How do I rig up a color filter for my Sigma 8-16mm Ultrawide?

Question

I am a bit confused... I'm not sure if the cokin system will work on my lens, as my lens doesn't have threads for a filter.

I've found that placing a Z series cokin filter in front of the lens will cover the area of vision on my D5100, but I'm not sure how placing a flat piece of glass or plastic in front of the lens will affect the optics (vignetting, bending, sharpness), as this lens is seeing much more than what's directly in front of it, even though the filter does cover the vision area.

Is there an ideal distance the filter should be from the lens? Or perhaps if I created a bubble shaped filter not just in front of, but extending behind the lens? Something just feels wrong... the front of my lens is a circular shape. And it's looking out of a flat piece of glass placed in front of it.

I'm just trying to place a blue filter in front of my camera so it captures moonlight at night. And I do not want to use white balance or Photoshop to mimic the effect. Thanks a lot for the help.

Answer 1

In fact there are threads on the Sigma 8-16 mm, but they are located in the slide on barrel that you mount over the petals of the built in lens hood. These threads are used primarily for the lens cap but can certainly be used for filters too. The downside of this is that you'll get vignetting in the short end of the zoom range and will be forced to remove the tube and the filter along with it.

There are other Sigma fisheye lenses that has rear gel filter holders that allow you to use gel filters that you cut according to the metal template included with that lens, but to my knowledge the 8-16 mm doesn't have this feature. I guess that you can engineer your own rear gel filter holder for your lens given there is enough room for the mirror of the camera body, but of course this introduces some risk.

Answer 2

Well, if you're willing to take a small risk, you can rig up a filter at the rear element. It really does depend on the lens in question and how close it comes to the mirror, but basically it's emulating some very large lenses that can't have a front filter out of sheer barrel size and so have drop in filters at the rear. Just tape a filter on the back. Now, you need soft, flexible, filters for this, but gels are not that hard to come by and can usually be purchased in photography stores or ordered online.

A demo of this, for a 3 stop ND filter on a fisheye lens, can be seen on PetaPixel. Do at your own risk!

Answer 3

The Sigma 6-16mm is threaded on the rear. You would need a filter that is threaded on the outside. It also isn't clear how you would get the filter out once you installed it. If the filter ring had tabs on it you could use that to gently screw it in and out.

I'm not sure where you would buy such a filter!