A while back I investigated the possibility of using the Lee filter system on my Fujifilm X-T1. As you can see, I invested in two Lee ND filters;
- Lee Big Stopper (110ND / 10 stops Neutral Density Grey filter)
- Lee Little Stopper (106ND / 6 stops Neutral Density Grey filter)
The thing with ND filters is that they reduce the light evenly. This results in (depending on the greyness of the filter) longer exposure times. With enough 'stops' in front of your lens, you can stretch the exposure from 1/200s to 10 or 15 minutes. Shooting with exposure times of minutes instead of the usual fraction of seconds results in motion blur in the photos (assuming that you're not shooting a stationary object indoors). Expose long enough, and the movement becomes a silky haze.
The photo below is a long exposure photo (60 seconds long to be exact), as you can see the sea already has a relatively smooth surface.
The exposure, however, wasn't long enough to create a real smooth/silky sea surface. For that to happen, you need minutes of exposure. And to accomplish that you need the help of ND filters. Especially during the daytime.
The following photos were taken in the afternoon, and the exposure times were over 10 minutes. Impossible without the use of ND filters.
To give you an idea of the exposure change; Without the ND filters, these photos would have had the following EXIF data (approximate):
- Exposure: 1/200s
- Aperture: f/11
- ISO: 200
Adding the 10 + 6 stops ND filter, this would theoretically create an exposure time of around 5:30 minutes, but the 10 stops ND tends to underexpose a bit (about 1-1.5 stops), so I usually add a 'stop' (adding a 'stop' on the exposure time means doubling the exposure time) in my calculations. The 6 stops ND filter tends to underexpose about half a stop. So the 10 stops is effectively an 11 - 11.5 stop ND filter.
I saw these variations on both B+W and Lee 10 stops ND filters, so I guess a 10% error margin is kinda normal.
So, if the strict calculation (based on the original 16 stops) ends up with an exposure time of 5:28, but the extra 1 - 1.5 stops increases the exposure time from 5:28 to about 15 minutes.
Thankfully, the longer the exposure the less influence a second more or less has on the total exposure of the photo itself. Exposing 14:50 or 15:10 minutes instead of 15:00 will have little to no effect on the end result.
The ND filters tend to introduce a colour cast and additional vignetting (especially on wide angles) on your photos. Converting to black and white is the easiest way of removing that. That's probably the main reason that most of these extreme long exposures, you find on the Interwebs, are in black and white.
TIP: If you're going to shoot long exposures in combination with a water surface, you should also shoot the scene without the ND filters. The reason is that objects in the water might move a bit (water flow and/or wind). Something the naked eye won't easily detect, but it will definitely turn up in a 15 minute exposure (and ruin your shot). You can use the object from the normal exposure, and use Photoshop to blend/mask/clone it in the long exposure photo.
There's nothing more annoying than to come home with to much motion blur in your photos.
Oh, and make sure you have room to crop, since removing vignetting tends to introduce noise in the photos. Better to cut a bit off and get a clean (and slightly lower resolution) photo than an image with annoying noise on the edges.