This is a handy 2-part print that recreates the housing of a typical watchmakers loupe.  It consists of a threaded base and top that will retain a single biconvex lens.  It's based off of designs used in the watchmaking industry, with a few improvements.  It features 3 different styles of bases: non-vented, 1-vent, 4-vents.  The vents help prevent the lens from fogging up and are large enough to allow for non-magnified vision on the periphery.  The top is available in 2 different styles: with 5mm shroud, without shroud.  The shroud is useful for high-power lenses such as 10x that are quite thick in the middle.  The shroud also helps reduce ambient light, leading to a clearer image.  The non-shrouded top will be good for lenses 5x or weaker.  

The 3 different types of bases (no vents, 1 vent, and 4 vents) and 2 tops (5mm shroud, no shroud).  

Non-shrouded tops with low power lens installed (left) and high power lens installed (right).  Note the high power lens has a much more pronounced curvature.

5mm shrouded tops using the same lenses as before (low power on left, high power on right).  

To print, I recommend placing the tops flipped upside-down (the flat end facing the printbed, the threaded end facing up).  The base can be printed normally (threaded end facing up, with the wider, flared out end facing the printbed).  

The dimensions of the loupe allow for biconvex lenses that are between 23mm to 25mm in diameter.  In many cases, lenses can be reused from damaged loupes (the original reason I made this design) or simply ordered directly from watch tool suppliers.  

Some examples of compatible loupes:

Horotec MSA00.031 series
Horotec MSA00.032 series
Horotec MSA00.001 series (ASCO)
Horotec MSA00.099G series (ARY MINI eyeglass loupe - small)
Horotec MSA00.100G series (ARY eyeglass loupe - standard)
Horotec MSA00.103G series (ARY MAXI eyeglass loupe - large)

Bergeon BG2611-N and BG2611-B series
Bergeon BG2611-TN and BG2611-TB series
Bergeon BG3611-N series
Bergeon BG1492-D (right side) and BG1492-G (left side) (ARY eyeglass loupe - standard)
Bergeon BG6223-D (right side) and BG6223-G (left side) (ARY MAXI eyeglass loupe - large)
Bergeon BG6222-D (right side) and BG62222-G (left side) (ARY MINI eyeglass loupe - small)
Bergeon BG7856 series

The above loupes are all designed with removable lenses.  The lenses themselves can be purchased as spares through Horotec and Bergeon under the following item numbers:

Horotec MSA00.030 series (-L1 = 10x lens, -L1.5 = 6.5x lens, etc.)
Bergeon BG1492 lens-only series (-L1 = 10x lens, -L1.5 = 6.5x lens, etc.)

This design will also work with the older Behr “Universal Easy-On” Model 55 loupes for eyeglasses/spectacles.  

A guide to understanding loupe numbers and their corresponding magnification levels:

#1 = 10x magnification  
#1.5 = 6.5x
#2 = 5x
#2.5 = 4x
#3 = 3.5x
#3.5 = 3x
#4 = 2.5x

Due to the nature of biconvex lenses, the higher the magnification, the larger the curvature of the lens.  This design was tested on a variety of lenses between 2.5mm and 7.5mm thick.

For those of you interested in the design thinking behind this model:

This loupe model went through a number of iterations to keep it functional, compact, and easy to print.  

The base has a 40mm diameter at the bottom, going to 28mm diameter at the top.  The height of the base is 30mm.  The lid is 30mm diameter with either 15mm height for the shrouded version or 12mm non-shrouded.  Overall, these dimensions are similar to most other watchmaking loupes.  It should allow for easy use with any standard loupe wire holder.  

The base has a 2mm thick ring that makes contact with the print bed.  This is critical to keeping the print secure during all the subsequent printing motions.  Any narrower and the print would come loose turning into a spaghetti monster.  

The interior slope of the base starts off around 45 degrees so the inside surface quality stays good.  The vents are squares rotated at 45 degrees to avoid extreme overhangs or bridging.  

The threaded portion uses a fine metric thread - M28x1.  There is an additional -0.05mm offset on the thread faces to improve thread interaction.  The fine thread is deliberate since it allows for a larger interior diameter for the base (larger aperture) and a smaller exterior diameter for the screw-on top when compared to using a coarse thread.  The threaded sections are 10mm in height.  This allows for lenses of varying thicknesses.

The sidewalls are 1.5mm thick which is a balance between sturdiness and weight.  Thinner walls would be lighter and more flexible, but are much more prone to cracking under normal use.  The 1.5mm thickness should hold up for years.  Without lens, the loupes are between 7-8 grams depending on configuration.  This is much lighter than the aluminum-housing loupes which are 12 grams minus lens.  

In terms of printability, I've done about 20+ prints of both the base and tops.  It's pretty consistent; as long as your first layer lays down nicely, everything else should be fine.  If you are printing multiple parts, I recommend doing sequential printing by selecting “Complete individual objects”.  This will minimize stringing.  

The models pictured above use Overture Matte PLA.  The various colors are somewhat translucent, except for the black which is opaque.  Depending on how much ambient light you want, you will want to experiment with colors, vents, etc.  Most watchmaking loupes are housed in opaque housings that are black, dark green, dark grey, etc.