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What do we mean when we say long exposure?

Interestingly it  refers to the intention behind the photograph rather than the actual time the shutter is open. In general the intention behind a long exposure is to show some motion, whether it’s the motion of the earth as in star trails, or of water or clouds. One notable exception is to make moving objects disappear from a very long exposure.

The simplest way to do a long exposure is to shoot in low light like in the image of the star trails. You don’t need any special filters or equipment, just you camera, tripod and a remote shutter release or self-timer.

Today’s DSLR’s are limited to a 30 second exposure when using the cameras built in meter. So for an exposure longer than 30 seconds you will need to use the cameras BULB setting and a locking remote shutter release. The bulb setting will lock the shutter in the open position as long as the shutter release button is depressed, thus the locking shutter release.

Often the light level is not low enough to get the 1 minute or longer exposure required to achieve the desired effect. In these circumstances a neutral density filter is required.

A neutral density filter has one purpose – to reduce the amount of light reaching the sensor, as the name implies it is neutral, affecting all wavelengths of light  equally with no change to color or hue. There are graduated neutral density filters, but they are used to help balance an exposure within the scene, and don’t really apply to long exposures.

Filters come in either the screw on type or the square filters that require a filter holder. ND filters come in a variety of densities denoted by the ND filter factor or optical density number. Unfortunately neither of these numbers equal the number of stops the filter will darken the exposure.

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Stops of Light Reduction      Optical Density      Filter Factor

      1                                                  .03                            2

      2                                                  .06                           4

      3                                                  .09                           8

      4                                                   1.2                          16

      5                                                   1.5                          32

      6                                                   1.8                          64

      7                                                   2.1                          128

       8                                                  2.4                          256

      9                                                   2.7                          512

     10                                                  3.0                          1024

So why are these numbers important?

Because for every 1 stop reduction in the amount of light entering the camera we need to decrease the shutter speed by one stop to maintain the correct exposure. For example:

Original Exposure                   ND filter              New Exposure

1 sec                                               1 stop                         2 sec

1 sec                                               2 stop                        4 sec

1 sec                                               3 stop                        8 sec

1 sec                                               4 stop                       16 sec

1 sec                                               5 stop                       32 sec

1 sec                                               6 stop                       64 sec

1 sec                                               7 stop                      128 sec

1 sec                                               8 stop                      256 sec

1 sec                                               9 stop                       512 sec

1 sec                                              10 sto                        1014 sec

Now to put this information to use.

Use your cameras  lowest native ISO, set the aperture to get the depth of field, somewher around f11

With your camera to manual mode, adjust the exposure based on the camera meter and take a test exposure. Check the histogram to be sure you are exposing to the right without getting any blown highlights.

Be sure you have the scene framed the way you want and the focus is correct, then set the focus to manual. It is important to do this before putting the filters on, because once you do it will be to dark to see through the viewfinder. Add the filter and determine the number of stops of light reduction, now you will need to do a little math.

Let’s use this example, If you exposure determined by the test exposure is 1/15 sec at ISO 100 at F11, and we add an 8 stop ND filter the new shutter speed will need to be:

1 stops 1/8 sec

2 stops 1/4 sec

3 stops 1/2 sec

4 stops 1 sec

5 stops 2 sec

6 stops 4 sec

7 stops 8 sec

8 stops 16 sec to achieve the same exposure as in the original test exposure.

Next time you out give it a try, you may be surprised by the results.