Lens extension tube or close up ring increases magnification

Summary at a glance:

Need a close-up image your preferred sensor and lens can’t quite deliver? A glass-free extension tube or close up ring can change the optics to your advantage.

C-mount extension tube kit – Courtesy Edmund Optics

What’s an extension tube?

An extension tube is a metal tube one positions between the lens and the camera mount. It comes with the appropriate threads for both the lens and camera mount, so mechanically it’s an easy drop-in procedure.

By moving the lens away from the optical plane, the magnification is increased. Sounds like magic! Well almost. A little optical calculation is required – or use of formulas or tables prepared by others. It’s not the case than any tube of any length will surely yield success – one needs to understand the optics or bring in an expert who does.

S-mount extension tube kit – Courtesy Edmund Optics

Note: One can also just purchase a specific length extension tube. We’ve shown images of kits to make it clear there are lots of possibilities. And some may want to own a kit in order to experiment.

Example

Sometimes an off-the-shelf lens matched to the sensor and camera you prefer suits your optical needs as well as your available space requirements. By available space we mean clearance from moving parts, or ability to embed inside an attractively sized housing. Lucky you.

But you might need more magnification than one lens offers, yet not as much as the next lens in the series. Or you want to move the camera and lens assembly closer to the target. Or both. Read on to see how extension rings at varying step sizes can achieve this.

Navigating the specifications

Once clear on the concept, it’s often possible to read the datasheets and accompanying documentation, to determine what size extension tube will deliver what results. Consider, for example, Moritex machine vision lenses. Drilling in on an arbitrary lens family, look at Moritex ML-U-SR Series 1.1″ Format Lenses, then, randomly, the ML-U1217SR-18C.

ML-U1217SR-18C 12mm lens optimized for 3.45um pixels and 12MP sensors – Courtesy Moritex

If you’ve clicked onto the page last linked above, you should see a PDF icon labeled “Close up ring list“. It’s a rather large table showing which extension tube lengths may be used with which members of the ML-U-SR lens series, to achieve what optical changes in the Field-Of-View (FOV). Here’s a small segment cropped from that table:

Field-Of-View changes with extension tubes of differing lengths – Courtesy Moritex

Compelling figures from the chart above:

Consider the f12mm lens in the rightmost column, and we’ll call out some highlights.

Extension tube length (mm)WD (far)Magnification
01000.111x
258.20.164
513.50.414
5mm tube yields 86% closer WD and 4x magnification!

Drum roll here…

Let’s expand on that table caption above for emphasis. For this particular 12mm lens, by using a 5mm extension tube, we can move the camera 86% closer to the target than by using just the unaugmented lens. And we quadruple the magnification from 0.111x to 0.414x. If you are constrained to a tight space, whether for a one-off system, or while building systems you’ll resell at scale, those can be game-changing factors.

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Any downside?

As is often the case with engineering and physics, there are tradeoffs one should be aware of. In particular:

  • The light reaching the focal plane is reduced, per the inverse square law – if you have sufficient light this may not have any negative consequences for you at all. But if pushed to the limit resolution can be impacted by diffraction.
  • Reduced depth of field – does the Z dimension have a lot of variance for your application? Is your application working with the center segment of the image or does it also look at the edge regions where field curvature and spherical aberrations may appear?

We do this

Our team are machine vision veterans, with backgrounds in optics, hardware, lighting, software, and systems integration. We take pride in helping our customers find the right solution – and they come back to us for project after project. You don’t have to get a graduate degree in optics – we’ve done that for you.

Give a brief idea of your application and we’ll provide options.

Related resources

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