# Variable Neutral Density Filter

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## Introduction: Variable Neutral Density Filter

[UPDATE: thanks to Zephyris, which has reminded me with his good Instructable that you first have to reverse one of the two CPL filters, of course I've found his Instructable shortly after publish mine, as often happens to me, and in this case I've also forgotten I already commented his instructable two years ago...]

A Neutral Density (ND) filter is a cool gadget, you should always carry it (at least daytime). You can find easily ND2, ND4, ND8 filters, these would darken your scene of 1, 2 or 3 stops respectively. When daylight is very strong it's difficult to keep a very large aperture, so you couldn't have low deep of field or nice bokeh. In these cases an ND filter should be very useful. I also use it to keep very long exposures in daytime. You can obtain unique effects, with cars and people moving as ghosts.
Of course you'll reach a better quality with traditional ND filters, but if you have two polarized filters you can experiment a really funny phenomenon. I suggest circular polarized filters (CPL) rather than linear ones, because with CPL you can keep automatic focus of your camera, whichever is the rotation angle of the filter on the lens. You can buy CPL filters on eBay, or at your favourite photography shop, you can find every diameter.

## Step 1: Taking a Closer Look

Polarized filters could rotate, so when you screw them one to each other, you could turn them mutually obtaining to let pass through all the light or block it, and also every step between two positions. This happens because light is normally formed by waves lying on various planes, and each polarized filter blocks all waves except ones on a typical plane. If you have two perpendicular filters the second would stop this wave too, and no light should pass. If the filters are aligned, all light which has passed through the first should be already on the right plane to pass in the second too. It's as making a toast pass through two barbeque grids...
Actually this is right for linear polarized, but for CPL is slightly different, and very difficult to explain. Anyway the idea at the base of this Instructable is the same.

## Step 2: Making the Tools

With CPL filters you have to reverse one of the filters. To know which is the right orientation of a CPL filter you could look at your PC monitor through it, and if monitor changes luminosity rotating the filter, the orientation is right, if it slightly changes colour so the filter is reversed. For our Variable ND Filter you need to dismount a CPL and reverse the glass. To dismount it, as I already explained in the other mandarin Instructable, I've built a pair of tools.

## Step 3: Some Test

Now you could screw up the reversed CPL filter over the original one, so that this last remains on the camera side, and the modified one stays outside. Then you can test your filter on a normal light source. You see very well the variable ND filter behavior in these images. Filters are aligned in first photo, and they're perpendicular in second one, where no toast would pass through the grids... nope! no light would pass through the glasses!

## Step 4: To the Extreme!

You'll run probably in strange unexpected effects if you'll photograph sky with perpendicular filters... because a little of light is still passing,  and you could capture it with long exposure times. If the filters' polarizations are not perfectly balanced you'll probably obtain something as in this my photo, where you see a strange blue X shape. Details of the shot are: 200 ISO, 1/30 s. and 4.5  aperture.

## Step 5: The Results

Now you can test your new Variable ND Filter directly over the clouds. Beware to not point directly to the sun before setting the filter to darken position, to avoid burn your sensor. This is my test photo, compared to the one without filter to compare image quality. Note how the shutter time of the first photo is much longer than second, despite aperture is wider. First day of sun I'll make some more interesting photo to show you ND filter behavior in a crowded "piazza".
And yes, I love mandarins! ;-)

UPDATE:
Dear followers, give a look at my Picasa Gallery to see last photos in Milan to test filter.

 ND filter Milan 2012

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## Questions

Wouldn't limo dark window tint on a UV filter work similar to ND filter?

"Making toast pass through two barbecue grids" is a bit over simplified and does not explain why if you have two crossed plane-polarisers blocking most of the light and put another in-between them at 45 degrees you get will get more light passing through.

Oh my god! How is it I've never though of this? Here I am using 4 different ND filters, when all I needed was this!

you should try this, but don't give your ND filters away, because their quality is probably much and much better ;-)

Isn't it possible to aproximate the same effect without a filter, at 100 iso, with a cheap dark lens and 16' aperture ?

Hi, typical exposure values with cloudy weather are 100 ISO, f8, 1/125 s. If you want to reach about 2-3 seconds of exposure time you need to move of 8 stops, you can't stop down your lens enough (neither with 50 ISO and f22).

What if...

What would the outcome be if you were to take the glass out of the one and put it into a 'regular filter' ring? that way you could have a non-rotating base. This surely would make it much easier to then adjust the ND level on the fly so to speak?

Yes, you're absolutely right, I've already thought that, but...
To semplify adjusting darkness you need to block the "regular" one and leave the "reversed" one rotating, but doing that you'll lose the possibility to use the unmodified one alone, as a polarized filter.

Is it really a ND effect or is it a polarizing effect mod? PL are supposed to cut back reflection. Where a ND is to darken the lens preventing over exposure reflection still get's through a ND filter. There are occasions you need to slow down the shutter so you get a blur effect but you have too much light to do that. in some cases you might want to keep the reflection such as in a waterfall photo.

Making toast pass through two barbecue grids? How close together are you placing the grids anyway? I've never managed it with grids farther apart than the toast height no matter how I align them.