Create Your Own Variable Neutral Density Filter
The following post was submitted by Matthew G. Monroe
Tucked away in my camera bag — just slightly behind the small headlamp that I always keep in there — are two of the most useful items in my photographic arsenal. No, it’s not a set of Pocket Wizards (pretty dang’ handy though), and it’s not the two-pack of Pop Tarts that I keep in the bag just in case I get hungry.
Without a doubt, the two most useful items in camera bag are…
A set of polarizing filters.
Specifically, I keep a really nice, really expensive German-manufactured filter in my bag — along with a really cheap, really lightweight polarizing filter that I bought second-hand for about five dollars.
On an individual basis, I think that polarizing filters are probably the most overused (and most inappropriately) used pieces of camera gear imaginable. Yes, they do have a time and a place — they’re an absolute lifesaver when photographing shiny objects or taking pictures of big puffy clouds. But honestly, there’s no reason at all to leave a dark piece of glass on your camera 24 hours-a-day, especially if you’re some wannabe’ photographer who’s trying to take family pictures in a dimly lit restaurant (not that I’ve EVER seen anything like that happen before).
The reason that I keep two polarizing filters in my camera bag — an expensive one and an “el cheapo” — is that when they are combined together (i.e.: stacked one on top of the other) they become the most incredible tool in my kit: a variable neutral density filter.
What is a Neutral Density Filter?
First off, a brief explanation… A neutral density filter is simply a dark piece of glass that’s placed in front of a camera’s lens, primarily to reduce the amount of light going into a camera. What a nuetral density filter (or “ND filter”) allows you as a photographer to do — that couldn’t be done otherwise — is to shoot in rather brightly lit situations, and yet keep a relatively slow shutter speed and/or wide open aperture. The reasons for doing this? Well, perhaps you want to shoot a landscape image during daylight hours, and for aestetic reasons you feel that a shallow depth of field is important — something that an aperture of f4 or so would provide. At the same time, it might be that a relatively slow shutter speed is what you think will make that shot work out best — let’s say around 1 or 2 seconds of exposure… The solution? A very dark neutral density filter cutting down on the amount of light going into your camera.
One big problem though… A dark filter over your lens means that you really can’t see what it is that your camera is aimed at, or even if you have your image in focus or not.
Hmmmm… Big problem.
My solution? Simple… Take two polarizing filters — one, a really nice, really expensive filter that also happens to be a “Circular Polarizing Filter” (a very technical term that again means “really expensive”) and stick that on your camera. Next, find a really cheap “Linear Polarizing Filter” at some camera store (you should be able to find dozens of these at any shop that sells used gear), and place this second filter (i.e.: “el cheapo”) on top of the first.
Now look through your camera…
Now spin that top filter around…
Amazing! You’ve just created a nuetral density filter which can vary the amount of light that it lets through, and you can vary that amount of light by spinning one polarizing filter on top of another. This is just like that experiment you did in your High School Physics Class, right? You remember… Two pieces of polarizing material? Set them off 90 degrees from each other and they go pitch black? Remember? Remember? Bueller?
Anyways, with your brand spanking new variable nuetral density filter, you’ll now be able to take relatively long exposures during the daylight hours (I get about 7-stops of range with my set-up) AND you’ll be able to actually focus your camera on whatever it is that is worthy of a long exposure. To focus, simply look through your camera’s eyepiece and then spin the top filter around (i.e.: “el cheapo”) until the image gets as bright as possible. Though it will be a bit darker than what you’re typically used to, you should still be able to see well enough to grab focus. If you’re an Autofocus kind of person, well, autofocus should do just fine also. Simply grab focus, and then turn your Autofocus off
Let me repeat this: Turn Your Autofocus Off
Now, just spin that top filter around until the image in your viewfinder is as dim as possible. Chances are pretty good that you’ll barely be able to see a thing.
Now comes the really hard part… Setting your exposure.
Exposure meters don’t work very well when two polarizing filters are stacked one atop another, and so you’ll probably have to do a bit of guestimating… Not a problem at all, in fact this an opportunity to really learn about the relationship between f-stops and exposure time. Take a wild stab at something — let’s say f5.6, with a shutter setting of perhaps 10 seconds — and then do a test shot. If the image on the back of your screen is overexposed, then just reduce your exposure time or tighten down the f-stops. By the same token, an underexposed image needs more light (duh), so just open up the aperture or increase the exposure time. With experience, you should be able to go from that initial test shot to an actual usable image within three or four clicks of the shutter.
