This useful set of three T-thread extension tubes will allow you to adjust your focus position etc by up to a maximum of 31mm in any combination of 6,10,15,16,21 and 25mm steps.

The three tubes in the set are 6mm, 10mm and 15mm long and each has a male T thread at one end and female T thread at the other.

For use with cameras and any device that uses standard T-threads (42mm x 0.75 pitch - so most camera adaptors, filter wheels etc).