Ever Get a Filter Stuck on Your Lens?
Sooner or later, everyone does. The pitch of filter threads is quite small so even a tiny grain of dirt can foul them. Even without any help you can accidentally cross-thread them. The big question though is not how they get stuck, but how you free them once they do.
Camera stores are happy to sell you a set of two plastic filter wrenches specifically designed to address this problem. You use them by holding onto your lens with one wrench and grabbing the filter with the other to give you more torque to hopefully loosen the offending filter. Unfortunately, these things are made to fit filters ranging from only 46mm to 55mm which means they are far too small for most modern lenses. While I do own a few 52mm threaded lenses and a number of 62mm ones, most Nikon lenses are now 77mm in diameter. No way could I get one of these wrenches around them.
A homemade solution often recommended for getting stuck filters off is to substitute two large rubber bands for the filter wrenches. This sometimes helps since your hands won't slip on the metal surface of the filter and lens anymore, but I've found that rubber bands are a bit too narrow to have much to grab hold of. Instead, I carry with me two pieces cut out from rubber material designed as pads to stop carpets from slipping. This same stuff is sometimes also sold for use under your plates and dishes in the cupboard as padded shelf liner. If you're not familiar with this open-weave waffle-looking stuff, it comes in rolls of various sizes that you can cut down to fit your needs. Two strips go with me in my camera bag, just in case. I use even fewer filters than I used to now that I'm shooting digital, but I suspect I'll always carry at least a polarizer.
If you find yourself with a stuck filter in the field and don't have any rubber bands or pieces of carpet pads with you, you might want to try this nifty trick. Take off your shoe or boot and make sure it is clean. Then hold the filter and lens with the front face of it parallel to the sole of your shoe. Lightly press the rim of the filter down onto the tread and rotate the lens to try and loosen the filter. Be sure that the glass of the filter doesn't contact your tread, but try to make sure the rim is in contact in as many points as possible so you will have even pressure all the way around. While I wouldn't recommend this approach if you're not desperate, it may well come in handy if you are in a pinch.
Here are a few other possible ideas to try:
You can buy kitchen jar openers with various sized jaws lined with rubber. When you think about it, the problem of loosening a stuck filter is not too dissimilar to that of loosening a stuck lid on a jar. This may not be convenient to carry with you on a trip, but if you can wait to you get home to remove the filter, this could do it.
An ordinary electrical cord can be wrapped around the filter to serve as a makeshift strap wrench to get better torque while keeping the pressure around the circumference even. Be sure to unplug the attached appliance first.
Metal contracts when it gets cold. Unless your filter and lens are made from the same alloy, they may well contract at different rates. Putting both in the freezer for a while might loosen things enough that you can free the filter.
Sometimes, if you ever so slightly tighten the filter even more, that may be just enough to allow you to then work things free. Trying to jiggle things a bit while you attempt to unscrew the filter could help too on occasion. Probably couldn't hurt at least.
One thing I would not recommend is the use WD-40 or any other sort of grease or lubricant. Not only is it unlikely to help, it could create enough of a mess on the filter or lens that you will never get the stuff off. The lubricant can seep into the threads and show up on the glass long after the filter problem has been resolved. There are limits to even the best lens cleaners.
If all else fails, don't force things or take any extreme measures. Take the stuck combo into a camera repair store and get them to fix things. I had a lens hood stuck on a polarizer once and put the two of them into a plastic bag while on a trip. When I got home, I brought them into Camera Techs here in Seattle and they miraculously separated them within a few minutes for no charge. While I am a regular customer there, this was still quite nice of them. I asked them how they did it and they answered that it was a "trick of the trade." Sometimes it pays to go to a trained professional I guess.
Update 8/23/2005 - Whereas I do mention pressing the rim of a stuck filter face down against the rubber of your boot sole if you are stuck (so to speak) in the field, I never mention the more sane approach of pressing it in a similar fashion against a rubber pad if you are at home. This can indeed be a good method since it keeps the pressure evenly distributed. Most methods of grabbing the stuck filter on the sides will result in uneven pressure. This will compress the filter where you are grabbing, and cause it to bulge outward (against the opposing threads of the mated lens), and thus defeat your efforts to loosen it. Filter wrenches are designed to minimize this problem. And by the way, while they are not easy to find, they do indeed make filter wrenches large enough for modern lenses. These ones at B&H for instance are sized to to work on lenses from 62mm to 82mm.