A German webshop called Astronomik sell filters for astrophotography. They have developed a rather clever way of placing a small filter within the camera bayonet of Canon EOS cameras, so that one filter can be used with lenses of all diameters. The range of filters they offer is great, everything from ND filters to bandpass filters tailored to specific wavelenghts. Its clip-on design makes changing filters as easy as changing lenses. I have to admit: in this regard, Canon owners are lucky.
The electronic contacts in the Pentax K-bayonet is arranged in a far more cluttered way. Astronomik encourage custromers to contact them with special needs, but I figured a custom job to fit K-mount would be either undoable or too expensive.
Instead, I took a long, hard look at my 645->K adapter. It's a "dumb" piece of metal without any electronic contacts or cluttering mechanic contraptions. Its inner diameter is a few mm larger than the EOS filter. Then I asked Astronomik nicely if they could envision a solution for using the EOS clip-on filters inside this adapter. They suggested fitting a small ring of hard plastic inside the adaptor to make the filter sit tight. It would only cost me a few Euros. I have to add also that the dialogue with Astronomik was very good; corteous and precise. Excellent service.
I took delivery of the ring and a filter yesterday, used a little bit of silicone to secure the ring inside the adapter, and clipped the filter in place. And with typical German precision, it was a snug fit!
A pot shot out the window in broad daylight assured me there was no vignetting. And adding to my enthusiasm, the weather forecast predicted clear skies for the night. Maybe I could test it for real straight away! -But alas. Local fog.
So my Little Gray Cell began churning. What else could I do to test this filter? A CLS-filter is designed to remove "airglow" (wikipedia-link) and glare from urban light pollution. So to push the filter to its limits, I decided to simply photograph one of the most obnoxious sources of light pollution and see what turned out. The result is the image above. As you can see, the streetlights appear white, despite being sodium lamps which normally appear intensely orange. At the same time, the front- and rear lights of the passing cars are relatively unaffected.
This could turn out to be fun.
Technical setup: Pentax K-5, 645->K adapter with CLS filter, FA645 45-85mm f/4.5 and tripod.
Exposure: Manual mode, 30s and f/8 at ISO 200.