Cosina (Cosinon) Cosina 50mm f2 Heritage

Overview

The Cosina Cosinon-S 50mm f/2 is a compact standard prime made by Cosina, a prolific Japanese optical manufacturer known for producing lenses under many house and third-party brand names. This particular optic was also sold rebadged as a Petri, and at least one example has surfaced under the Vivitar brand (noted with a higher serial number and slightly different internal construction, making infinity calibration harder). It was offered in Pentax K mount as an inexpensive kit-grade or budget standard lens. No established nicknames or community jargon (such as 'Bokeh King' or similar) are evidenced in the available reviews. Its cult appeal is entirely practical and value-driven: reviewers repeatedly note how cheap it is to acquire (averaging around $7.50) and how easy it is to disassemble, service, rebuild, and recalibrate. A recurring caveat is the black anodised mount, which prevents electrical contact with Pentax DSLRs, so the green button and catch-in-focus may not work unless the user creates a contact (e.g. with foil or by removing a little anodising). Some users have gone as far as converting the lens, which involves removing the aperture lever and its protective shield.

Verdict: The Cosina Cosinon-S 50mm f/2 is a budget standard prime for tinkerers and value hunters. It is center sharp wide open, very sharp stopped down, and delivers pleasant (sometimes swirly) bokeh, all for a few dollars. Its easy serviceability makes it ideal for those who enjoy rebuilding vintage glass, though DSLR users must accept modifying the black anodised mount to regain electronic aids.

Optical Character

Bokeh

Nice and smooth, can turn swirly wide open (disliked by some), rated highly by reviewers.

Sharpness wide open

Center sharp wide open; corners sharpen when stopped down, reaching total corner-to-corner sharpness by F8 on APS-C.

Flare resistance

Prone to flare/veiling in strong light when wide open, reducing contrast.

Contrast

Good contrast in normal conditions, but drops in strong light when shooting wide open.

Vignetting

No significant vignetting visible on APS-C.

Community Insights

What people love

Extremely cheap to acquire (average around $7.50) making it an outstanding value pick.
Easy to disassemble, rebuild, service, and calibrate infinity focus.
Center sharp wide open and totally sharp corners by F8 on APS-C.
Nice smooth bokeh, with high bokeh ratings from reviewers.
Fast enough f/2 aperture makes it quite easy to focus.
No significant vignetting or distortion on APS-C.

What people dislike

Black anodised mount prevents electrical contact with Pentax DSLRs, so green button and catch-in-focus may not work without modification.
Contrast drops in strong light when shooting wide open.
Swirly bokeh wide open, which not all users appreciate.
Some branded variants (e.g. a higher-serial Vivitar version) have different construction that resists easy infinity calibration.
Anti-Pentax focus rotation direction (~250°) may annoy Pentax users.

Pro Tips

Stop down to around F8 for total corner-to-corner sharpness on APS-C.
Avoid strong direct light or use a hood when shooting wide open to preserve contrast.
If using on a Pentax DSLR, create a mount contact (foil or lightly removing anodising) to restore green button functionality.
Since these are easy to service, don't fear a hazy or stiff copy—rebuilding and recalibrating is straightforward on most versions.
Embrace or avoid the wide-open swirly bokeh depending on your taste.

Sources (1)

Web-grounded synthesissecondary

The Cosina Cosinon-S 50mm f/2 is a compact standard prime made by Cosina, a prolific Japanese optical manufacturer known for producing lenses under many house and third-party brand names. This particular optic was also sold rebadged as a Petri, and at least one example has surfaced under the Vivitar brand (noted with a higher serial number and slightly different internal construction, making infinity calibration harder). It was offered in Pentax K mount as an inexpensive kit-grade or budget standard lens. No established nicknames or community jargon (such as 'Bokeh King' or similar) are evidenced in the available reviews. Its cult appeal is entirely practical and value-driven: reviewers repeatedly note how cheap it is to acquire (averaging around $7.50) and how easy it is to disassemble, service, rebuild, and recalibrate. A recurring caveat is the black anodised mount, which prevents electrical contact with Pentax DSLRs, so the green button and catch-in-focus may not work unless the user creates a contact (e.g. with foil or by removing a little anodising). Some users have gone as far as converting the lens, which involves removing the aperture lever and its protective shield.

https://www.pentaxforums.com/userreviews/cosina-cosinon-s-50-mm-1-2.htmlhttps://allphotolenses.com/lenses/item/c_745.htmlhttps://www.flickr.com/photos/tags/Cosinon%2050mm%20F2/https://www.dpreview.com/forums/thread/4238545