In general, start by selecting a camera. Pay particular attention to the size of the camera's pixels. Larger pixels will capture more light, and demand less performance from the lens. And that often lets you use a lower cost lens. Once you know the camera's specifications, it’s not hard to select a lens.

Next, use the width of the area you need to image to choose the general type of lens. Note there is some overlap in the area various lens types can image.

Object > 12 mm wide

Use a general-purpose entocentric lens.

Object 1 - 25 mm wide

Use a macro lens when you need to image a small area.

Object < 1 mm

Use a microscope lens. (Sorry, we don't offer any microscope lenses.)

Precision Gauging 1 - 240 mm

When you will be using the resulting image to measure features, you'll want to use a telecentric lens.

Regardless of the type of lens, our Machine Vision Lens Calculator will help you choose an appropriate model.

You'll need a lens having:

See the topics below for further information.

Optical format indicates the image circle of a lens, or the size of an image sensor measured diagonally. The lens's optical format must be at least as large as your camera sensor's format. If the lens has a smaller format, the image corners will be dark.

A lot of customers buy 2/3" format lenses for cameras that have 1/2" or smaller format sensors. There is usually little to no difference in price between a 1/2" and 2/3" lens, so 2/3" has become the standard. However, 1" and larger format lenses cost substantially more than 2/3".

You may notice that the optical format of image sensors and lenses are often expressed as a fraction of an inch, such as 1/2" or 2/3". Sensor sizes may also be specified as the diagonal measurement of the sensor, in millimeters. On the Machine Vision Store website, we tend to specify both inches and millimeters. For example, the LSKA211's specifications indicate it can be used with sensors up to 2/3", or 11 mm. But 2/3" is 16.93 mm, not 11 mm! What's up with that?

Well, the explanation takes us a ways back into the 20th century. Before there were CCD and CMOS image sensors, there were video camera tubes. Video camera tubes, such as the vidicon tube shown at right, were measured by their outside diameter. A 1" OD tube was said to have an optical format of 1". The portion of the interior of the tube actually sensitive to light was roughly two-thirds the tube's diameter.

Returning to our previous example, the LSKA211 has an optical format of 2/3". Multiply 2/3" by the portion of a vidicon tube that senses light (2/3), and we get roughly 11 mm.

Today's image sensors have little to do with vacuum tubes, yet this old standard still persists. This makes for some rather strange optical formats. Some years ago, sensors having a diagonal of 9 mm were introduced. Falling between 1/2" (8 mm) and 2/3" (11 mm), they were assigned an optical format of 1/1.8. We now have optical formats of 1/1.2, 1/2.5, and 1/2.3. And yet some image sensors don't fall into any of the standard formats. What a mess.

Use the sensor's actual diameter, or the lens's actual image circle, for all calculations. Nearly all lenses are manufactured in Asia or Europe, so the millimeter is the unit of choice.

We think its past time for the old optical formats to be discarded in the recycling bin of history.

Lens mounts define the interface between a camera and a lens. In machine vision, we are typically only concerned about a mechanical interface, as there is no electrical signal passed directly between the camera and lens. When selecting a lens, you'll want to be sure its mount is compatible with your camera's mount.

C-mount

The good old C-mount is the de facto standard for machine vision. Most cameras and lenses in our catalog are C-mount and, if it meets your needs, you really need not read any further. The C-mount interface has a 1-32 UN2A thread. C-mount cameras have a distance of 17.526 mm between the lens flange and their image sensor. (This distance is often called the "flange focal distance.") Generally, C-mount lenses support image sensors up to about a 1" optical format. However, we do offer some 4/3" format lenses having a C-mount.

CS-mount

The CS-mount was typically found on security cameras. Since most security cameras now have lenses built-in, the popularity of this standard has faded. CS-mount uses the same 1-32 UN2A thread as C-mount, but the flange focal distance is 12.50 mm instead. Note that C-mount lenses can be used on a CS-mount camera by adding a 5 mm extension ring.

