In this chapter, let me expand about neutral density filters (a.k.a. ND filters), as these filters play an essential part in long exposure photography, helping you achieve the particular “look” that you have in your mind. As mentioned, ND filters come in different strength (popular ones are 3 stop, 6 stop and 10 stop), and here I take a little deeper look at when to use which ND filter for your desired effect.
3 Stop ND Filter
I don’t use 3 stop ND filter when taking cityscapes “at waterfront”, as the strength is too mild to create silky smooth water effect. Hence, my use of 3 stop ND filter is limited for scenes that have no water to be smoothed out, such as the photo below with light trails of moving cars, which doesn’t require a very long shutter speed.
This light trails of moving cars (tail lights) was shot with 10 seconds of exposure using 3 stop ND filter (base shutter speed of 1.3 second) (Singapore Skyline, 18mm, f/13, 10 seconds [with 3 stop ND filter attached], ISO 100, shot 9 minutes before the end of dusk).
This mild strength of 3 stop ND filter (i.e. not-so-long exposure time) isn’t all bad, though, as it allows you to take quite a number of photos during blue hour, unlike more dense filter like 6 stop ND filter where you can take no more than a few photos due to a longer exposure time required per photo.
6 Stop ND Filter
6 stop ND filter is almost exclusively used for my photos that have water included. To create silky smooth water effect, slowing down 3 stop isn’t quite enough, but 10 stop is way too strong (for e.g. base shutter speed of 2 seconds [i.e. when no filter is attached] gets extended to 15 seconds [with 3 stop ND filter], 128 seconds [with 6 stop ND filter] and whopping 34 minutes and 8 seconds [with 10 stop ND filter] respectively).
Shot with 163 seconds of exposure using 6 stop ND filter (base shutter speed of 2.5 seconds) to achieve my desired effect of silky smooth water. Had I used 3 stop ND filter, the water wouldn’t have been smoothed out this much (Marina Bay [Singapore], 18mm, f/13, 163 seconds [with 6 stop ND filter attached], ISO 100, shot 7 minutes before the end of dusk).
If you’re to buy only one ND filter for cityscape photography at blue hour, I’d recommend nothing more than 6 stop ND filter. It’s a game changer for those who are interested to shoot long exposure photos at twilight and dusk.
10 Stop ND Filter
10 stop ND filter is an extreme filter that lets you expose super long (e.g. even 1 second of base shutter speed gets extended to over 17 minutes), hence requiring a careful planning. Personally, I don’t really find it useful for shooting cityscapes at blue hour, as digital noise caused by long exposure becomes unbearable when exposing for over 10 minutes leading up to the end of dusk (even with in-camera Long exposure NR [noise reduction] turned on).
Shot with 343 seconds of exposure using 10 stop ND filter (base shutter speed of 1/3 second). To keep long exposure noise at bay, this extreme filter’s use is rather limited to pre-dusk or even earlier in the day, not towards the very end of dusk (Singapore Skyline, 18mm, f/10, 343 seconds [with 10 stop ND filter attached], ISO 100, shot 19 minutes before the end of dusk).
While 10 stop ND filter helps you get a slow enough shutter speed for long exposure photography at pre-dusk (like the photo above, shot 19 minutes before the end of dusk), golden hour or even in daylight (e.g. to shoot motion blur of waterfalls in bright sun), it’s just not quite usable at the peak of blue hour.
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