When this lens was first manufactured in 1964, it was one of the sharpest 50mm lenses available. Even now it beats most commercial lenses, and I suspect more than one or two ‘pro lenses. In fact, this can be said about all of the Pentax Takumar lenses.
If you get the chance to buy any of the series you are unlikely to be disappointed. Even if less than perfect, they are still very good. You can sometimes find them in ‘car boot’ sales, at a range of prices and conditions, and various Internet auctions. I have even found them in the ‘surplus stock’ bin in camera shops! (It’s usually not worthwhile for the shop owner to spend time cleaning the dust of such an old lens. Besides, he might find something nasty under the dust anyway. So he won’t get very much for an example in anything but ‘mint’ condition. But I’m not complaining!)
The lens went through a number of variations and improvements until eventually discontinued in 1975. The original version had 7 elements arranged in 8 groups. This changed to 6 elements in 7 groups in 1966. In 1972 the lens gained a multi-coating and a lug for use with Spotmatic ES and F models. The later super multi-coated types are to be preferred over the earlier single coated types, simply for the sake of the better coating, but be aware that the lug may cause problems with some M42 mount cameras.
Other variations include different numbers of diaphragm blades, ranging from six to eight. This article shows the disassembly of a late 8 blade type.
Problems peculiar to this lens type.
Many examples show yellowing of the inner elements. This is due to the use of radioactive Thorium in the glass formulation. I believe that the cause of the discoloration may be attributable directly to the effects of the radiation itself as the Thorium decays. However, there is no need to worry, the level of radiation is very low, and is mostly stopped by the glass anyway. I am told that some lenses used for aerial surveillance in WW2, are hot enough to fog film after a few days!
This can (Reportedly) be rectified by exposure to UV for a period of two or three weeks. This method works quite well if the source of UV is Californian sunshine, but living in Britain I have not been available to verify the sunlight cure. It is claimed that artificial UV lighting will clear the glass in a little more time. My own experiments suggest that only ‘hard’ UV will suffice. (The sort that is produced by tanning lamps, or (more extreme) arc-welding equipment.) This type of UV can also be harmful and can cause severe burns and cataracts, so caution is advised.
However, for many users, this may not be a problem, as it is usually only a very light ‘straw’ shade, and at worst adds a slight warmth to Color Images.
As with other lenses problems encountered may include fungal growth and stuck diaphragms. Provided the fungal growth is slight the lens performance will not be discernibly affected, but cleaning the fungus out is still advisable. Stuck diaphragms are usually due to grease on the blades, which will need cleaning. Both of these operations will require dis-assembly of the lens.
The lens I am dismantling here came with an SP500 bought on the internet. It has a dented filter ring, a sticky aperture, discolored, elements, fungus, and it appears someone has already attempted some repair and scratched the front trim quite badly. All these factors usually add up to a junker, but it makes an ideal subject for this purpose!
This lens often baffles attempts to dismantle it, but with the right tools, and know-how it is not normally too difficult to do. In the case of this lens, the dent in the filter ring is going to be the most difficult obstacle.
As always in camera repair, the first place to start is with a careful examination of the subject.
OK, the filter ring and the front trim of this lens is damaged. The front trim damage probably results from a previous attempt to open the lens.
Notice that there are no obvious means of unscrewing the front element.