For the original playstation theres a screw just under the edge of the disk drive, turning it left or right can adjust the lense's ability to read the disk, it can also F'up the playstation pretty good so make sure you know where the screw was set so you can get back to where you started.
On the PSone I'm not sure, I haven't opened one up to look inside yet.
My experience has been that there are enough older systems out there that it's easier to buy one then to waste time with adjusting the lense, Just picked up a 5051 series with a new disk drive in it for $6 at Goodwill, whoever had it put the drive in but didn't get the power cable connected to the disk drive, all I had to do was connect it up and plug it in and ta da, a new Playstation I'll store until my others run out.
