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.

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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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