C-Mount Extension Tube and Spacer Ring Kits

  • Increase the Magnification of C-Mount and CS-Mount (1.00"-32) Camera Lenses
  • CMSP-KIT Contains Spacers with Thickness from 0.25 mm to 5 mm
  • CML-KIT Contains Extension Tubes with Lengths from 5 mm to 100 mm



Related Items

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Ray Diagram Demonstrating the effect of a C-Mount Extension Tube
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The Introduction of an Extension Tube or Spacer Ring Between a Camera and a Lens Decreases the Object Distance and Increases the Image Distance


  • Increase the Magnification of C-Mount and CS-Mount (1.00"-32) Camera Lenses
  • CMSP-KIT Contains Spacers with Thickness from 0.25 mm to 5 mm
  • CML-KIT Contains Extension Tubes with Lengths from 5 mm to 100 mm
  • Extension Tubes and Spacers Compatible with C-Mount Lenses and Cameras
  • Extension Tubes Compatible with Clamps and Slip Rings for Ø1" Lens Tubes

The C-Mount extension tubes and spacer rings included in these kits are designed to be used as spacers with C-Mount instruments such as Thorlabs' CMOS cameras and camera lenses. The use of these extension tubes and spacers increases the image distance between the lens and the camera sensor. This allows the lens to focus on objects at shorter distances than would be possible without a spacer. The decreased object distance increases the magnification of the image on the camera sensor while decreasing the depth of field.

These kits contain a variety of thicknesses/lengths to allow for fine/coarse adjustment of the image distance without having to search for a single piece to provide the right distance. Our CML-KIT includes extension tubes ranging in length from 5 mm to 100 mm for coarse adjustments of an optical system's length. Our CMSP-KIT includes brass spacer rings ranging in thickness from 0.25 mm to 2.00 mm that fit over external C-Mount threads. These rings fit between two C-Mount componenets and provide fine spacing adjustments. In addition, both kits contain a CS- to C-Mount extension adapter for compatibility with both types of cameras and lenses.

The 1.20" (30.5 mm) outer diameter of the C-Mount extension tubes is the same as that of Thorlabs' Ø1" lens tubes, so the C-Mount tubes are compatible with all clamps and slip rings for Ø1" lens tubes. However, the 0.97" (24.6 mm) inner diameter of the C-Mount tubes is too small to accept Ø1" (Ø24.5 mm) or Ø25 mm (Ø0.98") optics. However, optics with diameters between 0.87" (22.0 mm) and 0.97" (24.6 mm) can be secured inside the tubes with C-Mount retaining rings. For the construction of optical systems, Thorlabs offers Ø1/2", Ø1", Ø30 mm, Ø2", and Ø3" lens tubes, each of which is equipped with an SM thread standard that depends on the tube diameter.

CMSP-KIT C-Mount Brass Spacer Kit
Component Description (Item #) Qty.
0.25 mm Thick Spacer (CMSP025) 2
0.50 mm Thick Spacer (CMSP050) 2
1.00 mm Thick Spacer (CMSP100) 2
1.25 mm Thick Spacer (CMSP125) 2
1.50 mm Thick Spacer (CMSP150) 2
2.00 mm Thick Spacer (CMSP200) 2
CS- to C-Mount Extension Adapter (CML05) 1
CML-KIT C-Mount Extension Tube Kit
Component Description (Item #) Qty.
CS- to C-Mount Extension Adapter (CML05) 1
10 mm Long Extension Tube (CML10) 1
15 mm Long Extension Tube (CML15) 1
25 mm Long Extension Tube (CML25) 1
40 mm Long Extension Tube (CML40) 1
50 mm Long Extension Tube (CML50) 1
100 mm Long Extension Tube (CML100) 1
C-Mount Retaining Ring (CMRR) 2
Refills Available
C-Mount Compatible Products
CMOS Cameras Camera Lenses Zoom Lenses Thread Adapters Terminated Fiber Adapters

Threading Specifications

The following is a general overview of screw threading. For more details regarding specifications and dimensions, please consult the Machinery's Handbook, available for purchase at many bookstores.

Features of a Thread
A thread consists of three repeating features: a crest, flank, and root (see drawing to the right). Except in special cases, threads have symmetrical sides inclined at equal angles when a vertical line is drawn through the center of a crest or root. The distance between corresponding points on adjacent threads is known as the pitch of the thread. The flank angle is defined as the angle the flank makes with a perpendicular ray drawn from the screw axis. Unless otherwise stated, threads have a flank angle of 30°, resulting in a total angle between flanks of 60°. Each feature is shown in the diagram to the right.

