Photax 35 mm f/3.5
A Photax 35 mm f/3.5 type 1 is shown above. This lens model, usually in barrels identical to the illustrated one, was sold with a variety of brands. It has aperture preset and aperture rings at the front, followed by a focusing ring. It is equipped with a removable adapter carrying an M42 lens mount, attached to a T2 thread on the lens base and fixed with three set screws. The filter mount is 49 mm.
This is a lens with a 31.6 mm diameter of the front optical element (as visible at the lens front). It is relatively modern and displays a lower transmission of UV radiation and higher cutoff wavelength than the Kyoei types 1 and 2. The different UV transmission of the Photax type 1 with respect to these earlier models is detectable, but not substantial, at 365 mm, and reaches not more than 0.5-1 stops at this wavelength. At shorter wavelengths (330-340 nm) the difference increases to 2-4 stops. In my opinion, one can easily live with a difference of one stop and compensate for it in post-production. For wavelengths down to approximately 365 nm, the Photax type 1 is a suitable, easily obtained and cheap 35 mm UV lens. If shorter wavelengths are important, then one should look for some of the earlier lenses like the Kyoei. The UV performance of this lens is discussed more in detail here.
The brands listed below are found mostly on lenses with a front element diameter of 31.6 mm. I did not test lenses of all these brands, but some are obviously identical to the Photax type 1, except for the branding. For instance, most Photax, Paragon, Prinzgalaxy and Unistar 35 mm lenses are obviously from the very same series. Like the illustrated specimen, they are usually equipped with a fixed T2 mount and an interchangeable T2 adapter carrying M42 or Exakta lens mounts.
The lens in Figure 2 is branded Photax-Paragon, and has a fixed M42 mount and automatic aperture. The camera body closes the aperture to the value preset on the aperture ring by pushing on a pin that projects from the rear of the lens mount, in the 6 o'clock position. This particular lens model lacks a control to use the diaphragm in manual mode. This makes it a little difficult to use the lens on a non-M42 camera. Possible solutions are to use an M42 adapter with an internal flange that keeps the aperture pin pressed, or to modify the lens itself. Other Photax-Paragon 35 mm f/3.5 lenses in similar barrels have a slider near the lens mount to set the aperture to manual mode.
The diameter of the front lens element as visible from the front of the lens is 28.5 mm (23 mm in Kyoei-made models), and the filter mount 52 mm. The "looks" of this lens are more modern than those of the Photax type 1described above and of the Hanimex, Kyoei, Noflexar, Soligor and Optomax pages. Optically, this Photax type 2 lens obviously uses a different formula than the other 35 mm lenses described on these pages. The diameter of the rear element is also different (16 mm in the Photax type 2, about 14 mm in Kyoei-made models). The lens coatings of this Photax type 2 give decidedly bluish reflexes, unlike the golden reflexes of the Kyoei lenses known to perform best in UV photography.
The Photax type 1 was probably built at least a few years, and possibly up to 10-15 years, before the Photax type 2.
The UV performance of these Photax type 1 and 2 lenses is discussed more in detail here.