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Simple Variable Neutral Density Filter

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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!
 
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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.
jtorcello3 months ago

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

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 :)
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 (author)  akshayverma11 year ago
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?
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.
ykina2 years ago
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?
vader99002 years ago
Made one today for my 50mm f1.4, man I'm gonna have fun with this!
5810036322_d791c46833_o.jpg
arijbrown3 years ago
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.
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 (author)  arijbrown3 years ago
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...
dave774593 years ago
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 (author)  dave774593 years ago
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...
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.
tonebytone3 years ago
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 :-)
In Canada it is common to find Linear Polarizers cheaply on the clearance tables in camera stores. I just bought a 62mm B+W LP for this project for $20. Thanks for the idea. I could never afford a SinghRay, although I've drooled on one.
flamekiller3 years ago
Awesome instructable! I don't think I've actually heard of variable ND filters before, but I may have to make one for myself. I need a CPL anyway, and I found myself really in need of an ND trying to photograph waterfalls last weekend. Sounds like a nice easy project to undertake, that ought to come in cheaper than a 4-stop fixed ND anyway.
I had the same idea some time ago, and I've built this cool variable ND filter with two circular polarized ones. But I've noticed that when I exceed 80-90% of darkness, I obtain a strange effect in the photo: I see a big blue X shape all over the image... that could be because my filters are cheap. I'm interested to know if someone other had the same problem. I'll post soon an image to show you the effect. Bye
Two questions: (Assuming you have flipped one filter) Can you see the big blue X when you just hold the filter up in front of your eye? Does the X rotate when you rotate the filter on the camera? If you say yes to the fist question the X is almost certainly due to a poor quality circular polarizer. I see all sorts of odd colored effects when I pair up different combos of my collection of cheap polarizers. If you say yes to the second question you are probably seeing birefringence in one of the camera lenses (or filters - if you using a UV filter under this). Remove filters or try a test on a different camera. This is the main reason I use linear filters for this trick (btw when I got my LP they were cheaper than CPL). Light going through nearly crossed linear polarized filters is twerky enough let alone crossed circular polarizers. Expect to see all sorts of odd effects in transparent objects and reflected light. (Especially coatings like on those on lenses or in cheap plastic filters). I wish I had a budget to pick up some good quality polarizers to confirm if some of the odd things I have seen are not a problem with the better quality ones.
Thanks guys, for your exhaustive answers. I'll shot again a photo to the sky in these conditions, because I don't find anymore the other one. The effect I remember appeared only when the two filters are almost perpendicular one to each other, so with almost black surface, and very long exposure time. So this is not a problem because I rarely need so much darkness. I didn't remember if the effect is visible in front of the eye, I think not, because it's too dark. I didn't use other filter together with the two CPL. I'll try to rotate the filters together next time. Thanks, bye!
Not sure what causes it but you could probably correct it by taking a flat field image (uniformly lit across the frame) and processing in Photoshop or Gimp. 1.25" versions of this filter are sold as "variable moon filters" for telescopes. The Moon can be overly bright when looking through a large aperture. Great 'ible!
Zephyris (author)  andrea biffi3 years ago
I have a feeling that is to do with the angles the light is entering the lens; did zooming in reduce the effect? It probably isn't to do with the quality of the filters as I didn't see it and these were as cheap as they come...
Light_Lab3 years ago
Forgot to mention there is a way to check the quality of a CP. Hold it up in front of your eye (the right way around) and look at a mirror. A perfect CP will be so dark you can't see your eye in the reflection. If you can only get a slightly darker image or colors it is low quality. Remember to take a pocket mirror with you to the camera shop :-).
andyk753 years ago
Why do you need to turn the filter anyway? Use old linear polar filters! If you take two old linear polar filter you can just screw them together and have the desired effect. If you take expensive circular polar filter, you need to turn the filter in one of them, practically destroying this one for further use. But nice idea anyway!
Zephyris (author)  andyk753 years ago
