Using filters on a Sigma 12-24/f4.5-5.6 DG HSM Lens

This lens is the new ultra wide-angle zoom from Sigma, which roughly equates to a 19-38mm lens on a Canon D30/D60/10D with the 1.6x crop factor. Unfortunately, being such a wide-angle there is the problem of using filters without vignetting.

As sold, the lens has a fixed "petal" hood and is supplied with a removable circular hood that holds the lens cap in place. This removable hood is just short enough to avoid vignetting on a 1.6x crop camera so the lens can be used on such a camera with the removable hood in place. The hood is fitted with an 82mm filter thread to retain the lens cap, but this does not mean that filters can be used with it.

After some experimentation, I found that the removable hood only needs to be slipped forward from its "home" position by as little as 0.5mm and it starts to show up in the extreme corners of the images. Fitting a filter to this will reduce the internal diameter of the front of the hood and will make things worse, vignetting even with the hood fully located in its furthest back position. This will apply to both normal and "thin" filters - not good news for people like me who like using Polariser and Neutral Density graduated filters for landscape work.

So I started experimenting further and found that;

1. A Cokin P-series polariser (or a Kood equivalent - they are cheaper, LOL) will fit inside the fixed petal hood of the lens if approximately 0.5-1mm is trimmed off the filter all round and it will not vignette if it is located far enough inside the hood. All it needed was some form of holder.
   I made a sleeve to fit reasonably tightly inside the petal hood, 13mm long. When it's pushed right into the hood, the filter can sit on the front of the sleeve and is far enough back to avoid vignetting. It is also far enough forward that the edge can be accessed (to rotate it) over the shorter petals of the hood. I retained the filter with four guides located at the deepest points of the hood cutouts, so that the filter is centred, won't fall out, won't vignette and can still be rotated. Only two of the guides (opposite each other) need clips over the filter, so it can be "popped" out due to the flexibility of the sleeve but won't fall out of its own accord.
2. Cokin P-series 84mm square filters are large enough to use, provided they are fitted very close to the front of the petal hood. Time for another holder.
   This one is a sleeve that slips over the outside of the petal hood, with a thin lip to stop it sliding too far back. A sheet of material on the front has a hole slightly smaller than the sleeve diameter to form the lip and the outside is a suitable size to fix the filter holder slots. Using a sheet front also adds rigidity to the sleeve. I cut a slot through one side of the sleeve, so that I can still access and rotate the polariser if I wish to use both filters at once.

The material I opted to use is rigid polystyrene sheet (the same stuff as used for plastic model kits), which is easily cut and worked and easily glued using a solvent adhesive. In the Uk it's sold as Slaters Plastikard and is available in a wide range of thicknesses from good model shops or direct from Slaters.

I have included some photos of the finished (painting apart) holders below;

1 - Polariser holder on its own - the four guides, two with retaining clips can be clearly seen.

2 - Polariser holder with filter fitted.

3 - Polariser holder and filter fitted to the lens.

4 - Square filter holder - the filter guide slots and Polariser access slot are clearly visible.

5 - Square filter holder with a Cokin P-series ND Grad filter fitted.

6 - Square filter holder mounted on the lens, with a filter fitted

7 - Both Square and Polariser filter holders fitted to the lens, showing the polariser edge accessible
through the slot in the square filter holder.

8 - A different angle view of the above setup

Both these holders work well in practise - the polariser holder can even be left in place when the Sigma Lens cap adapter is fitted.

I have found that either filter will cause an increased risk of flare - putting a flat sheet of glass or resin on the front of such a wide lens is not the best way to avoid that - so care is needed, especially if shooting close to a 90 degree angle from the sun. Sometime I might experiment further with making a larger hood to reduce this problem.

Going even wider !!!! - Since I wrote this article I've upgraded to a Canon EOS 5D and have managed to work out how to fit a Cokin X-Pro filter in a similar fashion to use the lens at the full 12mm on the full-frame camera. This article describes how.

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All text and photos are copyright © Chris Maddock, 2004