I wanted to get an older lens (1950’s to 1970’s) as these are supposed to be better for infra-red photography. I managed to get an old screw-mount lens and adaptor to do some tests. This lens was a ‘Zodel’ brand (I haven’t heard of it either) of focal length 28mm (42mm equivalent on my camera). Here are the initial results:
With the focussing of this lens, the focus scale was way off (this might be the adaptor I used) – the infinity focus point was the 3m mark! I then tried it with the 850nm filter:
This lens seemed clear of the hotspot problem. This might be worth pursuing.