Originally posted by Hawkfan So is the ring flash realistically usable?
To affirm @Not_a_Number, the answer is a definite yes. It may not be adaptable to use with other lenses, but it will mount to your Dine macro and is will work just fine on your K-3 with the camera in M mode.*
Originally posted by Hawkfan I know nothing about flash really.
It might be a good time to learn and once the basics sink in, your whole view of lighting may change. Probably the most important principle is that flash exposure is based mostly on distance and lens aperture with shutter speed playing almost no role except for advanced technique using flash under bright conditions and other creative stuff.
Originally posted by Hawkfan Is P-TTL of the camera not compatible with the TTL of the flash?
That is quite true. Pentax used to control the flash duration during the exposure itself, "quenching" the flash when enough light had reached the film. That is true TTL flash and is very cool. Your flash was designed to work with those cameras. Sadly, there are cool things that can't easily be done with TTL, but can be done by measuring the required light for a flash setup
before the actual exposure. P-TTL measures the light from a short duration "pre-flash" and uses that to calculate flash duration for the actual exposure. P-TTL works well, but it is not something your flash knows how to do.
Originally posted by Hawkfan If the auto thyristor mode won't work very well, is there a real use of this old flash unit?
Yes! You can use it in full manual mode at either full intensity or 1/16 power. This is done by setting aperture (f-number) appropriate for the subject distance, calculated either from guide number or using the calculator table on the back of the flash. Alternatively, you can use the Auto mode with the aperture set to f/11 within the distances available for your choice of ISO(ASA) setting. Proper exposure is determined by distance, aperture and flash power(duration) for a given ISO(ASA) sensitivity.
Using the Auto mode with the camera in M mode and with the manual from the Butkus site available for ready reference would be a good starting place. When the flash is "ready", the camera should automagically switch to the 1/180s flash sync speed. Set the aperture to f/11 and take a shot a something a couple of feet away. With any luck, everything still works and you will have a big smile.
FWIW...I have several "old" flash that I use with my K-3, one of which is a Pentax AF280T. That flash has capabilities similar to your ring flash and works quite nicely.
Steve
* It will also work in the other modes, but best not to play in that space quite yet.