Tip - Variable Neutral Density Filter

[Lancashire Lad]

Earlier this week I decided to try something that I've had in mind for some considerable time - creating a fully variable neutral density filter from two circular polarisers. Useful for creating those "milky" water shots, or for allowing a lens to be used wide open (restricting depth of field) in very sunny conditions.

To do this, one filter had to have the glass "reversed", by unscrewing the retaining ring, flipping the glass, and then reassembling. (As this was to be permanent for this particular filter, I superglued the the whole thing together, to prevent the filter from being able to rotate when on the lens).

Now, the second, unmodified filter, was then screwed onto this reversed glass filter, (such that the reversed glass filter would be closest to lens), and only the unmodified filter would rotate when used on the lens.

Rotating the front polariser now allows "cross polarising", and can be incrementally adjusted to either allow or block out practically all the light passing through to the lens.

I'd previously read that this would normally only work if one of the polarisers was of the linear type, as autofocus may be affected otherwise.

Well - all I can say is that I've used two circular polarisers, and the autofocus still works fine on my camera with them used in this way.

I've attached some quick test shots below, taken just at the onset of a downpour - hence the raindrop flare, appearing in several of the shots. (No lens hood on at the time!). NB: the first photo, at 1/25th @ f8 was the unfiltered exposure setting.


01 - 1/25th sec. @ f8......02 - 1/3rd sec. @ f8......03 - 1/2 sec. @ f8



04 - 1 second @ f8........05 - 2 seconds @ f8.......06 - 4 seconds @ f8

Regards,
Mike.

[Tringa]

Interesting idea Mike that obviously works well. Why does one of the polarisers have to turned around?

Dave

[Lancashire Lad]

Originally Posted by Tringa

....Why does one of the polarisers have to turned around?....

Hi Dave,

If I remember correctly, it has to do with something called the "quarter wave plate" that will be etched onto the glass of each of the filters. (Obviously far too small to be seen by the naked eye).

I have no idea what it is, or why it works, ( a job for a quick websearch methinks!) - but you can easily check out the phenomenon by simply rotating one circular polariser whilst hand holding it in front of another one. You will notice only a very slight reduction in light levels when looking through.

However, if you then turn one of them "back to front", and rotate it whilst looking through both filters again, you will see that the light can be almost completely blocked out depending on how much you rotate the filter.

If you really take it as far as the light blocking will go, the photos take on a blue colour cast. At which exact point this occurs, I suppose will vary with different makes of filter. With the two I'm using, I've been able to go up to more than 20 second exposures before this becomes noticable.

Regards,
Mike.

[djeyewater]

Thanks for posting this Mike. You can buy these pre-built filters on ebay known as 'Fader ND', but they are quite expensive.

I'd like to have a go making one like you describe using a cheap polariser. Can I ask what brand / make it was that had a retaining ring you can unscrew? None of my current filters look like the ring can be unscrewed at all.

Thanks

Dave

[Lancashire Lad]

Originally Posted by djeyewater

....Can I ask what brand / make it was that had a retaining ring you can unscrew? None of my current filters look like the ring can be unscrewed at all....

Hi Dave,
One of the filters was a Hoya, and the other was a Jessop's branded one. (I modified the Jessops one as that was the least expensive, and I can still use th Hoya as a stand alone polariser when needed).

However, either filter could have been modified. - I'm not sure what type you have, but on both of these, when looking at them carefully, a small notch can be seen at either side of the inner securing/retaining ring. The notches are very small, only just big enough to get the tip of a penknife or something similar in, but the ring unscrewed quite easily. NB: only polarisers, which have the two glass plates will have these notches - none of my other single glass filters such as UV or skylight have them. (I suppose if you are intending buying for this purpose it would be best to check in store before purchase).

I suppose if you can't physically do that with your particular filters, you could acvhieve the same end result by purchasing a reversing ring (i.e a ring that has two male threads on either side of a central lip. That way you could simply screw the two filters face to face.

I have seen branded variable ND filters advertised but the cost was in the region of £200.00 each. - Although they purport to produce much better image quality than can be had by this DIY method.

However, the quality that I get is plenty adequate for my purposes, and since I had the two filters already, it didn't cost anything at all.

There is a slight loss of contrast as you may have noticed in the test images posted above, but that can easily be sorted by post camera image editing software.

Regards,
Mike.

[djeyewater]

Thanks for the info. I only have a B+W polariser at the moment, which doesn't seem to have any notches in it, and I wouldn't want to disassemble it anyway (too expensive). I will look on ebay for a cheap hoya or jessops filter.

While the expensive filters may be a bit better quality, I can't imagine they'd be much better than a DIY one made using branded filters (like Jessops and Hoya).

Dave

[Lancashire Lad]

Originally Posted by Lancashire Lad

....only polarisers, which have the two glass plates will have these notches - none of my other single glass filters such as UV or skylight have them....

I don't know what planet I was on when I wrote this - obviously polarising filters only consist of one plate of glass. (Mind you, I was up with a raging toothache at the time as a result of some root filling by the dentist!

What I should have said was only those filters with a fixed and a rotating ring would be likely to have the notches. The sheet of filter glass being held in place by means of a very slim screwed locking ring which has the diametrically opposed notches to enable it to be screwed into the outer part of the rotating ring.

Regards,
Mike.