F-mount

The F-mount, made popular by Nikon 35 mm format cameras, has a three-lug bayonet design. With a 44 mm throat, it accommodates larger sensors. The back focal distance is generally 46.5 mm. Since this mount uses springs to tension the lens against the camera body, we do not recommend using F-mount in locations prone to vibration. Screw mount lenses, such as the M42, offer better rigidity.

M42-mount

The M42-mount is sometimes called Pentax thread mount in the U.S., even though it was first used by Zeiss. It has a 42 mm diameter thread with a 1.0 mm pitch. The flange focal distance is typically about 45.5 mm, although this may vary from one camera model to the next.

T2-mount

T2-mount is another screw-type mount. It is similar to the M42-mount, but has a 42 mm diameter thread with 0.75 mm pitch instead. The typical flange focal distance is 55 mm.

M58-mount

The M58-mount is sometimes used by cameras that can be adapted for use with a variety of lens mounts. You'll generally need to purchase a mount adapter to use such a camera with your preferred lens. Be sure to purchase an adapter of the correct thickness; The camera-adapter combination must create a flange back distance equal to that specified by the lens.

MFT

The Micro Four Thirds ("MFT") system uses a bayonet-type lens mount with a flange focal distance of just under 20 mm. The MFT standard was created by Olympus and Panasonic. Both companies offer high-performance commercial cameras that feature a smaller, lighter design, as compared to traditional 35 mm cameras. We offer SVS-Vistek Tracer industrial cameras that use MFT lenses to quickly adjust zoom, focus and aperture.

TFL

The TFL-mount accomodates image sensors up to APS-C (27.9 mm diagonal) format. It uses a 35 mm thread with 0.75 mm pitch. The flange back distance is 17.526 mm, the same as C-mount lenses. This enables the assembly of compact imaging systems. However, there are currently not many TFL lenses available.

TFL-II

The TFL-II mount (sometimes referred to as "TFL-2") is a newer standard. Like the TFL mount, it has not yet found much support from camera and lens manufacturers. TFL-II uses a M48x0.75 thread. It accomodates APS-H (35 mm diagonal) image sensors with a flange back distance of 17.526 mm.

Industrial Lens Mount Summary

With the exception of C, CS, TFL and TFL-II mounts, you should always verify a specific camera's flange back distance, or FBD. (The flange back distance is sometimes called the flange focal distance, or the flange focal length.) It is not at all unusual to encounter industrial cameras that have a standard thread, but a non-standard flange back distance.

What determines how much resolution your lens should have? Usually, it is the camera sensor's pixel size.

We assume you're purchasing a camera having a certain number of pixels because you need that number of pixels. You probably wouldn’t buy a 12 megapixel camera if a 5 megapixel camera would suffice. And you therefore intended to use a lens that will not degrade the image quality produced by the 12 megapixel camera.

For example, maybe you're using a camera to measure the dimensions of a part. Based on the size of the part and the measurement precision required, you determine you need a camera having about 2300 pixels by 1700 pixels. That's roughly 4 megapixels ("MP"). So would we be safe with a 5 MP lens? Not necessarily.

It doesn't matter to the lens how many pixels the camera has. What matters is whether the lens can deliver sufficient contrast to each of the camera’s pixels. The smaller the pixels, the more lens performance is required. If the lens cannot "resolve" to the size of your camera's pixel, then your system will never achieve the accuracy expected. A lens must therefore be selected based on the size of a camera's pixels.

The megapixel rating may be useful in evaluating relative performance between two lenses from the same manufacturer. But, because pixel size can very greatly, it is of no value in comparing lenses from different manufacturers. And it is of no value in determining whether a specific lens is appropriate for a specific camera.

When deciding on lens resolution, it’s also useful to consider:

It seems we all have some natural understanding of "resolution" when it comes to cameras and displays. Camera sensors and LCD monitors are divided into pixels, and one way to evaluate resolution is to count the number of pixels a device has. But lenses are made from glass and don't have pixels. So, in some sense, it seems they should have infinite resolution. Unfortunately, that is not the case.

