Saturday, October 03, 2009

Long exposures: photographing a local stream

I have started gradually working my way  backwards through the episodes of the great photography podcast by Martin Bailey. I am only at episode 188 now, so still a long way to go.  On the way home on Friday I was listening to episode 190 on "Ten steps to great long exposure images", so when my boys wanted to go to the park today I though that this was a good opportunity to try out some long exposure photographs of the stream that runs through the park. So while the boys went fishing for zarigani I set up the tripod and took some shots.

One thing that I learnt from this podcast, which had been bugging me for some time, is the problem of using very small apertures. I had seemed to me that going to small apertures to increase the depth of field made sense, if there was enough light . For long exposures why not use f/22?  Why use a ND filter at f/11 instead? Then Martin said "diffraction" and it made sense. The small aperture causes diffraction which reduces the sharpness of the image. I had already heard other photographers talking about a "sweet spot" for sharpness around f/8 so this seemed to explain that as well.

The stream runs through some trees so there was a lot of shade. I used a ND 4 filter and polarizer to reduce the light and further reduce the shutter speed. For the first few shots I used the automatic focus but for the remainder I switched to live view and manual focus.

Not mentioned in the ten steps was need to cover the view finder to get an accurate light reading. On one of the first shots I took I forgot to do this was underexposed. Covering the viewfinder with the rubber cover changed the exposure time from 1 second to 3.2 seconds. If you shoot in manual then this is probably not a problem, but using Av mode with an ND filter this seems to be an essential step.

Also not mentioned was to bring some mosquito repellent. Standing by the stream for several hours I was getting constantly bitten....


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