The major diameter is taken from the crests of a thread while the minor diameter is taken from the roots. For most screws, crests and roots do not terminate at a sharp point, so crest and root truncation values are included in the definitions of major and minor diameter. The pitch diameter is approximately halfway between the major and minor diameters.

Thread Form
A thread form is a set of rules that define the features' scale relative to one another. Common thread forms include the Unified Screw Thread Form, used in the United States of America and measured in imperial units, and the ISO Metric Screw Thread Form, used in many parts of the world and measured with the International System of Units. There are many thread forms in the Unified screw thread standard designated by either UN, which defines a flat root contour, or UNR, which defines a round root contour. These can be further described by appending more letters. For example, an extremely fine thread with a flat root contour is designated UNEF. Those forms which are not standardized by the Unified screw thread system are designated UNS.

Thread Series
Most screws are identified by their thread series. Thread series are denoted by the major diameter and density of threads. Unified threads specify density in threads per inch, while Metric threads specify the thread pitch. For example, in the Unified nomenclature, a 1/4"-20 cap screw has a 1/4" diameter barrel and the pitch is 20 threads per inch (TPI). In metric nomenclature, an M4 x 0.7 cap screw has a 4 mm barrel and the pitch is 1 thread per 0.7 mm. The term M4 x 0.7 is often shortened to just M4.

Unified Thread Class Tolerancing
Location Loose Optimal Strict
Internal 1B 2B 3B
External 1A 2A 3A
Metric Thread Tolerance Positions
Location Loose Optimal Strict
Internal - G H
External e or f g h
Metric Thread Tolerance Grades
Dimension Location Tolerance Gradesa
Minor Diameter Internal 4, 5, 6, 7, 8
Major Diameter External 4, 6, 8
Pitch Diameter Internal 4, 5, 6, 7, 8
External 3, 4, 5, 6, 7, 8, 9
  • The tolerance becomes looser as the grade increases. The underlined grades are used with normal lengths of thread engagement.

Thread Class
The tolerances and allowances on a thread series are given by a thread class. Unified thread classes are alphanumeric identifiers starting with a number from 1 through 3, where 1 is the loosest tolerance and 3 is the tightest, and either A for external threading or B for internal threading.

Metric threads have a slightly more complex tolerancing method that uses tolerancing grades, designated by a number 3 through 9; and tolerancing positions, which use letters e through h. Grades provide a measure of the tolerance itself: the smaller the number, the tighter the tolerance. Positions denote the distance of the tolerance from the pitch diameter. Uppercase positioning letters indicate internal threads while lowercase positioning letters indicate external threads.

Quoting from the Machinery's Handbook, 29th Edition, p. 1885: "To designate the tolerance class, the grade and position of the pitch diameter is shown first followed by that for the major diameter in the case of the external thread or that for the minor diameter in the case of the internal thread, thus 4g6g for an external thread and 5H6H for an internal thread. If the two grades and positions are identical, it is not necessary to repeat the symbols, thus 4g, alone, stands for 4g4g and 5H, alone, stands for 5H5H."

Thorlabs' SM Series Threads
Threading specifications for our SM threads, utilized in our lens tube and cage system components, are given below so that you can machine mating components to suit your application. Most SM series threads utilize a non-standard Unified thread form, indicated by the letters UNS, with a 30° flank angle and a thread class of 2A and 2B. The exception is our SM30 series thread, which is a Metric thread form with a 30° flank angle and a tolerance of 6H/6g. We also offer products with C-Mount and RMS threads, and the specifications for these threads are given below for reference. Please note that other manufacturers may have different tolerances for C-Mount and RMS threads. For other thread specifications that are not listed here, please contact Tech Support.

