Simple Variable Neutral Density Filter

What is a variable neutral density filter? The neutral density bit means it is a filter simply designed to block some of the light getting into a camera. The variable bit means it is variable - you can control the darkness of the filter just by twisting one part of it. A proper variable neutral density filter can cost £100 or more!

Why would you want to block light getting into the camera? In short; control. A fully manual camera can be controlled via exposure time, aperture size and film speed. Adding a variable neutral density filter adds control of the amount of light entering the lens too. This lets you increase the exposure time and/or aperture while using the neutral density filter to prevent overexposure.

What effects are possible? The main tricks with a neutral density filter are to get shallow depth of field (a wide aperture) or long motion blur (a long exposure) under bright lighting conditions. This makes it very handy for taking portraits or nature shots, where you have bright lighting but want a shallow depth of field, or capturing the feel of a public event while bluring out individual people as they move.

How does it work? This method uses the properties of polarised light, specifically that two parallel polarisers will block very little light but two at 90 to each other will block nearly all light travelling through them. Find out more about polarisation, including in photography, here. This variable neutral density filter is far from perfect, but great if you want to make one cheaply!

Step 1: Raw Materials

All you need is two circular polariser filters (of the correct size for your lens of course!). There is no point spending too much on these, I used an old slightly broken one and one I picked up for about £5 on Ebay.

Step 2: Dissasemble One Filter

Pick one filter and dissasemble it. Filters are normally easy to take apart; just unscrew the retaining ring from the front size of the filter. Most have a couple of small notches in the retaining ring which you can push round with a small screwdriver. Remember righty tighty, lefty loosey...

Be careful with this step - it is very easy to slip and scratch the filter. Make sure you don't do this with a polariser you care about!

Step 3: Flip the Filter and Reassemble

Flip the polarising filter and then reassemble the lens by screwing the retaining ring back in.

It is easy to check the correct orientation of a circular polariser if you get confused. Find a polarising light source, most LCD screens should work, and look through the filter while twisting it. For the normal orientation of a circular polariser if the filter gets darker and lighter then you are looking through the camera-facing side of the filter. If filter stays the same lightness but changes colour slightly (normally yellow to blue) then you are looking from the other side of the filter. For the reversed orientation of the circular polariser (for this step) the opposite applies.

Make sure you mark which filter has been flipped so you don't get confused!

Step 4: Final Assembly

Screw the two polarising lenses together making sure the normal filter is on the camera side and the flipped lens is on the other side.

If you don't plan on using the unmodified filter as a polarising filter on it's own then you can glue the two together to make sure you don't get confused. If you do want to glue the filters together be careful; to use the filter you have to be able to twist the front filter while the back filter remains stationary - don't jam the twisting mechanism with glue! Also avoid using cyanoacrylate-based glues (eg. superglue and krazy glue), their vapours can fog the glass.

Step 5: Usage

To change the darkness of the filter just twist the front polariser while keeping the back filter stationary. A twist of 90° will take you from maximum darkness to maximum clearness. My filters gave me about a 10 f-stop range, from ~4 f-stops darker to ~14 f-stops darker.

The light from the filters entering the camera is circularly polarised so should work with all digital camera autofocus and metering mechanisms. Unfortunately, because this method is based on polarisers, you will see some of the normal effects of polarising lenses - bear this in mind if you are photographing reflective objects such as glass or water. The filter construction is also quite thick so you might get more vingetting, especially at short focal lengths on zoom lenses.

Depending on the quality of the filters you might see some colour changes depending on the orientation of the filters, blue in one direction and yellow in the other. The blue tint can normally be countered with a fluorescent light white balance setting.

Step 6: Applications

This filter is useful any time you want to capture shallow depth of field or motion under bright lighting conditions, things you might want to try are:

A flower under bright sunlight - use a wide aperture and a dark neutral density filter to capture the flower, without overexposure, with a nicely blurred background.
People in movement - use an extremely long exposure time and a very dark neutral density filter to blur the movement of people through a public space.

Comments

[PrathmeshK1]

Is there X pattern visible?

(it's a very common issue even with expensive variable nd filters.)

[Zephyris]

I think it is down to aperture, larger apertures allow the light to pass through the filters at more angles and generate this effect. Smaller apertures avoid that effect...

[PrathmeshK1]

Nope. As I know it happens on wide angle lenses like at 18mm (that is what you get with the kit lens for APS-C sensor cameras.) If you zoom in the X effect will not be visible but it will still visible in some cheap variable nd filters. It's not related to the aperture. The main use of Variable ND filter is to open the aperture (e.g. f/1.8) to get shallow depth-of-field on a sunny day.

[28122914]

i have a high power laser of 15w as nd-yag Laser. now i want to decrease the power of this laser without changing in raputation frequency and current. can any one suggest me how can i do it ???

