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Ed (Colorado, USA) asked : I have heard the term “dragging the shutter” quiet a few times from professional photographer. Do you mind explaining it with example may be?
When you are shooting (specially portrait) at night, you have to deal with different types lights; flash light (probably), street lights and environmental light. Sometimes you want to shoot without background object but there are some time when you want to capture beautiful background scene or nicely lit architecture along with your portrait. If you have noticed your point and shoot or entry level DSLR camera, there is a mode called “Night Portrait” which is auto mode (meaning camera automatically manages the camera settings) but the idea is same; to capture a portrait along with background lights or objects. Using one of these auto mode, you get good result sometimes but sometimes you might not be happy on what you get but there is nothing much you can do on these auto mode. If you want to control the lights and camera settings, you have to step up and dial into one of the semi-manual (S, A and P) or full manual (M) mode and set the values as you need.
You might have used this technique before knowingly or unknowingly and you are just not familiar with the terminology which happens to all of us at some point. It is all about balancing flash and ambient light (light that exist in the environment). When balancing ambient light with flash light, photographers choose camera settings to retain the mood of place, time and environment of shooting. While doing so, we have to allow ambient light by allowing shutter to open little longer (dragging the shutter) and apply the flash light at the end of the exposure (usually to freeze the motion).
Dragging the shutter is very simply and also commonly used technique in night portrait photography but understanding when and where to apply this technique is more important. It is important to know whether you are using Manual flash or TTL flash setting with your camera. If you are using Manual flash, usually Aperture, ISO, subject distance with flash and power of the flash affect flash exposure. Whereas ISO, Aperture and Shutter speed usually contribute to the ambient light exposure. That means you can use Shutter speed independently to control ambient light exposure only because changing Aperture and ISO will also affect Manual flash exposure. The environmental light (also called ambient light) is continuous source of light but flash light is gone with the blink of an eye which is the main reason why we say Shutter speed doesn’t contribute to the flash exposure.
But if you are using TTL flash, those four controls (Aperture, ISO, Distance and Power) have no control over flash exposure because your camera and flash talk to each other based on Aperture, ISO, distance to the subject and apply appropriate flash power to give correct exposure. That means you can now use Aperture or ISO or Shutter speed to control ambient light without affecting flash exposure. Using TTL flash is relatively easy and you don’t have to remember much while changing camera settings but if you are using Manual flash, you have to be very careful about what setting you have changed because if you change your Aperture, you have to change flash power or ISO or distance to compensate flash exposure but at the same time changing Aperture also affects ambient and hence you may have to adjust Shutter speed too. It’s kind of confusing but enough practice clears confusion.
Shutter Speed : 1/60 Aperture : f/4 ISO : 220
Usually dragging the shutter slower than 1/60th of second allows you to register ambient light but you can change shutter speed as you need more and more ambient light. I usually use rear sync flash with slower shutter speed so that your final moment would be frozen.
Shutter Speed : 1/5 Aperture : f/4.2 ISO : 1600
Shutter Speed : 1/3 Aperture : f/4 ISO : 800
Shutter Speed : 1/5 Aperture : f/4 ISO : 1600
These images are taken with TTL flash and you can see how changing camera settings is adopted automatically by flash unit and applying appropriate power to properly expose your image.