Link to actual product? Are you referring to this set of three, (one more than a pair).
ProOptic Auto Extension Tube Set
Sorry, no experience with Pro Optic brand extension tubes. However, the good news is that extension tubes are pretty simple devices
and not very expensive. <$20 seems quite reasonable. Reviews for ProOptic extension tubes in general would suggest inconsistent build
quality as the primary problem. A sticking or misaligned aperture arm might easily be bent back into working order if need.
Note that the genuine Pentax tubes are also not that expensive. I found several sets available for $60-$80.
I'll also say that extension tubes are one of those accessories that can often be found for virtually nothing when you're not looking for them.
I've acquired several sets over the years as part of camera lots. That includes several off brand sets (all worked fine), as well as a
set of genuine
Pentax Auto Ext. Tubes set K, all for no additional cost over the camera body or lens I was after. If you keep an eye on
shopgoodwill.com, local garage sales, etc, you will invariably find an extension tube set included in a bag full of equipment.
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