Some basic things to remember are:
1) Have your camera in Full Manual mode… Please don’t use that sissy “Auto” stuff.
2) Use a tripod. Long exposures demand it.
3) Have your camera batteries fully charged. Long exposure work is a big drain on batteries.
4) It’s very important that the “Circular Polarizing Filter” gets mounted on the lens first, and that the “Linear Polarizing Filter” then goes on top.
5) If you really want to capture motion blur, then you’ll want to set your ASA as low as it will go. On most digital cameras, this is a setting of 100, though on some of the DSLRs that are out there you can only drop down to an ASA of 200.
6) Have fun. You’re doing all this work because you enjoy taking pictures.
Just as a quick aside, the two samples images shown above (i.e.: the waterfall photos) were shot near Portland, Oregon at world famous Multnomah Falls. An interesting phenomena that took place while I was shooting the 30 second exposure is that all of the fog and mist surrounding the falls picked up a noticeable greenish tone — a tint that was created by daylight being filtered through nearby leaves and vegetation. In the past, I’ve noticed that swirling fog does seem to pick up — and amplify — color tones during relatively long exposures, tones that aren’t nearly as noticeable during short exposures. Though at first this might seem to be a bit of an annoyance, it can actually work in one’s favor when photographing a foggy scene at-or-before sunrise. Again, just remember to have fun while doing all of this.
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February 26th, 2008 at 12:41 am
This would have been great on my hike yesterday. I came across some waterfalls that I wanted to photograph with a long exposure, but there was snow everywhere. In order to get the desired effect on the water, the exposure was too long to keep the snow from blowing out. I ended up bracketing the exposures and combining the images, but it would have been nice to use something like this in the field.
February 26th, 2008 at 1:28 am
the writer of this entry writes like a douche bag. your pictures suck along with this entry
February 26th, 2008 at 1:45 am
Very nice Idea! I will give this a try. The only concern that I have is, that I’m not sure whether the Autofocus will work correctly in this setup.
One of the reason, why linear polarization filter are cheap and circular are not is, that the linear filters do not work with autofocus cameras. Thus it might be possible, that with that setup AF will not be reliable. Nevertheless, it should be possible to focus manually.
February 26th, 2008 at 1:54 am
Now this is one fun article. I’ve been looking for ND filters for a while now..this may just tip my hand.
February 26th, 2008 at 1:57 am
Great post! I already own a CPL and will probably be picking up a Linear Polarizer to try this out! Thanks!
February 26th, 2008 at 2:49 am
I never knew what Variable Neutral Density filter was. After I found out, I could never remember it. Now I call it Lord of the Rings filter. Can you see why?
February 26th, 2008 at 4:33 am
This was a post with a lot of good information I didn’t know, and I really appreciate that. However, the derisive tone regarding “wannabe” and “sissy ‘auto’ stuff” is completely unnecessary and counterproductive. You’re sharing information which is great, so your audience is by definition those who know less than you do. Insulting the audience is a losing proposition.
February 26th, 2008 at 6:35 am
I still need to get me a linear pol filter for this.
Be aware that this technique can also cause a blue/purple colour cast, so it’s not really a true replacement for a Neutral Density filter - after all, it’s supposed to be neutral.
February 26th, 2008 at 7:16 am
If you’re getting a strong blue or purplish cast from the “Variable Neutral Density Filter”®, chances are good that at least one of the filters isn’t all that good or well made. You can easily eyeball the set-up when it’s off the camera just by holding the two filters up to a light — with the linear filter in front of the circular polarizing filter — and then just spinning them around. Be sure that the two filters are facing in the proper direction (i.e.: the true “front” of each filter is facing away from you) and that — again — the linear filter is in front of the circular polarizer.
If you have an interest in seeing really weird color shifts, turn the circular filters around so that its back faces the back of the linear polarizing filter — crazy, crazy yellows and blues.
One thing that I should have perhaps mentioned while writing the article is that this is not the best set-up to use with wide lenses, as it’s very easy to catch the edge of the filter rings in all four corners of the frame. With my Nikkor 12-24mm lens, I will rarely go wider than 16 mm when using a stack of polarizing filters.
Matt
February 26th, 2008 at 9:54 am
So…what would happen if you used 2 circular filters?
