Use Neutral Density Filters to Extend Shutter Speed
Although it’s still possible to take “modestly” long exposure photos at blue hour without using a neutral density filter (a.k.a. ND filter), shutter speed won’t go slow enough to create silky smooth water effect and have clouds rushing across the sky.
This is where ND filters come into play, reducing the light that is coming through the lens in order to extend shutter speed (to minutes, not just seconds) and adding that dreamy feel typical of long exposure photos, like those you see on my Top 50 Photo Gallery.
ND filters help reduce the light that is coming through the lens, allowing shutter speed to be extended much longer to create silky smooth water effect in your photos.
Be careful with the size of filter you’re purchasing, though. Do check the filter thread size written inside your lens cap (e.g. 77mm for my trusty Nikon 18-35mm).
Each “Stop” of Neutral Density Filter Doubling Exposure Time
ND filters come in different strength such as 3 stop, 6 stop or 10 stop. The bigger the number, the darker the filter and the less light is let through. Let’s say you’re taking photos at dusk with a base shutter speed of 2 seconds (i.e. when no filter is attached), which is not quite long enough to smooth out the water.
However, by attaching 3 stop ND filter, the shutter speed of 2 seconds gets extended to 15 seconds, as each “stop” of ND filter approximately doubles the exposure time (2 seconds ► 4 seconds [1 stop] ► 8 seconds [2 stop] ► 15 seconds [3 stop]). Likewise, 6 stop ND filter gets shutter speed extended to 128 seconds (2 seconds ► 4 seconds [1 stop] ► 8 seconds [2 stop] ► 15 seconds [3 stop] ► 30 seconds [4 stop] ► 64 seconds [5 stop] ► 128 seconds [6 stop]), which is long enough to create silky smooth water effect.
Long Exposure Calculator Apps Make Our Life Easy
You must be wondering who can remember all these numbers! This is where long exposure calculator apps come in handy, as these phone apps automatically calculate a required shutter speed for you. Photographers used to have a cheat sheet for this, but phone apps have made our photography life easier. ;) My favourite app is Long Exposure Calculator (for iOS) by Junel Corales, which comes free (look for Android equivalent here ).
By setting your filter density (e.g. 6 stop) and base shutter speed (e.g. 2 seconds), Long Exposure Calculator app automatically calculates a required shutter speed to be used (2 minutes and 8 seconds [128 seconds] in this case).
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