When it comes to optics, there is no value in discussing resolution without considering contrast. Consider a camera imaging black and white stripes through a lens. The stripes may be radial, as shown at right. Due to lens performance, there will be less contrast between the stripes as the stripes become narrower. There is no way to decide what resolution is evident in this image without specifying a minimum level of contrast required. Resolution and contrast are intrinsically intertwined.

You'll notice the lenses in our catalog specify resolution, without stating contrast. We obtain this information from manufacturers, and it is a simplification based on certain unspecified assumptions. Manufacturers typically indicate resolution at 30% contrast but, since this often is not published, it may be different. The stated resolution is sometimes measured in the center of the field of view, where it is best. Resolution in the image corners will be inferior, sometimes substantially so. The actual image resolution delivered to the image sensor will depend on many other variables, especially the aperture setting. Nonetheless, lens resolution remains a better indication of lens performance than megapixel rating, and is far simpler than analyzing MTF curves.

For more complete information about a lens's performance, look to its Modulation Transfer Function, or "MTF." The MTF graph shows how much contrast a lens delivers at a certain spatial frequency under specified conditions. Here's an example. Note the MTF varies with light wavelength, the aperture setting, location of the lines within the image, and even the direction of the lines (radial or tangential.)

You may notice some lens manufacturers publish a Contrast Transfer Function instead of MTF. CTF is similar to MTF, but uses black and white stripes instead of a sine-wave variation from black to white. CTF is said to assign more favorable results to a lens. Others would argue CTF is more applicable to actual machine vision installations, where edges tend to have sharp contrast.

MTF or CTF curves are available for most lenses in our catalog. If there is not a download link on the item's page, please ask us for a copy.

The following factors control how large an area your camera will image. (Note this topic is not applicable to macro and telecentric lenses.)

Assuming the camera has already been chosen and the working distance has certain constraints, the lens's focal length becomes quite important.

Lenses having focal lengths of 8, 12, 16, 25, 35 and 50 mm are commonly available. We offer lenses having shorter focal lengths, but you'll notice that prices increase rapidly below 8 mm. We also offer some lenses having longer focal lengths, typically used to image objects from greater distances. You probably won't find a lens having the perfect focal length. In this case, it is better to adjust the working distance than to use extensions rings.

Some things to keep in mind when selecting a focal length:

Our Machine Vision Lens Calculator will calculate the necessary focal length for you.

Does your project require imaging objects at different distances from the lens? Or imaging features of the same object that are at different distances? If so, maximizing the lens’s depth of field becomes important.

A lens’s depth of field is the difference between the nearest and the furthest distance that objects will remain sufficiently in focus. For example, if the image is in focus as near as 40 cm and as far as 50 cm distance, then the depth-of-field is said to be 50 - 40 = 10 cm.

A lens’s depth of field is a function of the lens’s f-number, nominal object distance, the acceptable circle-of-confusion (generally, a monochrome camera’s pixel size), and the lens focal length. As the f-number, object distance, and circle of confusion increase, the depth of field also increases. As the lens focal length increases, the depth of field decreases.

You often have little control over the object distance and lens focal length. And in any case, these factors tend to cancel each other out. So how can you increase the depth of field?

Remember that when working to increase the depth of field, light is your friend, and camera resolution is your foe. Work to maximize the one, and minimize the other.

Learn more about how to maximize depth of field.

Improve vision system performance by more precisely focusing your lens. But before focusing, choose an appropriate test target. The target should have high-contrast monochromatic features of a size appropriate for the camera's world resolution. In other words, crisp black and white features that are each a few pixels in size. A classic USAF target often works well. Or, in a pinch, try a business card having a small font.

Place the face of the target on the same plane as the actual object you'll be inspecting. Keep it away from the image center (where focus is usually sharpest) to keep things real.

Most machine vision lenses have a thumb-screw that secures the focus mechanism. Loosen the screw without removing it. Adjust the focus ring back and forth, hunting for the best focus. The optimal focus will reveal the best contrast between the small features on your target.

Secure the thumb-screw finger tight. Check the focus by rotating the target and moving it around the field of view.