When we talk about exposure, we’re talking about light being exposed to our camera’s sensor via the shutter, aperture and ISO. When we talk about how long that exposure lasts, we’re only really talking about the shutter speed (speed being the key word here). We vary the length of time that the shutter stays open to give us more or less light, and if the shutter stays open for more than a 1/60 of a second, that is considered to be a long exposure. The effects of long exposures will be different depending on whether you’re shooting during the day or at night.
The reason for using a longer exposure at night is obvious – more time is needed for the available light to reach the sensor because it's darker. It’s not uncommon to utilise a shutter speed as slow as 10-30 seconds (the standard maximum for many camera models is 30 seconds), or on BULB mode, with the use of a remote trigger or shutter release.
Long exposures can be used at night simply to get good focus and avoid blur, or they can be used creatively, like in the photos below. The wonderful thing about shooting at night is that the available light sources are artificial, but very colourful. Using a long exposure will increase the vibrancy and haloing effect of these lights in a way that we would never see in reality. City streets come to life as car headlights produce ghosting and streetlamps cast halos on footpaths.
Image taken by Harold Abner
Image taken by Mark Howells-Mead
If you use external flash with with a long exposure, such as an on-camera Speedlite, you will also be able to sharpen parts of the scene, illuminating the subject and the foreground, while still having an atmosphere of low light and a blurred background. This can also create light trails from any other light source in the frame, like in the photo below. Just set the flash to fire on slow sync mode several times throughout the exposure.
Image taken by Lee Charlton
During the day
If the lack of light at night is the reason why people use longer shutter speeds, is there any need to use them during the day? Actually there is, for the purpose of capturing motion blur, whether human motion or the motion of water and clouds in landscapes and seascapes. For these examples we would make use of shutter speeds anywhere between 3-10 seconds, depending on how much blur you would like to show.
Image taken by Luc Wave
But the problem with using long exposures in daylight is that you are almost always going to end up with a very bright image because there is so much light reaching the sensor that it leads to overexposure. There’s a few steps we can take to remedy this. First of all, make sure that your ISO is as low as it needs to be. Secondly, make sure your aperture number is as high as it can be. Changing both of these settings will cut down on some of the light reaching the sensor, however you will probably find that your image is still a little on the bright side. If this is the case, try using a neutral density (ND) filter. These handy little attachments will decrease the light by a few f-stops in addition to using a high aperture number.
At the very least, you’re going to need to invest in a good tripod. While motion blur is a deliberate effect, camera shake is a blur you don’t want. The steadier and more secure your tripod is, the crisper your photos will be. By using a remote or cable shutter release as well, you can be sure your hands aren’t shaking the camera when pressing the shutter button. Finally as mentioned above, you may also want to invest in a set of ND filters and an external flash to give you even more options.
For some ideas on fun exercises you can try using long exposures during the day and at night, read this article and this article.