My solution to Filter Problems for the Canon FDn 300mm f/4 Lens
Rodney M Letts.
rodletts@gmail.com
This is a great telephoto lens to use with mirrorless cameras. It weighs under 1000grams, uses rear earth glass and is sharp as a tack. The effective focal length when used with an APS-C sensor camera gives it a crop factor of 1.5 and hence an effective focal length of 450mm. The lens comes with a detachable tripod mount. It is fast enough to use handheld with care and makes a great telephoto lens to use at night if you bump the ISO up a few stops to around 800 or so, most decent digital cameras have low noise sensor performance well beyond 800 ISO.
You can still get hold of this lens in mint condition for around $250 on ebay but as always, expect to pay a lot more for the L version.
OK now for the bad news:
This lens uses 34mm diameter exclusive filters and usually comes with the X1 (clear) filter, these filters are difficult to get and are expensive, just to make matters worse Canon designed the front filter thread to be 78.5mm which is non-standard and now impossible to get any filters of this size or even step-up or step-down rings to interface with it.
My solution to the problem:
I wanted to use all sorts of add on filters and supplementary close-up lenses and started to look into machining up a suitable adapter, after drawing up a few mechanical drawings it soon became apparent that this was going to be a fiddly job and it looked like I might get 90% through it and have a disaster with the rather delicate machining operation and perhaps finish up with a crumpled mess; so I put it on the back burner.
Just by chance I needed to buy some lens hoods for a few of my other Canon lenses and concluded that the rubbery ones on ebay were the best value for money, as the original Canon FD ones were way overpriced. Finally the lens hoods arrived and looking at the 77mm diameter one I noticed that the metal ring had both a male thread that normally screws to the lens you want to use it on but it had a female thread on the other side of the ring. Between these threads was a shoulder that measured 79.7mm diameter. Hmm the light went on, if the 77mm male thread was just undersize to fit my 300mm lens, perhaps I could turn the ring down so that it was a snug fit leaving the 77mm female thread facing out from the lens so that standard 77mm diameter filters could be screwed on as required. Carefully machining the 79.7mm diameter down to 77.40mm produced a neat fit into my 300mm lens and still allowed the built in lens cap and hood to work as the adapter OD is smaller than the ID of the cap or hood. I used a few drops of Loctite 406 Superbond to permanently fix it in place on my lens. Cheaper Ethyl cyanoacrylate instant adhesives are available but I use this one because it works really well and is worth the extra $s. Dont get it on your fingers or on the lens glass, you only need several small drops on the ring before gently pushing in place and make sure you get it on straight. Try the ring for fit prior to putting the adhesive onto the ring because once its in place thats it. If everything goes to plan its ready to use straight away. Check that the lens hood slides over the adapter, it should even with a 77mm filter screwed onto the front of the lens.
If you dont have a lathe, I may be able to supply these rings already machined to size at a small cost plus postage if there is enough interest. You supply your own instant adhesive.
My total cost was $28 and could have saved about $20 if I had used a lower cost adhesive.
77mm Rubber lens hood $4, Loctite 406 $23.86
Setting up the lathe and measurement time etc. was about 15 minutes.
