making a variable ND filter

4 replies [Last post]
Claire
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Joined: Apr 28 2001

Has anyone tried making a variable ND filter out of a couple of ordinary circular polarisers?

I did this today using one Jessops ordinary screw in circular polariser bought years ago together with an old Cokin circular polariser and it's square holder (found in the loft!). I mounted this in front and it all worked, rotating the Cokin filter gave me variable exposure. I was then able to drop my shutter speed on the GH3 to more sensible figures for video recording in daylight and also gain stepless control over the light level.

All except for the colour! This had dropped away horribly,  not sure why, anyone know?

Claire

tom hardwick
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Joined: Apr 8 1999
Re: making a variable ND filter

The colour bias (I assume you mean by 'dropped away horribly') is due to the fact that your neutral density filters are simply not neutral; they're not blocking the colour spectrum equally across the spectrum.  It's a very common problem, and the less you pay for your NDs, the less N they are.

tom.

Claire
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Joined: Apr 28 2001
Re: making a variable ND filter

Hi Tom, thanks for the reply.

I got the impression the whole colour saturation had dropped away rather than a colour shift but after you mentioned it I had another good look. Edius vectorscope helped me see it's actually the blue channel that is attenuated leaving dark muddy reds and greens. Oh well, you are right it seems. thanks. Well, it was only an experiment.

After doing some reading I see there can be quite a hit on quality with a variable ND as opposed to some fixed ND's so  I have now ordered a couple of  Tiffens (0.6 and 0.9).

The GH3 won't go lower than 200 ISO, do you think these sizes will be strong enough for shooting video in summer sunshine at  1/50 or 1/100 shutter speeds?

PS.. my lenses will be F2.8, which I will obviously be wanting to use for narrower DOF purposes as and when required.  

Claire

tom hardwick
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Joined: Apr 8 1999
Re: making a variable ND filter

A 0.9 ND is only 3 stops and as you don't want to stack filters (do you) then I suggest you get a stronger one. The small chip in the GH series needs all the help it can get in the limited dof stakes, so you'll be wanting to shoot wide open as often as possible. ISO 200 is high and 1/50th is low

Claire
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Joined: Apr 28 2001
Re: making a variable ND filter

Thanks for the advice Tom.

Claire