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Using a Cokin X-Pro filter on the Sigma 12-24 lens

Since I first discovered how to fit Cokin P-series filters onto my Sigma 12-24 EX DG lens I have upgraded to a Canon EOS 5D, a camera with a full-frame 24x36mm sensor, which makes this lens really wide. However, there is no way that P-series filters are large enough. I spent a while using Lee 100mm filters, following a suggestion from a fellow 5D/12-24 user, Gareth Cooper, but was still restricted to using the lens in the 20-24mm range.

This page shows how, after considerable experimentation, I managed to get a Cokin X-pro filter mounted in such a manner that it can be used on a full-frame camera, such as the Canon EOS 1DS or 5D - or, even, a film camera. Also, fitting Lee 100mm filters to the same I can now use them down to about 16-17mm full-frame or the full 12mm on a 1.6x cropped sensor camera.

Firstly, I bought a replacement Front Cap Adapter, product code 512520, from Sigma Imaging (UK) Ltd. As at Jan 2006 the price was £19.99 including postage. Once it had arrived, the butchery began.

Firstly, I cut off the front of the adapter - using a hacksaw I cut along the line of the front one of the three grooves on the outside of the adapter. This lines up with just inside the front strip of felt on the inside of the ring. Once deburred this allows the ring to slide right back on the lens. The protoype involved simply filing the inside lip off the adapter but this was unsatisfactory as it was impossible to avoid some damage to the felt lining and made it a poor fit on the lens.

I then put a lip on the outside of the front edge, just small enough for the Lee standard filter holder to fit on and lock onto what is now the front groove on the sleeve. This lip was made from a ring of plastikard, backed with epoxy resin. Putting a sheet of plastic onto the Lee holder between the body and the filter guides with an appropriate sized hole ensures that the holder cannot slip too far back whilst the lip on the sleeve prevents it coming off on the front.

Finally, I glued four short strips of plastic to the sleeve barrel at 90 degree intervals, with a 5mm gap between them and the aforementioned lip. These will hold the X-Pro adapter in place.

 

This image shows the modified sleeve, with both the lip and the plastic lugs visible.

 

whilst this one shows the sleeve on the lens. It is slipped right back so that the front edge is level with the lowest points of the petal hood.

When used with the Lee holder it will not be necessary to slip it so far back but for the X-Pro it is essential

Next I made up a frame with a hole the right diameter to fit on the sleeve and cutouts at the 45 degree points to correspond with the lugs on the sleeve. On the extremities of the wings I built up blocks to which the armas of the holder will be mounted. I decided to make the arms detachable as the whole assembly wil be some 150 x 225 mm in size, a problem to store in the camera bag as one unit. The outside faces of the mounting blocks are 148mm apart. The mounting hole is lined with several layers of plastic to bring it up to just under 5mm and make it a good fit on the sleeve, whilst the edges of the frame are lines with more plastic to provide stiffening.

The following photos show the front and back views of the frame.

The next task was to make two arms to hold the filter. these are a box construction for stiffness and slot over the mounting blocks on the frame, fixed in place using two spare tripod mount thumbscrews I happened to have lying around. The threads in the mounting blocks are a couple of 3/8"-1/4" adapter that I also happened to have.

The arms of the holder are 225mm long and the top faces stand 14mm proud of the front edge of the sleeve when the holder is mounted on it. The inside faces of the arms are 128mm apart when the whole frame is assembled.

To the very ends of the arms I then fixed a set of Lee filter guides (one guide only), with a "pad" of plastic underneath to make the lower surface of the guides. The inner edges of the guides are 120mm apart which allows a slight "slop" for the filter that I received which turned out to be 128mm wide instead of the nominal 130mm, and the inside spacing from bottom to top of the guides is 3.5mm to accommodate the thickness of the filter. The inside upper edge of the arm assemblies and the inner ends of the filter guides are bevelled to ensure that they don't appear in the image area under any circumstances - the amount of bevel was determined by trial and error on the camera.

Here we see the filter holder assembled;

and here it is mounted on the lens, without and with a filter in place;

Once the construction was completed and I was testing it I found that it was possible for light to reflect off the back of the filter onto the front of the lens in some circumstances, so I made up two sheets of plastic which can be slipped into guides on the back of the frame when required, as seen here;

The final pic shows the side elevation of the completed filter holder mounted on the lens, it's a big beastie but I'm looking forward to using it ;-)

 

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All text and photos are copyright © Chris Maddock, 2006