Tube extension

I know nothing about these, except the fact that they seem cheap to buy. What should I buy to double the magnification of my 105 macro?

Tubes are good and are probably the easiest / most cost-effective way to gain magnification, but try to get some that have aperture contacts. These are more expensive, but maintain the auto-aperture settings, which can be very useful, or even critical if your lens doesn't have an aperture ring.

You can also try more extreme measures, such as bellows, reversing your lens (using a reversing ring), or even a butchered zoom with a reversed enlarger lens fitted to it if you want huge magnification, but I think the tubes are the best place to start.

Regards
David

There is a set for sale here
https://www.pentaxuser.com/forum/topic/fs--macro-kit--af-160-ring-flash--m-bello...

Well this has the pttl contact transmission if this is what you mean

KOOKA KK-P25 Copper Macro Extension Tube with Close-up Image with TTL Exposure for Pentax K1 K3 K5 SLR Cameras (25mm) https://www.amazon.co.uk/dp/B06X3ZYDG1/ref=cm_sw_r_cp_apa_nI3gzbS59HDY3

However I am not sure what 25mm stands for

I suspect the 25mm would be the length. though the TTL exposure bit sounds like rubbish. You can get TTL metering using a toilet roll for extension!
The contacts for aperture control are very useful especially if you're using modern lenses without an aperture ring.

To increase the magnification of your lens by 1 you would need to add the lenses focal length in extension. Bare in mind that many macro lenses reduce their focal lengths drastically on close focusing...

That tube would certainly help but it would be better if you could find a set of three with different sizes. Just a single tube would be somewhat limiting. At least that one has all the electrical contacts and aperture lever so it's worth considering.

Have a look at link It sounds full on expert but this one time Pentax User member is a wonderful teacher.