Your Nikon D3200 offers four special sync modes within the Fill Flash and Red-Eye Reduction Flash modes on your camera, where the flash and shutter are synchronized so that the flash fires at the exact moment the shutter opens.
Technical types refer to this flash arrangement as front-curtain sync, which refers to how the flash is synchronized with the opening of the shutter. Here's the deal: Your camera uses a type of shutter that involves two curtains moving across the frame each time you press and release the shutter button. When you press the shutter button, the first curtain opens, allowing light through to the sensor. At the end of the exposure, the second curtain draws across the frame to once again shield the sensor from light. With front-curtain sync, the flash fires at the moment the front curtain opens.
Here's how these four special sync modes work:
Slow-Sync: This mode, available only in the programmed (P) and aperture-priority (A) exposure modes, also uses front-curtain sync but allows a shutter speed slower than the 1/60 second minimum that's in force when you use Fill Flash and Red-Eye Reduction flash.
The benefit of this longer exposure is that the camera has time to absorb more ambient light, which in turn has two effects: Background areas that are beyond the reach of the flash appear brighter; and less flash power is needed, resulting in softer lighting.
The downside of the slow shutter speed is, well, the slow shutter speed. The longer the exposure time, the more you have to worry about blur caused by movement of your subject or your camera. A tripod is essential to a good outcome, as are subjects that can hold very, very still. A good practical use for this mode is shooting nighttime still-life subjects like the one you see in the following figure. However, if you're shooting a nighttime portrait and you have a subject that can maintain a motionless pose, slow-sync flash can produce softer, more flattering light.
Slow-sync flash produces softer, more even lighting than normal flash in nighttime pictures.Some photographers, on the other hand, turn the downside of slow-sync flash to an upside, using it to purposely blur their subjects. The idea is to use the blur to emphasize motion.
Note that even though the official Slow-Sync mode appears only in the P and A exposure modes, you can get the same result in the manual (M) and shutter-priority (S) modes by simply using a slow shutter speed and the normal, Fill Flash mode. You can use a shutter speed as slow as 30 seconds when using flash in those modes. In M mode, you can even use flash with the shutter speed set to Bulb, the setting that keeps the shutter open as long as you hold down the shutter button.
Rear-Curtain Sync: In this mode, available only in M and S exposure modes, the flash fires at the very end of the exposure, just before the shutter closes. The classic use of this mode is to combine the flash with a slow shutter speed to create trailing-light effects. With Rear-Curtain Sync, the light trails extend behind the moving object, which makes visual sense. If instead you use slow-sync flash, the light trails appear in front of the moving object.
You can set the shutter speed as low as 30 seconds and as high as 1/200 second in this Flash mode.
Rear-Curtain and Slow Sync: Hey, not confusing enough for you yet? This mode enables you to produce the same motion trail effects as with Rear-Curtain Sync, but in the P and A exposure modes. The camera automatically chooses a slower shutter speed than normal after you set the f-stop, just as with regular Slow-Sync mode.
Note that as you scroll through the available Flash modes, the symbol for this mode initially shows just the flash symbol and the word Rear; after you finish selecting the setting, the label changes to Slow Rear.
Slow-Sync with Red-Eye Reduction: In P and A exposure modes, you can also combine a slow-sync flash with the red-eye reduction feature. Given the potential for blur that comes with a slow shutter, plus the potential for subjects to mistake the prelight from the AF (autofocus)-assist lamp for the real flash and walk out of the frame before the image is actually recorded, this Flash mode is one of the most difficult to pull off successfully.
All these modes are somewhat tricky to use successfully, however. So have fun playing around, but at the same time, don't feel too badly if you don't have time right now to master these modes. In the meantime, search the web for slow-sync and rear-sync image examples if you want to get a better idea of the special effects that other photographers create with these Flash modes.