What Is a Varifocal Lens?

A varifocal lens is a lens whose focal length can be adjusted, so its field of view can be changed after the lens is fitted. A lens with a shorter focal length gives a wider view; a lens with a longer focal length gives a narrower view. A varifocal lens lets the user move between those views, rather than being fixed at one of them.

This post explains the varifocal idea in plain language. It states no focal length figure, no model and no specification, because figures belong to a model specification, and no model specification is published on this site. Its purpose is to give a buyer the understanding to decide whether adjustment is needed, and what to ask if it is.

What adjustment means in practice

In practice, adjustment means the installer fits the camera, looks at the picture, and then changes the view until the scene is covered the way the buyer wants. That is a different experience from fitting a fixed lens, where the view must be guessed in advance and can only be changed by refitting a different lens.

The value of that experience depends on how well the view can be known in advance. Where the camera position is fixed, the scene is known, and the view will not change, adjustment adds little, and a fixed lens may be simpler. Where the position is uncertain, the scene is unclear, or one camera model must serve several positions, adjustment is genuinely useful, and the installer will be glad of it.

Adjustment can be made by hand at the camera, or by motor from elsewhere, and the difference matters. Hand adjustment suits a view that is set once at installation. Motorized adjustment suits a view that must change during use, or a camera that is hard to reach. The motorized varifocal page on this site covers the motorized case separately, and buyers who need adjustment during use should read it.

Varifocal, zoom and fixed: clearing up the words

Three words cause confusion in this area: varifocal, zoom and fixed. Varifocal names the adjustable category: a lens whose focal length can be adjusted. Fixed names the opposite: a lens whose focal length cannot be adjusted. Zoom names the application of changing the view, and it overlaps with varifocal in everyday use, since a zoom lens changes its view in the same way.

On this site, the distinctions are kept practical rather than pedantic. Varifocal pages describe the adjustable category in the company's listed range. Zoom pages describe the application of changing the view, for CCTV cameras and for machine vision cameras. Fixed appears only as a comparison concept in the varifocal versus fixed guide, not as an offered category, and buyers should not mistake the comparison for an offering.

A buyer does not need to master these words to buy well. What matters is describing the need: must the view be set once, must it change during use, or is it fixed and known? Those three needs map to hand-set varifocal, motorized varifocal and fixed respectively, and an inquiry that states the need will get a better answer than an inquiry that argues about terminology.

When a varifocal lens is the right choice

A varifocal lens is the right choice where the view is uncertain at ordering time, where one camera model must serve several positions, where the scene may change after installation, or where the installer needs to fine-tune the view on site. In those cases the flexibility of adjustment is not a luxury but the practical answer to a real uncertainty.

A varifocal lens is not the right choice where the view is known, fixed and unchanging, and where simplicity and repeatability matter most. In those cases a fixed view is simpler, there is nothing to drift out of adjustment, and the buyer should not pay for flexibility the job does not need. The varifocal versus fixed guide on this site explains that trade-off in full.

A varifocal lens is also not a substitute for planning. Adjustment can fix a view that is slightly wrong, but it cannot fix a camera in the wrong position, a scene that was never described, or a picture question that was never asked. The camera position and the scene should still be planned first, and adjustment should be used to fine-tune a planned view, not to rescue an unplanned one.

What to confirm before ordering

Confirm how adjustment is made for the exact lens concerned, whether by hand or by motor, and confirm that the adjustment method suits the buyer's camera system and job. Confirm the model, if one is offered, and its specifications, in writing, for the exact lens concerned, together with the minimum order quantity, lead time and price for the quantity required.

Confirm compatibility with the buyer's camera and mounting arrangement, in writing, before payment. Confirm which documents are supplied with the order. On certificates, the position on this site is: Certificates available upon request. No certificate name, number or issuing body is stated here, and none should be assumed from any other statement on the site.

Finally, confirm how the adjusted view is held in place, so that it does not drift after installation. A view that is set correctly and then drifts is worse than a view that was never quite right, because it fails silently, and the failure may only be noticed when the picture is needed. Ask how the setting is secured, and what keeps it secure over time.

Setting the view and keeping it set

Setting a varifocal view is only half the job; keeping it set is the other half. A view that is set correctly at installation and then drifts, because the adjustment was knocked, because vibration moved it, or because the setting was never secured, will fail silently, and the failure may only be noticed when the picture is needed and the scene is no longer covered. A buyer should therefore ask how the setting is secured for the exact lens offered, and what keeps it secure over time.

The setting should also be recorded. A note of the view chosen for each camera, made at installation, lets a later check confirm that the view is still the view intended, rather than relying on memory. That record costs little and prevents a common failure: a camera whose view changed unnoticed, and whose pictures no longer show what they were installed to show. Adjustment is a tool for setting a view correctly, and like any tool, it needs a record of how it was used.

Frequently asked questions

Is varifocal the same as zoom?

The two overlap. Varifocal names the adjustable category; zoom names the application of changing the view. Describe the need and ask which term applies to the lens offered.

Does a varifocal lens need adjustment after installation?

Only if the view must change during use, in which case motorized adjustment may be needed. Hand-set varifocal lenses are adjusted at installation and then left.

Can adjustment fix a camera in the wrong position?

No. Adjustment fine-tunes a planned view; it cannot rescue an unplanned position or an undescribed scene.

Related pages

• Varifocal Lenses
• Varifocal vs Fixed Lens Guide
• Motorized Varifocal Lenses
• Varifocal CCTV Cameras