Filters for new 300mm L?

<p>I need a circular polarizing filter for my 300mm L lens. The 34mm drop-in filters are like hens' teeth so I was wondering if it is possible to fit one on the front of the lens. I read that the front diameter is 78.5mm so non-standard but I have found a machine shop that can make me a step-down to 77mm. Is this a feasible option? Has anyone tried it?</p>

<p>FYI...what looks like 'threads' on the front of Canon's large lenses like the 300 are not screw-type threads...just concentric circles as light reducing baffles...not designed for filters to screw into them.<br>

One of the clamp on filter frames (or DIY) with drop-in square filters might work.</p>

<p>Ah, thanks for the heads-up. I'll investigate the clamp-on frames. </p>

<p>My idea (for sometime in the future) is to cut out the face of an extra lens cap I purchased on ebay and then screw in/tack weld on a filter ring (without glass) on to the remaining ring of the cap. Then I will have a slip on filter holder...</p>

<p>I read an internet post a while back about a guy that had a thread adapter made so he could use front filters. Very specific how he did it.<br>

Cant find it now but its on the internet.<br>

The OD of a 77mm filter is slightly smaller than the inside of the lens food so maybe you could do something with that.</p>

<p>Large-format photographers often face problems like this when they're using old lenses. If you're looking for a cheap solution, you can make a jury-rigged adapter using stepping rings, series adapters, lens caps, and epoxy. It doesn't matter if it's ugly, as long as it works and it doesn't damage the lens.</p>

<p>If you want to use a conventional round filter, an alternative (but expensive) solution would be to get your machine shop to make a slip-on adapter that goes over the outside of the hood or housing, with a 77mm or 82mm female thread on the other side.</p>

<p>This would be a bit pricey, probably $150 - 250, but it's a thought. One company that does beautiful work of this type is <a href="http://www.skgrimes.com/">S K Grimes</a> in Rhode Island. They could probably craft it out of hard rubber or plastic, so it wouldn't scratch your lens housing.</p>

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<p>34mm drop-ins : If you can find just one 34mm drop-in and wish to change the glass, you can remove the original glass (the filter unscrews) and replace it with any Tiffin 37mm round filter. I've read that the fit might be a tad loose. When I tried it the fit was not loose, but I understand that a small drop of rubber cement will secure it if it doesn't fit securely. </p>

<p>That would be the optimum solution. I just removed the 1x (placeholder) filter from the lens and I'm looking at it now. The glass will rotate in the holder but it doesn't unscrew. Are the Canon additional filters different from the placeholder filter?</p>

<p>If the filter's not in a threaded mount then I suspect the glass is held in with a "c"-type wire retaining clip that might look something like one of these:<br /> <img src="https://www.mikesxs.net/parts/img600/27-1063.jpg" alt="" width="600" height="450" /><br /> <em>Usually </em>you can just pry out a retaining ring like this using some type of pick or small screwdriver, and then the glass just pops out. I've done this before myself without too much trouble, but I will say you need to be very careful! It is easy to slip with the tool and scratch the glass or to put too much stress on it and break it.<br /> <br />In any event, it seems like the bigger issue is how to make the filter easily rotatable. If the glass can rotate around in the standard holder as you said, I suppose the simplest plan would be to just remove the entire holder each time, sight through it visually while setting the clocking for the effect you want, and then re-insert it. That's certainly inconvenient and fiddly, but maybe your only choice unless you can find a way to MacGyver you're own drop-in mount that would allow you to rotate the filter without removing it. I can <em>imagine</em> all sorts of ideas to make that work, but it would require some good machining/fabrication skills.<br>

<br /> Jeff</p>

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<p>The glass in the 34mm drop-in filters are not threaded filters; however, the 48mm drop-in in filters are special, thin 48mm filters threaded in. I am not sure how the 34mm filters are mounted but Canon did make a gel filter holder for the 34mm drop in filter (not sure about the 48mm drop-in filters). When I bought my 400mm f4.5 from KEH, it came with the gel filter holder (without a gel filter) installed instead of a filter so I had to buy a 1X filter.</p>

<p>Although the 48mm drop in filters are screwed in, they are not standard thickness filters although they are standard filter threads. I found this out with a little experimentation: I found a set of Canon 48mm filters on ebay for cheap just to see. They threaded in just fine but the holder would not fit the slot with the standard filter.</p>