I've heard this term of "dragging the shutter". What does this mean and why would I use it?
It just means using a long exposure, usually in the context of mixing flash with ambient light, in order to let in more ambient light to better expose the background. See this related question:
What they said.
You need to be aware that in some situations this effectively occurs unintentionally as a consequence of the way most modern shutters work - and the effect on images can be other than intended.
The following applies mainly to "aperture priority" mode of operation - but similar effects can occur if care is not taken in other modes.
When flash is used, maximum available shutter speed is limited by the nature of the commonly used "two curtain" / focal plane / roller blind shutter. For practical purposes these typically have a maximum speed in the 1/100th - 1/200th second range when using flash. (Multi-flash flash is a seperate situation and much less common).
Excellent super slow motion video of a 2 curtain shutter in action
I'll just use the term "shutter" here to mean a two curtain shutter of some form.
A shutter fully uncovers the sensor (or film) only up to a preset maximum shutter speed - typically in the 1/150th to 1/200 second range but sometimes faster. For faster shutter speeds (or shorter exposure times) an effective faster shutter speed is achieved by "sliding a partially opened window" across the sensor. If the "window uncovers only say 1/4 of the sensor at any given moment then the effective shutter speed is increased by a factor of 4.
For ambient light the window can be as small a fraction of the sensor width as desired.
But, for a single-flash flash the shutter must be wide open when the flash occurs so that the whole sensor sees the light from the single flash event. This means that the maximum shutter speed when using flash may in some situations be slower than the camera in aperture priority mode would use if ambient light only was being used.
If the shutter speed with flash is say 1/200th second max ( = 5 milliseconds) and conditions are very dark then without a flash in the same conditions you might require a shutter speed of say 1/10th second. In that case, as the actual shutter speed is 1/200th second with flash = 20 times faster, the ambient light will contribute minimally to the overall exposure.
However, if you are photographing a stage show with eg brightly spotlit dancers, or are using the flash as fill flash to reduce facial shadowing on a bright day, them if using the ambient light only, your shutter speed might be 1/100 th second, or 1/250th or even 1/500th. Actual value will depend on aperture, ISO and light level. If you now enable the flash the shutter speed will be forced down to the maximum value that thecamera supports in flash mode. As you have set the aperture manually and as the shutter speed is now similar tp or even slower than the value that you would use with ambient light only, the ambient light will contribute substantially to the end result.
On sunlit day with fill flash the result can be massive overexposure as the camera is not able to increase the shutter speed adequately.
With a stage show with fast motion the effect can be more subtle but just as undesirable. The flash will now expose the scene about correctly but the overall slow shutter speed and high ambient light levels will allow extended images of moving dancers to be recorded. You may get a sharp image from the flash with the dancers surrounded by a movement "halo" from the lower speed ambient exposure. Wit care this can be used tp produce very "arty" and desirable effects. Without care this can produce a blurred and fuzzy mess. Actual result depends on actual shutter speed, aperture and ISO and amount of movement.
If yu wish to avoid or at least minimise this sort of effect set aperture and ISO such that the requried shutter speed with ambient light is far lower than the max flash-sync shutter speed. eg if ambient light requires 1/200th second at ISO 400, f/2.8, then using ISO 100 and f5.6 would push the required shutter speed down to about 1/15th second. If the camera is now arranged to expose correctly with flash at 1/200th second the ambient light will provide only about 5% of the light. Smaller apertures will further reduce ambient contributions.
Even if the scene is relatively static, varying desired results may be achieved by adjusting flash and ambient balance. A photo of a speaker at a podium lit with a halogen spot and with other dignatories seated close behind may benefit with more flash and less ambient to achieve better overall colour balance and colouring. A DJ at a party with several coloured spots washing his equipment may require some flash to allow equipment details to be seen but benefit from substantial mounts of ambient light so that the spot colours are not totally washed out by the flash. Dancers will generally look better if at least some of the lighting comes from coloured spots - while limiting the blurring which will occur if too much ambient effect is allowed.
By careful juggling of aperture & ISO and possibly also adjusting the flash power level you can produce any mix of flash and ambient in the image that you desire, with appropriate effects accordingly.
Dragging the shutter refers to allowing the shutter to stay open a longer time in flash photography to allow more of the ambient light to be captured. By only partially exposing the shot with a brief burst of intense light, the shutter can be left open longer than the flash burst to allow more influence from the ambient light and potentially give a more natural feel while still limiting the duration the shutter has to be open for a standard exposure.
There is still a slight trade off in the time stoping properties of a fast shutter, but since a lot of the light is captured in a fraction of the shutter time (from the flash) there is less impact than would occur in an exposure without a flash at all.
Dragging the shutter (also known as slow-sync) is a term used in the context of flash photography. It refers to the practice of leaving the shutter open longer than required by the camera's sync speed. The sync speed is the shutter speed needed to allow the relatively short duration flash to fire while the sensor is completely exposed between the time the first curtain has completely opened and the second has not yet begun to close.
Many times dragging the shutter is combined with second curtain sync.
This video explains in simple terms how a focal plane shutter works and why sync speed is important when a focal plane shutter is used in concert with a portable flash unit.
The purpose of dragging the shutter is to allow more of the ambient light in the scene to strike the sensor. Areas of the scene that would be too dark because they are too far away from the flash unit that is adjusted to properly expose the main subject can then be seen in the photo.
Another use of dragging the shutter is to allow light trails. Imagine a car driving from left to right at night. By dragging the shutter the lights on the car will create a trail of light across the frame. The short duration flash will illuminate the car and freeze it for the instant the flash is lit. If first curtain sync is used, the flash fires as soon as the first shutter curtain is open. The result of this looks unnatural to our eyes because that leaves the light trail leading in front of the car as the lights continue from left to right after the flash has fired. By using second curtain sync, the flash will fire just before the second curtain begins to close. Thus the light trails are exposed before the flash fires, and they appear to be trailing the car.