If you use two linear filters you will confuse the autofocus and metering mechanisms in digital cameras. If you have one linear filter floating about you can use it for the front les with the same effect (without having to flip the filter), it is quite hard to get a linear polariser cheaply if you need to buy one...
You are basically very correct but have you actually tried a LP on your camera. When I got a few cheap LP's a couple of years ago I was amazed how many cameras they work fine on (actually better than the cheap CLP). There has to be a front surfaced mirror involved in your cameras optical path before a LP will mess up the automatic metering. In my experience only about 1 in 10 camera models have this problem.
Light_Lab3 years ago
I have been using a trick like this for many years; I use two linear polarizers so I don't need to flip one lens. One trap with all crossed polarizers though is, if you are shooting water falls you loose a lot of sparkle and shine. I have quite a few circular and linear polarizers, all low cost ex China. I have noticed that the cheap linear ones are far better than the cheap circular for everything from darkening skys to beating reflections. The quarter wave plate in circular polarizers is wavelength (color) dependent. Various color corrections are made and this is where the quality suffers with the cheaper circulars. Many people use circular polarizers that don't have to, it all depends on if your light metering is off a mirror in your camera. If you get a chance try a linear filter and see how it works. All my cameras work better with the linear polarizer.
very nice. But the perfect usage of this is on water. i rate ND filter as water filter. The perfect matching conditions for this filter are on beaches, waterfalls and lakes. But are you sure that it gives 10 stops under exposure? if it is so than you have done a great job. Because 10x ND filter is very expensive.
To put it simply I can use a 40 second exposure in direct summer sunlight with the widest aperture! It does give a distinctly blue hue with the two filters fully cross-polarised though...
Gene3 years ago
After reading this article, I went looking and found that you can get brand new Tiffen linear polarizers for very cheap from Amazon ($20 for a 77mm, was something like $8 for 62mm.) Use one in front of a circular polarizer, and you can keep your AF and metering without having to do any surgery.
yamahito3 years ago
Nice. You can also get away without modifying the front polariser if you can get your hands on a linear rather than circular polariser. However these are quite rare these days as they would normally interfere with TTL measurements.
Zephyris (author)  yamahito3 years ago
If you can get a linear polariser then that is ideal; you will find that the quarter wave plate on the back of the circular polariser will allow the automatic metering etc. to work fine. Linear polarisers are really quite rate these days, the cheapest I could find was around £25 which defeated the low-cost aim!
A.Mac Zephyris3 years ago
Considering the Singh-Ray variable ND filter runs between $350-$400, I think £25 still counts as pretty low-cost. Regardless, thanks for the 'ible
Silence3 years ago
This is brilliant. Ive been looking at ND filters and the prospect of carrying around a half dozen filters and snapping them on to the desired stop seemed a little inconvenient. I'm gonna have to try this once i get some bills outta the way.
CyberBill3 years ago
This is great! I've got a 16" telescope that nearly burns my eyeballs out of my head if I try to look at the moon or Venus or other bright objects, so using something like this (with a smaller filter set) would solve my problem! :)
lennyb3 years ago
i remember reading somewhere once about a high speed movie camera shutter made of polarizing filters. the speeds available in such a camera were ridiculously high{if i remember correctly} but the drawback being it needed a hell of a lot of light. i was going to make a simple shutter for a homemade view camera like this but managed to find a 100 year old lens shutter set to use instead{much cheaper than cpol filters}
Zephyris (author)  lennyb3 years ago
Are you thinking of the rapatronic camera? The shutter speeds with those are pretty insane! http://en.wikipedia.org/wiki/Rapatronic_camera
lennyb Zephyris3 years ago
yup thats the puppy alright. thanks for digging up the reference for us. well they certainly had enough light to expose that seeing as how the picture in the wiki is of the first few billionths of a nuke going off. by the way the shutters had to use linear polarizers instead of circular polarizers. i should have mentioned that in my post above but i didnt notice until just now. shouldn`t write theese things when im sleepy i guess.
guitar753 years ago
Thank you for this. I had an extra filter collecting dust on my desk. I performed your modification and it worked like a charm. Looks like that filter is going back into my camera bag!
I agree with Andy. Unless the camera has some funky gismos which detect circularly polarized light, there would be no reason to bother with the headache of circularly polarized light. Furthermore, two linear polarizers aligned would have the same transmission as one, so they could be permanently attached without worry.
Zephyris (author)  Ellen the Generous3 years ago
The reason I used circular polarisers was because linear polarisers mess with the autofocus and metering on most digital SLRs, and circular polarisers are more commonly available and cheaper. As I said to andyk75 the ideal setup is a linear polariser then a circular polariser, you just can't get hold of linear polarisers cheaply these days.
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