Simple Variable Neutral Density Filter

Step 6 Applications

This filter is useful any time you want to capture shallow depth of field or motion under bright lighting conditions, things you might want to try are:

A flower under bright sunlight - use a wide aperture and a dark neutral density filter to capture the flower, without overexposure, with a nicely blurred background.
People in movement - use an extremely long exposure time and a very dark neutral density filter to blur the movement of people through a public space.

Comments

[arijbrown]

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.

[McGrep]

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]

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...

[dave77459]

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]

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...

[arijbrown]

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.