Category: Lenses

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.

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.

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.

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:

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
0	100	0.111x
2	58.2	0.164
5	13.5	0.414

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.

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.

Related resources

You might also be interested in one or more of the following:

• Depth of Field (DoF)
• Guide to machine vision lens selection

1st Vision’s sales engineers have over 100 years of combined experience to assist in your camera and components selection.  With a large portfolio of cameras, lenses, cables, NIC cards and industrial computers, we can provide a full vision solution!

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Most who are involved with imaging have at least some understanding of depth of field (DoF). DoF is the distance between the nearest and furthest points that are acceptably in focus. In portrait photography, one sometimes seeks a narrow depth of field to draw attention to the subject, while intentionally blurring the background to a “soft focus”. But in machine vision, it’s often preferred to maximize depth of field – that way if successive targets vary in their Z dimension – or if the camera is on a moving vehicle – the imaging system can keep processing without errors or waste.

Making it real

Suppose you need to see small features on an item that has various heights (Z dimension). You may estimate you need a 1″ depth of field. You know you’ve got plenty of light. So you set the lens to f11 because the datasheet shows you’ll reach the depth of field desired. But you can’t resolve the details! What’s up?

So I should maximize DoF, right?

Well generally speaking, yes – to a point. The point where diffraction limits negatively impact resolution. If you read on, we aim to provide a practical overview of some important concepts and a rule of thumb to guide you through this complex topic without much math.

Aperture, F/#, and Depth of Field

Aperture size and F/# are inversely correlated. So a low f/# corresponds to a large aperture, and a high f/# signifies a small aperture. See our blog on F-Numbers aka F-Stops on the way the F-numbers are calculated, and some practical guidance.

Per the illustration below, a large aperture restricts DoF, while a small aperture maximizes the DoF. Please take a moment to compare the upper and lower variations in this diagram:

If we maximize depth of field…

So let’s pursue maximizing depth of field for a moment. Narrow the aperture to the smallest setting (the largest F-number), and presto you’ve got maximal DoF! Done! Hmm, not so fast.

First challenge – do you have enough light?

Narrowing the aperture sounds great in theory, but for each stop one narrows the aperture, the amount of light is halved. The camera sensor needs to receive sufficient photons in the pixel wells, according to the sensor’s quantum efficiency, to create an overall image with contrast necessary to process the image. If there is no motion in your application, perhaps you can just take a longer exposure. Or add supplemental lighting. But if you do have motion or can’t add more light, you may not be able to narrow the aperture as far as you hoped.

Second challenge – the Airy disk and diffraction

When light passes through an aperture, diffraction occurs – the bending of waves around the edge of the aperture. The pattern from a ray of light that falls upon the sensor takes the form of a bright circular area surrounded by a series of weakening concentric rings. This is called the Airy disk. Without going into the math, the Airy disk is the smallest point to which a beam of light can be focused.

And while stopping down the aperture increases the DoF, our stated goal, it has the negative impact of increasing diffraction.

Diffraction limits

As focused patterns, containing details in your application that you want to discern, near each other, they start to overlap. This creates interference, which in turn reduces contrast.

Every lens, no matter how well it is designed and manufactured, has a diffraction limit, the maximum resolving power of the lens – expressed in line pairs per millimeter. There is no point generating an Airy disk patterns from adjacent real-world features that are larger than the sensor’s pixels, or the all-important contrast needed will not be achieved.

High magnification example

Suppose you have a candidate camera with 3.45um pixels, and you want to pair it with a machine vision lens capable of 2x, 3x, or 4x magnification. You’ll find the Airy disk is 9um across! Something must be changed – a sensor with larger pixels, or a different lens.

As a rule of thumb, 1um resolution with machine vision lenses is about the best one can achieve. For higher resolution, there are specialized microscope lenses. Consult your lensing professional, who can guide you through sensor and lens selection in the context of your application.

Lens data sheets

Just a comment on lens manufacturers and provided data. While there are many details in the machine vision field, it’s quite transparent in terms of standards and performance data. Manufacturers’ product datasheets contain a wealth of information. For example, take a look at Edmund Optics lenses, then pick any lens family, then any lens model. You’ll find a clickable datasheet link like this, where you can see MTF graphs showing resolution performance like LP/mm, DOF graphs at different F#s, etc.

Takeaway

Per the blog’s title, Depth of Field is a balancing act between sharpness and blur. It’s physics. Pursue the links embedded in the blog, or study optical theory, if you want to dig into the math. Or just call us at 987-474-0044.

1st Vision’s sales engineers have over 100 years of combined experience to assist in your camera and components selection.  With a large portfolio of lenses, cables, NIC cards and industrial computers, we can provide a full vision solution!