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Ø1" N-BK7 Best Form Spherical Lenses, Uncoated

  • Best Possible Performance from a Spherical Singlet
  • Optimized for Infinite Conjugates
  • Diffraction-Limited Performance at Small Input Diameters
  • Uncoated for High-Power Applications




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Uncoated N-BK7 Best Form Lenses
Lens Shape Convex/Convex
Substrate Material N-BK7a
Design Wavelength 587.6 nm
Diameter Tolerance +0.0 mm / -0.1 mm
Index of Refraction 1.5167 at Design Wavelength
Surface Quality 10-5 Scratch-Dig
Spherical Surface Power 3 Fringes
Spherical Surface Irregularity λ/4
Centration <3 arcmin
Clear Aperture ≥90% of Diameter
Focal Length Tolerance ±1% @ Design Wavelength
  • Click Link for Detailed Specifications on the Substrate
Zemax Files
Click on the red Document icon next to the item numbers below to access the Zemax file download. Our entire Zemax Catalog is also available.


  • Best Possible Performance from a Spherical Singlet
  • Uncoated Design for High Power Applications
  • Ideal for High-Power Applications
  • Diffraction-Limited Performance at Small Input Diameters

Thorlabs' N-BK7 Best Form lenses are designed to minimize spherical aberration while still using spherical surfaces to form the lens. They are typically employed at infinite conjugates in high-power applications where doublets are not an option. The lenses presented here are uncoated. Versions of our best form lenses are also available with one of three antireflection coatings deposited on both surfaces to reduce the light reflected from each surface of the lens (see table to the right).

For spherical lenses, a given focal length can be produced by more than one combination of the front and back radii of curvature. Each combination of surface curvatures will result in a different amount of aberration caused by the lens (see the Graphs tab). The radius of curvature for each surface of these "best form" lenses has been chosen to minimize the spherical aberration and coma produced by the lens, optimizing it for use at infinite conjugates. This process makes these lenses more expensive than plano-convex or bi-convex lenses, but still significantly less expensive than our premium line of CNC-polished aspheric lenses. Since the lenses are optimized for minimum spot size, they can theoretically reach diffraction-limited performance for small input beam diameters (see the table below for details).

For best performance in focusing applications, place the surface with the shorter radius of curvature (i.e., the more steeply curved surface) towards the collimated source. Lens kits are also available. Please click here for more information.

N-BK7 Best Form Lenses Selection Guide
-A Coating (350 - 700 nm)
-B Coating (650 - 1050 nm)
-C Coating (1050 - 1700 nm)
Optic Cleaning Tutorial
Item #Input Diameter for
Performance @ 632.8 nm
Input Diameter for
Performance @ 1064 nm
LBF254-040 5.0 mm 6.0 mm
LBF254-050 6.0 mm 7.0 mm
LBF254-075 8.0 mm 9.5 mm
LBF254-100 10.0 mm 11.5 mm
LBF254-150 13.5 mm 16.0 mm
LBF254-200 17.0 mm 20.0 mm
Optical Coatings and Substrates
Lens Tutorial
Quick Links to Other Spherical Singlets
Plano-Convex Bi-Convex Best Form Plano-Concave Bi-Concave Positive Meniscus Negative Meniscus
N-BK7 Transmittance
Click to Enlarge

Click Here for Raw Data
The transmission curve for N-BK7, a RoHS-compliant form of BK7, is shown above. Total transmission is given for a 10 mm thick, uncoated sample and includes surface reflections.
lens shape

Lens Shape vs.
Spherical Aberration

The plot to the right shows coma and spherical aberration plotted as a function of the curvature (c) of the front face of the lens for an N-BK7 lens with focal length of 100 mm, the stop set at the front surface of the lens, and a ±20 field. The curvature (c) is related to the more familiar radius of curvature (r) by c = 1/r. The minimum of the spherical abberation almost coincides with the zero coma location. The lens shape where the spherical aberration is at a minimum provides the basis for a "best form" design.

