Daylight Long Exposures Using a ND Filter

How to get smooth moving water and silky skies in your photos

By mattb123 in Articles and Tips on Jun 5, 2014

Have you ever wanted to get the effects you see in nighttime long exposures like silky smooth moving water or streaking clouds, but during daylight hours? The key is to reduce the amount of light reaching your camera's sensor so you don't let too much in, which overexposes your shot. The easiest way to do this is to stop down. Stopping down to f/22 or even f/32 works, but only so much and the image quality will most likely suffer from diffraction. Compared to how long you can leave the shutter open at night, you still end up with a relatively quick exposure too. If you want to take even longer exposures and/or maintain a wide aperture you need a neutral density (ND) filter.

A ND filter works by reducing the amount of light getting into the camera. They have different ratings and sizes; some even allow you to vary the amount of incoming light. For this article we will be discussing use of a 49mm filter thread B+W ND 3.0 10 Stop filter which very conveniently fits many classic Pentax lenses as well as the wonderful DA Limited primes. A 10 stop filter means it will reduce the light by a factor of 1/1000. That is quite a lot so this will allow for much longer exposure times that you would typically use during the day.

To shoot with this filter you will need to compose and focus before putting the filter on or use Live View and manual focus. AF will have a very hard time and can’t be counted on. Once the filter is mounted, looking through the viewfinder will just look dark.

Exposure times can be calculated for precision or since this is digital they can be pretty easily estimated and chimped. Once you have a feel for it it’s not hard to estimate how long you will need.

B+W 3.0 10-stop ND Filter

Here's how it's done. Besides the camera, you will need the following items:

• Camera support – a sturdy tripod is best.
• The ND Filter. The examples seen here were done with a B+W ND 3.0 49mm filter. 
• Something to cover your viewfinder. There are covers that come with our Pentax dSLRs which work great but unfortunately I have trouble keeping track of mine. This item can be pretty easily improvised if necessary. Just draping a dark colored lens bag over it will usually do the trick too.

Or you can improvise like I did here when I didn't have anything else handy. The fourth image here shows what you may see if you use no viewfinder cover at all. That shot is ruined.

   
Click on any thumbnail to enlarge

1. Compose your image before putting the filter on. For the best effect be sure to have something with some motion included in your composition. This can also be done using Live View with the filter on but that can be challenging in bright light. Clouds and water are probably the most common subject for this technique.
2. Focus, then switch your camera to manual focus so it doesn’t try to AF once the filter is in place.
3. Put the filter on the lens.
4. If you are in Av mode you can half-press the shutter to get a meter reading. This reading will typically underexpose so I either use Exposure compensation or switch to manual mode and add a couple of seconds to the metered exposure.
5. Now, put the camera in 2s timer mode (or use a remote, making sure SR is off) and make an exposure.
6. Cover the viewfinder. This is especially important if there is a light source like the sun behind you.
7. Once it’s done have a look on the screen to see what you got. Look for over and underexposed areas and use the histogram to see how well exposed your shot is. Adjust your exposure as needed and shoot again if the first attempt didn’t give the results you wanted.

Here are two examples of the same scene taken with and without the ND filter. The clouds, water, and the weeds (just a little because it was breezy) all look different between shots. 

     
Click on any thumbnail to enlarge

Here are some tips to keep in mind:

Many ND filters add a color cast to the image that is often not all the pleasing or accurate. This is pretty easy to fix most of the time in your editing software. Something like using the WB dropper in Lightroom to click on a neutral portion of the photo to set the white balance in post will usually do the trick. 

Different subjects can look better at varying shutter speeds. If your shot looks too smooth, speed the shutter speed up by opening up the aperture or increasing ISO to compensate. 

Don't forget that viewfinder cover.

And finally, experiment and have fun!