[amanieux]

i did it with 2 tiffen cpl and it worked well (i got between 2 and 10 stops) much sharper than my $10 fotga vnd. at first i tried to unscrew a cpl with a screwdriver but it was impossible to do - then i used 2 nails in a piece of wood (the sharp part of the 2 nails one inch out of wood so you can easily bend them to adjust the distance between the 2 nails. but i had a problem with b&w and heliopan : they don't have the 2 opposite ridges as in the tiffen cpl, does someone knows how to unscrew these kind of brass filters ? thanks (male to male ring adapter is a simple idea for a few bucks but it add some thickness to the final vnd filter so i'd rather revert one cpl)

[Jaswant61]

You can use a male-to-male reverse ring to mount one filter backward (instead of opening the mounting threadings). That way, both of your polarizers remain functional, if used individually. There are many sizes available on ebay for cheap. e.g.:

52mm male to 52mm male ring:

http://www.ebay.com/itm/Filter-Coupling-Reverse-Ring-Adapter-for-52mm-to-52mm-Black-/131163018425

52mm male to 72mm male ring:

http://www.ebay.com/itm/Male-52mm-72mm-52-72-mm-52mm-to-72mm-Macro-Reverse-Ring-reversing/290944033394?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20131003132420%26meid%3D27a7c67e765945f2abcc5c7a95240178%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D221282084359&rt=nc

[jtorcello]

Use a 77mm-82mm step-up adapter, and somehow stick or glue the reversed 77mm polariser into that?

[akshayverma1]

Yes, it happened! But, it took finger-bleeding-force to do it. It was easier to hold it with a cloth and push with the screwdriver at an angle, instead of down :)

[akshayverma1]

I tried a lot but am not able to remove the inside ring. I am trying to push it anti-clockwise with a small screwdriver, but it's just not coming off.

[Zephyris]

They normally just unscrew, but there is a possibility that your filter has a clip together design. Does it look like there is a screw thread?

[andrea biffi]

I'd forget to have already asked you an advice about variable ND filters, and I wrote an Instructable too. It's very similar, but thanks to yours I reminded to tell about reversing one of filters, if not anything should have worked! :-) I also retrieved my blue X shape photo.

[ykina]

Is there a difference between a linear polarizer and an SR linear polarizer?

In looking around online, I've seen both methods of making a variable ND - two circulars or a circular and a linear. However, using the linear/circular method seems as if it does not suffer from that "blue hue" curse (check this video for proof: http://www.youtube.com/watch?v=Jmwpi0RRuyo). I purchased a cheap 77mm linear and circular off eBay and when trying to combine the two, I only get a very weak darkening. Nowehere near the total blackness you see in that video, or with two circulars.
I managed to get my hands on another linear polarizer (unfortunately, it's a 49mm, so it's useless to me) and when I pair that one with my circ, it works! Looking at my 77mm linear package, it actually calls it an "SR Polarizer." I've found only a few uses of the term "SR polarizer" online and none that seem to dictate any difference between SR and linear.

Anyone got anything on this?

[vader9900]

Made one today for my 50mm f1.4, man I'm gonna have fun with this!

[arijbrown]

I built this with a cheap circular polarizer off ebay. When looking through the glass, the idea certainly works - it goes from almost clear to totally dark with a turn. However, somehow my camera won't meter through it. Is that to be expected? Makes it much harder to use. Also, the color cast I get isn't subtle. I could probably lose a few stops of light without ruining it, but when I stop it WAY down, it turns super intense blue - the kind I'm having a hard time correcting. Let me know if this is what you'd expect or if I'm doing something wrong.

[McGrep]

With polarizing filters, if they are oriented a certain way they can actually warm or cool the image. It may be because of different wavelengths of visible light being blocked (I assume). Experiment with the orientation and it should work better (e.g. turn one filter around, switch one's position with the other, switch both, etc.) Of course, this means just manually holding up the filters until deciding absolutely that you need to switch the threading like in the above Instructable.

[Zephyris]

I haven't experienced metering problems, not sure what could be going on there. As for the colour hue it depends a lot on exactly the filters you are using, with my particular combination it wasn't too bad - as I always shoot outside the blue hue can be removed with the fluorescent white balance. You could always try adjusting the RGB in the final image to manually fix the white balance...

[dave77459]

I use Cokin filters rather than screw on types. Can I simply reverse one in the mount to achieve this effect? I guess the problem is that the Cokin circular polarizers are very expensive. But they are handy for the variety of filter thread sizes I have on different lenses.

[Zephyris]

Any pair of circular polarisers with the front one reversed will work. If you are using a flexible filter system like that then it is probably worth trying to get hold of a cheap linear polariser to use as the front polariser, that will work just as well. The linear polarisers are just hard to get hold of for screw mounting...

[arijbrown]

Thanks for your feedback. Maybe it is just the extreme-cheapness of the filter or maybe I just have to use it at a less extreme setting. I think that if I was only dropping the light a bit the blue might have been correct-able, but at the far end, the only option is going to black and white. Still confused about the in-camera metering though.

[tonebytone]

I have a lens with a 52mm circular polarizer mounted on it. With the camera on a tripod, I held a 67mm CPL from another lens reversed in front of the 52mm one, being careful not to actually touch the larger glass to the rim of the smaller filter. I can turn either the smaller or the larger filter, depending on which is easier at the time, to the point where I get a totally black view thru the viewfinder. I have not yet made any images using the two CPLs. Doing it this way, tho, at least I do not have to destroy one CPL to make the variable ND filter permanent. I have a Kodak 13-stop ND filter which makes the scene in the viewfinder black. So I'm assuming that when the 2 CPLs are positioned to get the black-out in the viewfinder, this amounts to at least a 12-13-stop ND filter. BTW, the Kodak gel filter is not quite neutral, as it makes the images slightly sepia-toned - which is nice for some images. Anyway, thanks for this tip :-)