The (badly translated) instructions that came with the 560mm seem to imply that you should not use that lens without a filter installed.

The (badly translated) instructions that came with the 560mm seem to imply that you should not use that lens without a filter installed.

Actually, the instructions said both that the filter is optional and also that the lens cannot be used without the filter. Some ambiguity there, so I wrote to Ricoh, which, after about six weeks, wrote back and said, "follow the instructions".

Update: while fooling around trying to figure out how to do astrophotography, I experimented a bit with the 560mm.

Now, I'm one of those people who use a UV filter on the front if it's threaded and leave it on all the time both for physical protection of the objective element as well as to cut haze and such. So I'd replaced the one that came with the 560mm from the factory (in the back-mounted holder) and put a new one on the front (like, 112mm diameter), using Chiaro 99-UVbts filters.

So, in my low-light experiements, even though these filters are supposed to attenuate the light by only one percent each, I pulled 'em both out. In part because a UV filter is designed to suck up light at the blue end of the spectrum, and I figured there are likely to be stars out there of such color that the filters would interfere.

When I pulled the back-mounted one, I realized that the holder the filter screws into is itself screwed into a ring-gear, and they're on opposite sides of a plate, so they hold each other in place. When you unscrew the filter, the holder-plate may be unscrewed from the ring-gear, rather than the filter from the mount. When that happens, the ring-gear can be inadvertently left in the lens, and when it pops out from the mounting plate, it can rattle around loose inside the lens, and that can't be good.

So I came to the conclusion that the reason the instructions are ambiguous is that (1) optically, the filter really is optional; and (2) you should not remove the filter because that could cause internal damage to the lens if you don't get things exactly right and the ring-gear gets loose.

My solution in this case was to take the filter, the filter-mount, and the ring-gear out, all of 'em, and stick 'em in plastic bags, and returned the empty holder to the lens slot.

[angerdan]

Here's how the filter looks like:

Unfortunately the image has gone.
The filter holder for the 645 D FA 25mm is identical (besides the black paint instead of white).
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ephotozine.com/articles/pentax-smc-da645-25mm-f-4al--if--sdm-aw-lens-18808/images/highres-pentaxDFA25HOLDER2_1332489458.jpg
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Pentax-DA 560mm F5.6 ED AW Review - Construction and Handling | PentaxForums.com Reviews