CXY1 in 30 mm Cage System
Click to Enlarge

CXY1 Translation Mount and
SM1 Lens Tube Mounted in a
30 mm Cage System
Threaded Mounting Adapter
Click to Enlarge

Ø1" Optic Mounted in a
ST1XY-S XY Translator

Click to Enlarge

LMR1 Fixed Mount with Ø1" Lens

Click to Enlarge

LM2XY Translating Mount with Ø2" Lens
Recommended Mounting Options for Thorlabs Lenses
Item # Mounts for Ø5 mm to Ø10 mm Optics
Imperial Metric
(Various) Fixed Lens Mounts for Small Optics, Ø5 mm to Ø10 mm
(Various) Small Optic Adapters for Use with Standard Fixed Lens Mounts, Ø5 mm to Ø10 mm
Item # Mounts for Ø1" (Ø25.4 mm) Optics
Imperial Metric
LMR05 LMR05/M Fixed Lens Mount for Ø1/2" Optics
LM05XY LM05XY/M Translating Lens Mount for Ø1/2" Optics
SCP05 16 mm Cage System, XY Translation Mount for Ø1/2" Optics
(Various) Ø1/2" Lens Tubes,
Optional SM05RRC Retaining Ring for High-Curvature Lenses (See Below)
Item # Mounts for Ø1" (Ø25.4 mm) Optics
Imperial Metric
LMR1 LMR1/M Fixed Lens Mount for Ø1" Optics
LM1XY LM1XY/M Translating Lens Mount for Ø1" Optics
ST1XY-S ST1XY-S/M Translating Lens Mount with Micrometer Drives (Other Drives Available)
CXY1 30 mm Cage System, XY Translation Mount for Ø1" Optics
(Various) Ø1" Lens Tubes,
Optional SM1RRC Retaining Ring for High-Curvature Lenses (See Below)
Item # Mounts for Ø2" (Ø50.8 mm) Optics
Imperial Metric
LMR2 LMR2/M Fixed Lens Mount for Ø2" Optics
LM2XY LM2XY/M Translating Lens Mount for Ø2" Optics
CXY2 60 mm Cage System, XY Translation Mount for Ø2" Optics
(Various) Ø2" Lens Tubes,
Optional SM2RRC Retaining Ring for High-Curvature Lenses (See Below)
Item # Adjustable Optic Mounts
Imperial Metric
LH1 LH1/M Adjustable Mount for Ø0.28" (Ø7.1 mm) to Ø1.80" (Ø45.7 mm) Optics
LH2 LH2/M Adjustable Mount for Ø0.77" (Ø19.6 mm) to Ø2.28" (Ø57.9 mm) Optics
VG100 VG100/M Adjustable Clamp for Ø0.5" (Ø13 mm) to Ø3.5" (Ø89 mm) Optics
SCL03 SCL03/M Self-Centering Mount for Ø0.15" (Ø3.8 mm) to Ø1.77" (Ø45.0 mm) Optics
SCL04 SCL04/M Self-Centering Mount for Ø0.15" (Ø3.8 mm) to Ø3.00" (Ø76.2 mm) Optics
LH160C LH160C/M Adjustable Mount for 60 mm Cage Systems,
Ø0.50" (Ø13 mm) to Ø2.00" (Ø50.8 mm) Optics
SCL60C SCL60C/M Self-Centering Mount for 60 mm Cage Systems,
Ø0.15" (Ø3.8 mm) to Ø1.77" (Ø45.0 mm) Optics


Mounting High-Curvature Optics

Thorlabs' retaining rings are used to secure unmounted optics within lens tubes or optic mounts. These rings are secured in position using a compatible spanner wrench. For flat or low-curvature optics, standard retaining rings manufactured from anodized aluminum are available from Ø5 mm to Ø4". For high-curvature optics, extra-thick retaining rings are available in Ø1/2", Ø1", and Ø2" sizes.

