Where to find rear slot filter

[OldFox]

I have an EF17-40mm f/4L USM. In the specs reads:

Gelatin filter holder provided at the rear
The lens has a drop-in gelatin filter holder at the rear. The holder can holds up to three 31 x 31mm gelatin filters.

Where can get these filters? I have looked 'everywhere', but all that I can find are filters that put in front, not in the rear.

The one I am searching would be ND filter.

[PeterS45]

Take a look at this site http://www.fotoinfo.com/info/technicalinfo/gelfilters1.html for info on these kind of filters. Every good photoshop should be able to order them.

[slin100]

There's one thread on photo.net that says you simply take a standard filter and cut it down to size.

[scottbergerphoto]

I wasn't sure if you were looking only for where to get the filters or also where is the filter holder located. So, if the latter is true: On the 16-35, f/2.8, at the back of the lense around the opening is a black, metallic, incomplete, circular ring, held down by three small screws. There are two white corners stenciled on the black outer ring that has the contacts. These white corners identify the opening in the ring where you insert the filters. As already mentioned, you can buy a gelatin filter and cut it to size.
Scott

[Canuck]

This is not the best pic I have ever taken by any stretch of the imagination, but is for illustrative purposes. This was taken using the Sigma 28-80 F3.5-5.6 cheap as chips lens and it shows! I think it is relatively self explanitory. The 2 arrows are where the filter will use to stay put. This is the back of the Canon 16-35mm F2.8L lens.

http://images.fotopic.net/?id=2418964

[CyberDyneSystems]

Gelatin filters come in sheets,. and you cut to size.

I'm suure there are other brads as well,. but some names are Rosco, Lee, and I'm pretty sure Cokin makes then too???