Roland W Offline Upload & Sell: On
|
p.1 #6 · p.1 #6 · Drop-in H-A, O III, and S II filters for 600mm f/4 IS | |
I have looked in to this issue in detail, and know of some things to share. First off, the slot in the lens body that the drop in filter holder goes in to is quite narrow on the two Canon IS super telephoto lenses I own, and I suspect that is the same on all modern lenses that take the 52mm drop in holder. It is so narrow that many astronomy 2" (48mm) filters will not even go in to the open slot, much less if you were to add the thickness of the step down adapter and the thickness of the drop in holder. Many mounting metal of astronomy filters is usually much thicker than a photographic filter is, and that is why they are so far from fitting.
There is for sure a drop in holder that is made for 52mm screw in filters, and it works well for a single 52mm photographic filter. I now have a couple of these, and the price seems to vary a bit from different sources. With the clear filter that comes with it un screwed and removed, you end up with a round pocket that is 51.0mm in diameter, which is exactly the right size to set in an un mounted astronomy filter glass like the ones that Baader sells, that are listed as 50.4 mm in diameter. It is like a perfect fit, except of course there is no direct way to keep the un mounted filter glass in place. My first thought was to perhaps use a dab of silicone adhesive at a few spots to keep it in place. That would likely require one holder per filter you want to use, which adds to the cost a bit.
In looking at the empty holder just now, I have realized that if the filter glass is thin enough, it may be practical to use a 52 to 48 step down adapter to screw in on top of the un mounted filter, and if so, that would hold it in place but still allow changing it, plus the assembly should fit in the slot in the lens. My guess is that 1.5mm glass thickness would work to be held in by screwing in the step up adapter, and that 2mm thick glass would likely be just a bit too thick. I have not had a chance to look up the thickness of the Baader filters, but will do that soon, because I like you would like to set up one or too drop in filters for astronomy.
By the way, the gelatin drop in holder that comes with each lens is not going to work out for being modified to accept a glass filter unless is was only about 44 mm in diameter. So go for the "52mm screw in" holder if you want to try this, not the gelatin holder. And thank you for getting me back on track on this project. The big Canon lenses have a lot of coma distortion that shows up, especially toward the corners, but they can still be useful for astronomy in many cases.
|