How to Choose a Machine Vision Lens

Choosing a machine vision lens is different from choosing a CCTV lens, because the picture must do different work. A CCTV picture must be watched; a machine vision picture must support a decision, often a measurement, and it must do so reliably, every time, under the same conditions. This guide walks through the machine vision choice step by step, for buyers who are new to it and for buyers who want to check their process.

The guide states no model and no specification, because no model specification is published on this site. It ends with an inquiry checklist, because the final step in choosing a machine vision lens is always the same: describe the object, the decision and the conditions, and confirm the exact lens, its specifications, its terms and its documents in writing before payment.

Step one: name the decision the picture must support

The first step is to name the decision. A machine vision picture may support an inspection decision, whether a product passes or fails, a measurement decision, what size or position something is, a reading decision, what a mark or code says, a guidance decision, where a tool or robot should go, or a counting decision, how many items are present. Each decision asks a different question of the picture, and the lens must serve that question.

Naming the decision also means naming what counts as pass or fail, or what counts as a correct reading. A decision without a threshold is not yet a decision: a buyer who cannot say what passes and what fails cannot judge whether a lens gives a picture good enough to decide. The threshold should be stated in the inquiry, in plain terms, so that the supplier can judge the lens against it.

A buyer should also say how the decision is made: by a person looking at the picture, or by software analysing it. A picture for a person and a picture for software have different requirements. A person tolerates some distortion and unevenness; software reads geometry as data and may fail on a picture a person would accept. Naming the decision-maker is therefore part of naming the decision.

Step two: describe the object and the conditions

The second step is to describe the object: what is being looked at, how big it is, what it looks like, and how it is presented to the camera. A small, dark, shiny object asks different things of a lens from a large, bright, matte one, and an object that arrives in a fixed position asks different things from an object that arrives anywhere on a belt or table.

The conditions must be described alongside the object: the light, the speed, the working distance in general terms, the camera position, and the environment, including dust, vibration and temperature change. A machine vision setup is repeated many times, so the conditions that matter are the conditions across repetitions, not the conditions at a single demonstration. An inquiry that describes only the demonstration conditions will get a lens that works once and fails in production.

Consistency deserves its own paragraph in the description. How much variation is acceptable between repetitions? Must every picture be identical, or is some variation tolerable? A lens and lighting combination that gives a good picture once but a different picture next time has failed the machine vision test, however good the single picture looked, and the acceptable variation should be stated before the lens is chosen.

Step three: choose the family, then the view

The third step is to choose the lens family, then the view within it. The family follows the camera: an industrial camera points toward the C Mount industrial pages, a board camera points toward the M12 pages, and a general camera points toward the CCTV pages. Within the family, the view follows the object and the decision: a wide view for a whole area or product, a narrow view for a detail, an adjustable view where the object or feature varies, and a close view where the subject is small.

Distortion should be considered at this step, not afterwards. Where the decision involves shape, size or position, the low distortion category on this site should be read, and the distortion behaviour of the exact lens offered should be confirmed, with its measurement method. Where the decision involves only presence or colour, distortion may matter less, and the buyer should not pay for a property the job does not require.

The zoom question should also be settled at this step. Where the object or feature varies in size, or where one camera station must handle more than one task, a zoom lens may be justified. Where the object and feature never change, a fixed view may be simpler and more repeatable. The machine vision zoom page on this site covers the zoom case, and the fixed-versus-adjustable trade-off should be weighed honestly, not assumed in favour of flexibility.

Step four: confirm the exact lens before ordering

The fourth step is to confirm the exact lens, if one is offered, together with its specifications, in writing, against the object, decision and conditions described in the inquiry. The confirmation should cover the lens, the camera it fits, the working arrangement, and the repeatability of the picture across repetitions. A general assurance that a lens suits machine vision is not a confirmation.

Confirm the commercial terms at the same time: the minimum order quantity, the lead time and the price for the quantity required, and what the price includes and excludes. Confirm which documents are supplied with the order. On this site the certificate position is stated plainly: Certificates available upon request. No certificate name, number or issuing body is stated here.

Confirm compatibility with the camera and mounting arrangement, in writing, before payment, and confirm how the lens's performance will be checked on delivery, if it can be checked at all. A machine vision lens whose performance cannot be checked on delivery should be ordered on terms that describe its performance in checkable words, or the buyer should accept explicitly that performance rests on the supplier's statement alone. Either way, the basis should be explicit before payment.

Frequently asked questions

What is the biggest difference from choosing a CCTV lens?

The picture must support a decision reliably, every time, not merely be watched. Repeatability matters as much as sharpness.

Do I need to know the object size precisely?

You need to describe the object, its size in general terms, and what the picture must show of it. Precise figures belong to the model specification, confirmed for the exact lens offered.

Can I choose from this site alone?

This site explains the choice and the categories, but it publishes no models or specifications. The final choice must be confirmed with the supplier.

Related pages

• Machine Vision Lenses
• Machine Vision Zoom Lenses
• Low Distortion Lenses
• Machine Vision Camera Lens Guide