February 26th, 2008 at 10:55 am
Great article Matt… it was Matt that wrote it, right? I didn’t see any introductions and the voice of the article definitely sounded different than Darren’s.
At any rate, this is an awesome little project for the filter-lovers. I was actually just having a discussion with another photographer about ND filters and I revisited the ol’ VariND that still goes for $300 or $400. That one is terribly difficult to justify, so this polarizer trick might actually be a good alternative… especially since a quality polarizer is a good one to have in the bag anyways.
February 27th, 2008 at 12:58 am
I am trying to find a photo lab to print my pictures and am having mixed results with quality, price, and service. Who are the pro’s using to print the pictures they are selling?
February 27th, 2008 at 1:34 am
Gwegner: Autofocus will work just fine so long as the linear filter is in front of the circular filter.
Pablo: Hmmmm… If you were to ever meet me in person, I speak exactly the same way I write (which is either a good thing or a bad thing). Unfortunately, stuff like ton and inflection doesn’t come across very well in the written word, and so I do apologize if a phrase which — when spoken in an obviously joking manner — doesn’t translate very well to the printed page. On the other hand, anyone who walks around with a polarizing filter on their camera 24 hours a day (which I have seen more times than I can remember) does deserve a bit of ribbing.
god: The first letter in any sentence is always capitalized… Just a quick writing tip.
Matt
February 27th, 2008 at 4:24 am
I agree, many a time business has been lost and people almost fired in the company I used to work for because the creative department dont really care about email etiquette and dont realise that the nuances of their jokes and normal speaking style doesnt translate well.
February 27th, 2008 at 4:26 am
Matt, as much as I admire professional photographers like yourself, many amateurs, including myself read this blog to learn more and many of use do use the automatic mode when the surroundings allow it because we want to get the best shot possible and our skills don’t permit us to use the manual settings properly. Having said that, I shoot manually like a douchebag.
February 27th, 2008 at 5:13 am
M: Regarding e-mail etiquette and nuance: Point taken, and appreciated.
Regarding manual versus auto modes: The beauty of digital photography (and I’m a guy who has shot a lot of film) is that — under most circumstances — there really isn’t any reason to be shooting in full auto mode. That LCD screen on the back of the camera is one of the best light meters ever made — better than most incident and in-camera meters that I can remember — and the feedback you get from it is immediate. Yes, under rapidly changing light conditions there are certainly reasons to be in some sort of automatic setting. But when shooting long-exposure images (which is what a neutral density filter is primarily used for — and what the article was basically about), the automatic modes are more than likely to screw your images up. More importantly, it’s good to work in manual mode because it teaches — and reinforces — the relationship between f-stop and shutter speed.
February 27th, 2008 at 9:28 am
Good point Matt.
As you’re not paying for film, shoot away. If the point of this site is to learn, let’s stick our necks out and learn photography! I too am a douche bag and shoot either in M (manual), A(aperture), or S(shutter). Frankly, I detest auto mode. You might as well not own a DSLR and get a nice Lucia or canon PS. so let’s learn something new!
If I slap a polarizing filter on top of an ND filter what kind of results can I expect? I see from the link that the more F-stops (thats stops-up, right?) you go the more the color(blue, purple, etc…) changes.
http://www.dimagemaker.com/article.php?articleID=1201
So, if I start with a ND filter that is good for 2-4 stops will the polarizer make a difference?
February 27th, 2008 at 9:30 am
sorry, thats Leica.
February 27th, 2008 at 10:02 am
That’s a great tip. I will definitely try this. Both photos are beautiful.
February 27th, 2008 at 12:42 pm
YOU STILL WRITE LIKE A DOUCHE BAG. THE PEOPLE HAVE SPOKEN (PABLO, AND M.)
thanks for the writing tip but I’m still not going to capitalize my letters after a period. just for you buddy. learn some etiquette. degrading the people that take their time to read your crap won’t make you look any cooler. sissy.
February 27th, 2008 at 3:02 pm
oh my god.
LOL
February 28th, 2008 at 2:25 am
I’ve seen pictures of this scene before. Where is it located/what’s its name?
February 28th, 2008 at 7:19 am
god: I dont see how insulting Matt and namecalling puts you above him. If you don’t like the blog feel free to leave.
great article and BTW my english is not good enough to be offended by the jokes in it. :) I want an ND filter on my lens all the time it looks soooo cool. like the lens is wearing sunglasses :D (or sunglass or whatever)
February 28th, 2008 at 10:53 am
Hey god, it’s the new millennium, come in from the old testament, would ya? Didn’t you die or something? And how very ee cummings of you, refusing to use proper written grammar and whatnot. Nice.
