I have been playing around with a neutral density filter in the last few trips. An ND filter is basically a filter that reduces the intensity of all wavelengths of light equally thereby ensuring no changes in the rendition of colours. So, the basic purpose, given that the ND filter reduces the intensity of light entering the lens is to slow down the shutter speed. Think of it as sunglasses for your lens.
Now, why would you want to slow down the shutter speed, you may be tempted to ask. And I can think of a couple. (a) You might want to deliberately achieve a motion-blur effect (moving objects, falling water, flowing stream, etc) (b) you might want to get a shallower depth-of-field (esp. in bright daylight) than what the conditions are allowing you to.
This stream in North Pullu was a perfect setting to try and use the filter to slow down the shutter speed and try and smoothen out the flowing water in the frame. So, I plonked the tripod at the edge of the stream and tried various settings. I was using a variable ND filter with which I could adjust how much I was throttling the light entering the lens – took several attempts to finally get the hang of it and get this image. I prefer the B&W version though as the flowing water does grab attention in that image.
But there was something wrong, atleast to me. I could discern vignetting (or light fall-off) in parts of the image that would progressively increase as I dialed up the throttling on the variable filter. It is only much later, while looking up the tech specs for the filter, that I figured out that ND filter (esp. the Variable ND ones) need to be checked for compatibility. I had picked up this Kenko 77mm PL fader for use with my 70-200 & 300mm lenses and read that below 28mm some mechanical vignetting was expected. And I was shooting this landscape with my 11-16mm lens. So, there it was. I now knew what went wrong.
Slow down, you move too fast
You got to make the morning last
Just kicking down the cobblestones
Looking for fun and feelin’ groovy
Ba da da da da da da, feelin’ groovy