From a previous post. I have also used Canon FD close up lenses which are available in 52mm size & are doublets.
With the release of the K Series in 1975 Pentax introduced a new set of close-up lenses.
Six were available in two filter thread sizes and all had SMC coatings:
S25 (52mm: K50/1.4 & K55/1.8)
S40 (52mm: K50/1.4 & K55/1.8)
T80 (52mm: K85/1.8, K105/2.8, K120/2.8, K135/3.5 & K150/4)
T95 (58mm: K135/2.5 & K200/4)
T160 (52mm: K105/2.8, K120/2.8, K135/3.5 & K150/4)
T183 (58mm: K135/2.5 & K200/4)
“S” indicates the close-up lens is for “standard” lenses and “T” is for “telephoto” lenses. The “number” indicates the focal length of the close-up lens in cm. (The distance from film to subject, the shorter the focal length the bigger the magnification.) Usable/recommended K Series lenses are in brackets.
These are great close-up accessories that look & work exactly like a Pentax SMC fitter. They take up little space in your camera bag and are perfect for close-ups if you don’t own a macro lens or extension tubes. Unlike a macro lens or extension tubes there is no exposure factor or light loss.
The highest magnification you can get with a K Series close-up lens is 0.42x with a S25 and a K55/1.8 lens. That’s close to a K50/4 or K100/4 Macro lens, which both have 0.5x magnification.
Read more at:
PENTAX Close-up Lens reviews - Pentax Camera Accessory Review Database