a longer focal length mean a better surveillance camera?

No. It means a narrower view with more detail for the part covered. Better depends on what each camera's picture must show.

Can I plan focal length without visiting the area?

You can plan from a plan, but the camera position and the area must be known. A view discussed without the area is an abstraction, not a plan.

Should every surveillance camera have an adjustable lens?

No. Use adjustment where the view is uncertain or must change. Fixed views suit fixed, known jobs, and they cannot drift.

Related pages

• Surveillance Camera Lenses
• CCTV Camera Focal Length Guide
• Surveillance Lens Guide
• Varifocal Lenses