Originally posted by Nick Siebers That lens is prone to separation/deterioration between the cemented elements, and that looks like an example. I took mine to a local repair shop and they felt it couldn't be fixed.
If you are
really determined to repair the lens back to its former glory you could try this :
The SMC 100/4 is a pretty old lens Pentax would probably have used Canada balsam to cement them - you can separate the elements by softening the adhesive by heating at 150°C, and after separating the elements cool them down (
carefully) and soak the separated lens elements in acetone. This should remove the balm from the lens surfaces and then you can attempt re-cememting them with a modern UV curing optical cement*- be warned this does require some specialised equipment and skill also re-centering the lens elements after separating them can be very difficult.
Some more modern lenses use UV curing cements which have to be removed by soaking in acetone,or in a solution of 50% NAoH** (sodium hydroxide) I have used cryogenic methods (liquid nitrogen) to physically shatter optical adhesives between lenses - but I do not recommend this.
* UV curing cement is easier to use than Canada basalm - because it retains its low viscosity until it is exposed to concentrated ultraviolet light - so you have plenty of time to get the lens aligned perfectly. The materials I use to de-cement and re-cement lenses are available
here
** Be careful when mixing up this solution, remember to add the NAoH to the water - not the other way around. When NAoH comes into contact with water a considerable amount of heat can be released which can be hazardous. Also be warned that NAoH is rather corrosive, so wear appropriate protective equipment. I have heard that 3M Safest Stripper Paint and Varnish remover can be used to remove optical cement but it takes
considerably longer than concentrated solvents.