SM05 Threading: Ø1/2" Lens Tubes, 16 mm Cage Systems
External Thread, 0.535"-40.0 UNS-2A Internal Thread, 0.535"-40.0 UNS-2B
Max Major Diameter 0.5340" Min Major Diameter 0.5350"
Min Major Diameter 0.5289" Min Pitch Diameter 0.5188"
Max Pitch Diameter 0.5178" Max Pitch Diameter 0.5230"
Min Pitch Diameter 0.5146" Min Minor Diameter (and 83.3% of Thread) 0.508"
Max Minor Diameter 0.5069" Max Minor Diameter (and 64.9% of Thread) 0.514"
RMS Threading: Objective, Scan, and Tube Lenses
External Thread, 0.800"-36.0 UNS-2A Internal Thread, 0.800"-36.0 UNS-2B
Max Major Diameter 0.7989" Min Major Diameter 0.8000"
Min Major Diameter 0.7934" Min Pitch Diameter 0.7820"
Max Pitch Diameter 0.7809" Max Pitch Diameter 0.7866"
Min Pitch Diameter 0.7774" Min Minor Diameter (and 83.3% of Thread) 0.770"
Max Minor Diameter 0.7688" Max Minor Diameter (and 64.9% of Thread) 0.777"
C-Mount Threading: Machine Vision Lenses, CCD/CMOS Cameras
External Thread, 1.000"-32.0 UN-2A Internal Thread, 1.000"-32.0 UN-2B
Max Major Diameter 0.9989" Min Major Diameter 1.0000"
Min Major Diameter 0.9929" Min Pitch Diameter 0.9797"
Max Pitch Diameter 0.9786" Max Pitch Diameter 0.9846"
Min Pitch Diameter 0.9748" Min Minor Diameter (and 83.3% of Thread) 0.966"
Max Minor Diameter 0.9651" Max Minor Diameter (and 64.9% of Thread) 0.974"
SM1 Threading: Ø1" Lens Tubes, 30 mm Cage Systems
External Thread, 1.035"-40.0 UNS-2A Internal Thread, 1.035"-40.0 UNS-2B
Max Major Diameter 1.0339" Min Major Diameter 1.0350"
Min Major Diameter 1.0288" Min Pitch Diameter 1.0188"
Max Pitch Diameter 1.0177" Max Pitch Diameter 1.0234"
Min Pitch Diameter 1.0142" Min Minor Diameter (and 83.3% of Thread) 1.008"
Max Minor Diameter 1.0068" Max Minor Diameter (and 64.9% of Thread) 1.014"
SM30 Threading: Ø30 mm Lens Tubes
External Thread, M30.5 x 0.5 – 6H/6g Internal Thread, M30.5 x 0.5 – 6H/6g
Max Major Diameter 30.480 mm Min Major Diameter 30.500 mm
Min Major Diameter 30.371 mm Min Pitch Diameter 30.175 mm
Max Pitch Diameter 30.155 mm Max Pitch Diameter 30.302 mm
Min Pitch Diameter 30.059 mm Min Minor Diameter (and 83.3% of Thread) 29.959 mm
Max Minor Diameter 29.938 mm Max Minor Diameter (and 64.9% of Thread) 30.094 mm
SM1.5 Threading: Ø1.5" Lens Tubes
External Thread, 1.535"-40 UNS-2A Internal Thread, 1.535"-40 UNS-2B
Max Major Diameter 1.5339" Min Major Diameter 1.535"
Min Major Diameter 1.5288" Min Pitch Diameter 1.5188"
Max Pitch Diameter 1.5177" Max Pitch Diameter 1.5236"
Min Pitch Diameter 1.5140" Min Minor Diameter (and 83.3% of Thread) 1.508"
Max Minor Diameter 1.5068" Max Minor Diameter (and 64.9% of Thread) 1.514"
SM2 Threading: Ø2" Lens Tubes, 60 mm Cage Systems
External Thread, 2.035"-40.0 UNS-2A Internal Thread, 2.035"-40.0 UNS-2B
Max Major Diameter 2.0338" Min Major Diameter 2.0350"
Min Major Diameter 2.0287" Min Pitch Diameter 2.0188"
Max Pitch Diameter 2.0176" Max Pitch Diameter 2.0239"
Min Pitch Diameter 2.0137" Min Minor Diameter (and 83.3% of Thread) 2.008"
Max Minor Diameter 2.0067" Max Minor Diameter (and 64.9% of Thread) 2.014"
SM3 Threading: Ø3" Lens Tubes
External Thread, 3.035"-40.0 UNS-2A Internal Thread, 3.035"-40.0 UNS-2B
Max Major Diameter 3.0337" Min Major Diameter 3.0350"
Min Major Diameter 3.0286" Min Pitch Diameter 3.0188"
Max Pitch Diameter 3.0175" Max Pitch Diameter 3.0242"
Min Pitch Diameter 3.0133" Min Minor Diameter (and 83.3% of Thread) 3.008"
Max Minor Diameter 3.0066" Max Minor Diameter (and 64.9% of Thread) 3.014"
SM4 Threading: Ø4" Lens Tubes
External Thread, 4.035"-40 UNS-2A Internal Thread, 4.035"-40.0 UNS-2B
Max Major Diameter 4.0337" Min Major Diameter 4.0350"
Min Major Diameter 4.0286" Min Pitch Diameter 4.0188"
Max Pitch Diameter 4.0175" Max Pitch Diameter 4.0245"
Min Pitch Diameter 4.0131" Min Minor Diameter (and 83.3% of Thread) 4.008"
Max Minor Diameter 4.0066" Max Minor Diameter (and 64.9% of Thread) 4.014"