Anyway, good article. Quite informative.
February 29th, 2008 at 9:32 am
A couple things:
1st: Sticking cheap glass on a nice lens, negates having nice glass.
2st: Doubling your filters will cause vignetting on a wide-angle lens.
If you’re going to spend the money on a DSLR, spend the money on a a ND and GND. It’s not like they’ll break the bank.
February 29th, 2008 at 9:33 am
very interesting! i’ll have to play with this. too bad there are no waterfalls around here…
February 29th, 2008 at 10:58 am
very informative!
February 29th, 2008 at 4:53 pm
Nice tip, but would like to ask the ff:
What would be the effect if 2 CPLs were used instead of 1 CPL + 1 Linear ?
March 2nd, 2008 at 3:13 am
Interesting article, sound like a great idea. I too would like to know what the impact of having two circular or two linear filters would be.
March 2nd, 2008 at 3:43 am
Sorry that I haven’t followed up on comments in a while. I’m on the road right now (Southern California), and just haven’t had a whole lot of free time.
Rungakutta: The location is Multnomah Falls. It’s located in the the Columbia Gorge, in the Northwest portion of the United States, on the Oregon side of the river, about 35 miles from downtown Portland, Oregon.
Balls: Cheap glass does not negate nice glass. Using bad glass (i.e.: scratched surfaces, uneven coloration, etc) does negate bad glass.
Also, doubling filters on a wide angle lens will not automatically cause vignetting. It might cause vignetting depending on how wide of an angle you end up using, though I have no problems at 16 mm, and I consider that to be a decently wide lens setting.
didjman: Two CPLs? Not much effect… That’s why a linear filter is placed in front.
Matt
March 24th, 2008 at 11:15 pm
Note: 2 CPL’s will work the same way - just reverse the front one (furthest away from the lens)
April 25th, 2008 at 6:48 am
I just bought a canon rebel xli and didn’t even know what to do with the filter that came with the camera so this is good info for me. I cannot believe that you guys are arguing over grammar, that is ridiculous. I am hear to learn how to better utilize my camera, not how to use capitalization.
May 2nd, 2008 at 3:04 pm
AMAZING,
did you know that Singh-Rays Variable neutral density filter costs 369 dollars!
May 30th, 2008 at 3:45 am
Not sure if this thread is still alive or not, but a couple of points come to mind:
- Are the color shifts correctable in Photoshop? At what point do they become irrecoverable?
I’d personally compose, pick the focus points I want and auto-focus on them, then move the lens into manual mode, and leave it there while I recompose (all atop a tripod of course) and fiddle with the filters. Not sure if this is clear in your article because of your emphasis on “turn autofocus off” that stands out on its own
June 11th, 2008 at 1:44 pm
@Ner:
I’ve experimented with this a little bit. I found color shifts weren’t a problem until I started slowing things down past 3-4 stops. After that, it quickly becomes apparent. It is correctable to a certain extent in pp, but I found it difficult to completely remove.
I think the point where it becomes unrecoverable depends on the subject matter and the color of the shift itself. I’d imagine a green color shift in a forest photograph would be easier to deal with than one of people. I also imagine that shooting in b&w might help…
June 12th, 2008 at 9:45 am
Maybe I’m a Luddite, but I’m still shooting film with a Canon AE-1 Program film camera (circa 1984). I already have a Cokin filter set-up and a rotating polarizing filter. (I don’t leave the filter on all the time.) It looks like this idea out to work just as well with a film camera. Any thoughts?
June 14th, 2008 at 10:22 am
I got some results with 2 circular filters. But the front one had to be turned around (with thread looking out) and there was strong color cast. So I guess for BW only.
June 25th, 2008 at 6:07 am
Wow! Who thought a wee bit of humor could cause a flurry of attitude driven responses. Matt is a very talented, funny, and sharing kind of dude. He’s more than happy to open up with any knowledge he has. And that’s the key–it’s only knowledge. As a–what seems like–life long photographer I am fully aware that knowledge can only take you so far. At some point wisdom needs to(hopefully)seep forth and the combination can take you to unthought of heights in any discipline, especially photography.