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How To Create a DIY Low-Cost Neutral Density Filter


Neutral density filters are one of my favorite creative tools. With the ability to cut down light and stretch your exposure times for a creative effect, using an ND filter creates new opportunities to expose scenes creatively. However, their purposes are fairly limited and ND filters can be rather costly. Today, we'll look at how to construct a very low cost alternative.

Why Neutral Density?

Each tool in our photographic kit can open up new creative possibilities. Neutral density filters have the special ability of reducing the amount of light that reaches our camera's sensors.

If you aren't familiar with the creative applications of neutral density (commonly called ND) filters, it can be hard to understand why less light could be handy. If you're anything like me and love fast primes with wide apertures, it might be hard to imagine why you would want less light to work with.

Creating a long exposure in low light situations such as these is easy, but what if we wanted to stretch out our exposure time when we have "too much light" available?

However, it is occasionally true that we have more light than we want. Let's envision a scenario of trying to make a long exposure during the day. On a bright sunny day, even closing the aperture down very small to f/22 and reducing the ISO as low as it go may not give us the shutter speed we want. You may have never run into this, but the possibility exists that those settings still might get us a shutter speed of only 1/2 of a second - not usually slow enough to convey a sense of motion.

These situations are when ND filters are essential to achieving the outcome we desire. The idea here is that when we can't just use settings to reduce visible light, we can employ a filter to do the job. Any situation in which we need to achieve certain settings in terms of exposure and have too much light to work with is where neutral density filters are powerful creative tools.

This piece of welding glass (which smudges easily) is ideal for keeping costs low while creating a makeshift ND filter.

However, one hang up about ND filters is that they are fairly special purpose tools. If you aren't shooting long exposures in bright light everyday, you might not be able to justify the cost of buying one. That's why I'm such a big fan of using welding glass as a makeshift neutral density filter to achieve the same outcome. Welding glass is used by welders to protect their eyes from the sparks produced while fusing metal.

Getting the Tools

To get started with this project, I took to eBay to purchase the necessary welding glass. I found a shade made by a company called Harris. I chose the "shade 14" filter, which I found to reduce the available light by 18 stops of light. This filter cost me around $9, which is obviously less expensive than almost any neutral density filter.

This Harris filter worked well for my purposes. It's a shade 14 and cuts 18 stops of light.

Although there are a number of ways that you can put together this setup, I'm always looking for the most direct and inexpensive route to setting gear up. For my purposes, there are few things as simple and low cost as using a couple of rubber bands to keep the glass attached to the front of the filter. This makes the glass compatible with any size filter thread on the lens, and is easily adaptable as you change the lens.

Using rubber bands to attach the filter to the front of my Canon 40mm lens was ideal for keeping cost and setup time low.

As you can see here, it's still completely possible to use the controls on your camera. My main tip here would be make sure to carry lots of spare rubber bands, as they will slowly wear down and eventually snap while removing and attaching the filter. (You shouldn't have any issue with this happening while the filter is attached to the front of the lens.)

One other possible method is to attach it to a filter ring. If you have a spare filter ring, you could easily use an adhesive to attach the welding glass to the ring. This is a more solidly put together tool, but lacks the flexibility given that it will only fit a certain filter thread size. If you do choose this option, you can pick up a cheap filter, under $1 at a thrift shop. You can then carefully knock out whatever filter is already there.

The last method is using a filter holder like the Cokin system. These slotted brackets that screw into your lens are made to accept sheet of plastic already. The welding glass will slip right in. I would of course try to find one of these on the used market.

Calculating Exposure

As we mentioned earlier, the amount of light blocked by the filters can vary greatly. That's why I like to spend some time testing out the welding glass and getting a good feel for how much light the filter is blocking. Making some calculations and applying them to the exposure is essential to ensuring you can calculate and utilize the filter properly.

Again, most of these pieces of glass are sold with a "shade" rating that describes how much light they block. However, it's not necessarily useful to us in terms of stops of light.

My friend Paul and I spent significant time calculating the light cut by the filter. Conversion tables exist to convert "shade numbers" of welding glass to stops of light, but we found they weren't accurate with our samples. We used sunny days, and started by making a photo without the shade attached, and then attempted to achieve a similar exposure with the welding glass in front of the lens.

There are conversion tables you can find with various tests on how to convert the "shade number" to stops of light. However, your glass of choice might not have a shade number with it. Even if it does, it might not be calculated correctly. Therefore, I always try to spend the time calculating the light reduction myself.

My standard procedure when figuring out the effects of the filter is to try to create two photos that are very close in terms of exposure, the first without the filter attached, and the second with the welding glass in front of the lens.

I usually choose a sunny day and select a low ISO. Then, I attach the filter and start guessing the exposure. Obviously, with the filter blocking light, you'll select progressively slower shutter speeds and more open apertures. I also typically will turn the ISO all the way up.

Make sure you note the settings of the first exposure, as well as the settings of the exposure with the glass attached. Then, you can do the math and see how many stops of light separate the two frames. Smartphone apps exist that can help you calculate the number of stops of light, and then apply that "light difference" to future exposures.

Color Corrections and Limitations

In the sample images, you might notice that using a piece of welding glass as a neutral density filter actually isn't so neutral in color. Because of this, we need to apply color correction to offset the tint applied by the piece of welding glass. We can either apply this in the camera by adjusting the white balance, or correcting it in post-processing.

One note, this is one situation where you should definitely shoot RAW if you aren't already. As long as your camera has the ability to shoot in RAW, it is very helpful to do so in order to adjust the white balance in post production without losing quality. Because we have a color shift present, it's necessary to color correct to achieve a more realistic outcome. (However, some of the pieces of glass shift the color in a visually appealing way that you may not want to correct.)

Most welding glass pieces will apply the green tint visible in this photo.

Having used a number of these shades, the color effects seem to differ from one piece of glass to another. Typically, however, the effect is green. I use Lightroom to correct most of my images, but the same functionality of adjusting white balance can be found in many photo editors, including Adobe Camera RAW. Utilizing the sliders, I typically will just eyeball the photo and move the white balance and tint sliders to correct the color shift.

It's also possible to color correct by adjusting the white balance in camera. Again, this is typically something that requires some experimentation due to the variation in color tint of the glass. Setting custom white balance, measured in degrees Kelvin is the best way to try and correct it. Personally, I would rather just shoot in RAW and correct later.

With some tweaking of white balance and tinting in Lightroom, the "after" result was usable for my purposes.

Finally, you may notice that your photos aren't all razor sharp while processing them. This is simply the limitation of using a low cost filter option. Optics like this aren't designed with imagery in mind, so the sharpness can be lacking.

I often find myself compensating by increasing sharpening on my photos. If you find yourself needing neutral density capabilities often enough and aren't pleased with the sharpness, then investing in a true ND filter is probably the best option.

Wrapping Up

If you're looking to construct an inexpensive neutral density filter, a piece of welding glass can effectively provide the light reduction at a much lower cost. If you're willing to correct the color differences and put together the kit yourself, you can create a low cost yet effective replacement for a neutral density filter.

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