Thorlabs' Camera Lenses for Machine Vision each have a characteristic range of object distances. Objects placed within this range can be brought to a sharp focus on the sensor of the C-Mount camera. As seen in the image sequence below, the addition of an extension tube between the camera and the lens changes this range of object distances and allows the system to focus on objects closer to the lens. This increases the magnification of the image on the camera while decreasing the depth of field. The following table lists the range of possible object distances for various combinations of Thorlabs' C-Mount extension tubes and machine vision lenses.

Note that Thorlabs' camera lenses are designed to correct for optical aberrations witin a specified range of object distances. The modification of the specified minimum object distance with an extension tube may reduce the ability of these lenses to eliminate aberrations.

14" (356 mm) Object Distance
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4" (102 mm) Object Distance
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4" (102 mm) Object Distance
5 mm Spacer

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5/8" (16 mm) Object Distance
20 mm (15 mm + 5 mm) Spacer

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RSP1X15 rotation mount imaged using a previous-generation scientific camera (Item # 4070M-GE) and MVL25M43 Lens at different object distances. A 20 mm spacer was created by using CML05 and CML15 spacers together.
Object Distance and Magnification for Lens and Spacer Combinations
Camera Lens Focal Length Design Format No Spacer 1 mm Spacer 5 mm Spacer 10 mm Spacer 20 mm Spacer
Minimum Object Distance Magnification Minimum Object Distance Magnification Minimum Object Distance Magnification Minimum Object Distance Magnification Minimum Object Distance Magnification
MVL8M23 8 mm 2/3" 120 mm 0.07 30 mm 0.2 - - - - - -
MVL12M23 12 mm 2/3" 150 mm 0.08 70 mm 0.16 - - - - - -
MVL12M1 12 mm 1" 300 mm 0.04 93 mm 0.12 - - - - - -
MVL12M43 12 mm 4/3" 100 mm 0.1 48 mm 0.19 - - - - - -
MVL16M23 16 mm 2/3" 200 mm 0.08 110 mm 0.14 - - - - - -
MVL16M1 16 mm 1" 300 mm 0.05 134 mm 0.11 - - - - - -
MVL25M23 25 mm 2/3" 200 mm 0.12 150 mm 0.16 73 mm 0.31 - - - -
MVL25TM23 25 mm 2/3" 100 mm 0.24 79 mm 0.28 46 mm 0.44 29 mm 0.63 - -
MVL25M1 25 mm 1" 300 mm 0.08 200 mm 0.12 83 mm 0.28 - - - -
MVL25M43 25 mm 4/3" 150 mm 0.15 115 mm 0.19 56 mm 0.35 31 mm 0.55 12 mm 0.96
MVL35M23 35 mm 2/3" 200 mm 0.18 175 mm 0.21 115 mm 0.32 81 mm 0.46 51 mm 0.73
MVL35M1 35 mm 1" 300 mm 0.12 243 mm 0.15 138 mm 0.26 91 mm 0.4 - -
MVL50M23 50 mm 2/3" 200 mm 0.3 190 mm 0.32 160 mm 0.39 135 mm 0.48 105 mm 0.65
MVL50TM23 50 mm 2/3" 100 mm 0.46 91 mm 0.48 76 mm 0.58 64 mm 0.7 49 mm 0.94
MVL50M1 50 mm 1" 500 mm 0.11 424 mm 0.13 269 mm 0.2 189 mm 0.3 124 mm 0.5
MVL75M1 75 mm 1" 1000 mm 0.078 858 mm 0.091 553 mm 0.14 389 mm 0.21 251 mm 0.34
MVL75M23 75 mm 2/3" 1200 mm 0.07 1010 mm 0.08 630 mm 0.13 440 mm 0.2 285 mm 0.34
MVL100M23 100 mm 2/3" 2000 mm 0.05 1700 mm 0.06 1060 mm 0.1 740 mm 0.15 480 mm 0.25

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C-Mount Extension Tubes and Spacer Ring Kits

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
CML-KIT Support Documentation
CML-KITC-Mount Extension Tube Kit: 5 mm - 100 mm
CMSP-KIT Support Documentation
CMSP-KITC-Mount Brass Spacer Ring Kit: 0.25 mm - 5 mm