Extra-thick retaining rings offer several features that aid in mounting high-curvature optics such as aspheric lenses, short-focal-length plano-convex lenses, and condenser lenses. As shown in the animation to the right, the guide flange of the spanner wrench will collide with the surface of high-curvature lenses when using a standard retaining ring, potentially scratching the optic. This contact also creates a gap between the spanner wrench and retaining ring, preventing the ring from tightening correctly. Extra-thick retaining rings provide the necessary clearance for the spanner wrench to secure the lens without coming into contact with the optic surface.

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Posted Comments:
Posted Date:2012-04-05 18:28:00.0
Response from Tim at Thorlabs: Thank you for helping us improve our presentation! For these lenses, Zemax files are available by clicking the red icon next to the part number. Using these, theoretical focal length shifts and OPD graphs can be generated. We also provide data on diameters needed for diffraction-limited performance for our Best Form Lenses under the “Specs” tab and we are able to provide focal shift graphs for our Achromatic Doublets. I will talk to our web team to see how we can better integrate this data together within the presentation to make the performance between the two more obvious. For any immediate needs you can always contact us at where an Applications Engineer will be able to help you.
Posted Date:2012-04-02 10:55:49.0
Trying to compare the best form lenses to an achromat for focusing a laser beam to the smallest possibe spot. It would be great to have some data that allows us to see when one is better than the other.
Posted Date:2008-06-05 17:20:44.0
Response from Inge at Thorlabs: Thank you for your feedback. We removed the page break from the product table on this page. We liked the solution and reviewed several other product pages and did the same. Please let us know what you think of the solution.
Posted Date:2008-03-30 09:09:37.0
Since there are only 4 lenses on the second page can you just have them all appear on one page.

Ø1" N-BK7 Best Form Spherical Lenses, Uncoated

Item # Diameter Focal Length Dioptera Radius of
Curvature 1
Radius of
Curvature 2
Back Focal Lengthc Reference
LBF254-040 1" 40.0 mm +25.0 24.02 mm -134.60 mm 6.5 mm 2.3 mm 36.3 mm Best Form Lens Drawing
LBF254-050 1" 50.0 mm +20.0 30.06 mm -172.0 mm 6.5 mm 3.2 mm 46.4 mm
LBF254-075 1" 75.0 mm +13.3 44.50 mm -289.00 mm 5.0 mm 2.9 mm 72.1 mm
LBF254-100 1" 100.0 mm +10.0 60.02 mm -353.30 mm 4.0 mm 2.4 mm 97.3 mm
LBF254-150 1" 150.0 mm +6.7 89.35 mm -570.49 mm 4.0 mm 3.0 mm 147.5 mm
LBF254-200 1" 200.0 mm +5.0 121.50 mm -684.50 mm 4.0 mm 3.2 mm 197.8 mm

Suggested Fixed Lens Mount: LMR1(/M)

  • Reciprocal of the focal length in meters.
  • Edge thickness given before 0.2 mm at 45° typical chamfer.
  • Measured at the design wavelength, 587.6 nm. Since the index of refraction for N-BK7 is inversely proportional to the wavelength, the focal length of each lens increases with increasing wavelength.
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
LBF254-040 Support Documentation
LBF254-040N-BK7 Best Form Lens, Ø1", f = 40 mm, Uncoated
LBF254-050 Support Documentation
LBF254-050N-BK7 Best Form Lens, Ø1", f = 50 mm, Uncoated
LBF254-075 Support Documentation
LBF254-075N-BK7 Best Form Lens, Ø1", f = 75 mm, Uncoated
LBF254-100 Support Documentation
LBF254-100N-BK7 Best Form Lens, Ø1", f = 100 mm, Uncoated
LBF254-150 Support Documentation
LBF254-150N-BK7 Best Form Lens, Ø1", f = 150 mm, Uncoated
LBF254-200 Support Documentation
LBF254-200N-BK7 Best Form Lens, Ø1", f = 200 mm, Uncoated
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