Blurring the sky...

I've never felt the need to put a virtually opaque piece of glass in front of my lens before now until another photographer I know asked me whether it was possible to use a slow shutter to blur clouds in a landscape shot. I had to think about this one for a while. How do you get an exposure time of around 30 secs in broad daylight ? f32 using 6 ISO film should do it but alas, my digital camera only goes to 100 ISO. I cobbled together a red, blue, green and polarising filter and measured the light-blocking strength and it came to around 10-stops which should easily give me an acceptably slow shutter speed.

This image is the result of a 30-sec exposure.Couple of issues with this approach. Firstly, 3 colour filters jammed together gives a yucky overall colour cast so I had to shoot in B+W. Secondly, all that light entering then exiting, not one, but 4 filters means that sharpness is affected. Thirdly, its very difficult to see through the filter stack to compose which means removing it to focus and compose, then refitting it to shoot, and lastly, if you want the clouds to blur, it has to be on a windy-ish day so I needed to carry around a jolly heavy tripod to avoid camera movement ! Now, with a bit of research, I found out that there are several filter manufacturers that produce 10-stop, Neutral Density (ND) filters but its important that you understand first how filters work so here is the techy stuff.

A filter is a piece of glass which goes in front of your lens and its purpose is to affect the light passing through it. There are hundreds of different types of filters, all with different purposes.

A Neutral Density (ND) filter is designed to reduce the amount of light passing through it without altering any of the colours hence the 'Neutral' tag. They have their own nomenclature which can be confusing but essentially, ND filters are graded in 'stops' i.e a 1-stop reduction, 2-stop, 3-stop etc. and given a number i.e 0.3 or 3.0 to indicate its stopping power. The varied filter manufacturers list their filters similar to this....

0.3 / ND2x / 101 Neutral Density Filter reduces the light by one f-stop 1/60th becomes 1/30th 0.6 / ND4x / 102 Neutral Density Filter reduces the light by two f-stops 1/60th becomes 1/15th 0.9 / ND8x / 103 Neutral Density Filter reduces the light by three f-stops 1/60th becomes 1/8th 1.8 / ND64x / 106 Neutral Density Filter reduces the light by six f-stops. 1/60th becomes 1 sec. 3.0 / ND1000x / 110 Neutral Density Filter reduces the light by ten f-stops. 1/60th becomes 15 secs

Here's a couple of links so that you can get an idea of the purpose and costs of ND filters....

http://www.bpluswfilters.co.uk/index.php?page=products&sec=140

http://www.warehouseexpress.com/circular-or-threaded-filters/b3072