I am often asked why you should use your camera in Manual Mode when shooting flash in low ambient conditions, and why is that better than shooting in Aperture priority. This study will explain the reasons, plus I will show the effects of Rear Sync versus Front Sync when there is any subject motion involved in flash pictures.
But first it is important to fully understand that when you use your camera in Manual mode and your flash in i-TTL, the flash handles the exposure of the subject, as long as the ambient is dim, and the subject is in the center of the frame. So the image is still exposed automatically even though the camera is in Manual mode.
Here are my conditions that do not change during this study:
- Dim ambient light
- ISO 400
Image 1 below: Test Image, Flash off, 1/60th, Manual Mode.
The purpose of this image is to indicate how much ambient light there is. You can see that at 1/60th, it is about two stops underexposed. When using the camera in Manual mode and the flash in i-TTL mode, you want the camera controls to always underexpose the ambient by about two to three stops. Also, I moved my hand, so you can see what motion blur looks like without the flash.
Image 2: Flash ON, Aperture Priority.
I was moving my hand toward my face. Look closely at my fingertips. That orange blur on each finger is called ghosting or ghosting trails. The flash froze my moving hand and then the ambient caused the ghosting. Also, noticed the direction of the ghosting trails. The ghosting trails appear to preced the motion of my hand. This is what we call ghosting trails on the 'wrong' side of the motion. The position of the ghosting trails was caused by using Front sync.
Image 3: Flash ON, Camera Manual Mode, 1/120th shutter.
I was moving my hand toward my face. Notice that there are no ghost trails. This is because the shutter speed was increased high enough to eliminate the ambient from the image.
Image 4: Flash ON, Camera Manual mode, Rear Sync, 1/60th shutter.
Again, I moved my hand toward my face. Look closely at my fingertips. Now, you can see ghost trails going in the opposite direction. This is because Rear Sync was used, which caused the flash to fire at the end of the shutter cycle rather than the beginning of the shutter cycle as in the previous images. Now, the ghost trails are on the 'correct' side of the motion.
Image 5: Flash ON, Aperture Priority mode, Rear Sync, 1/15th.
Again, I moved my hand towards my face. Notice the much greater amount of ghosting. This is because when you select Rear Sync in Aperture Priority, you also get Slow Sync, and Slow Sync, causes the shutter speed to reduce to fully expose the ambient. This also caused overexposure.
Image 6: This has all the same settings as Image 5.
I just wanted to show the great sensitivity to movement. All I did was move my head.
1) When shooting flash in low ambient conditions, like normal indoor artificial lighting, it is best to use Camera Manual mode with Rear Sync to control ghosting.
2) Using Aperture priority with Rear Sync is not a good choice for dim ambient conditions. It will lead to overexposure and way too much ghosting and general motion blur.
3) Using Aperture Priority is also not a good choice when using Front Sync when shooting in dim ambient conditions, because the camera will always select 1/60th sec shutter (Flash Shutter Speed). This is too slow to stop ghosting during motion like a speaker waving his arms or a couple dancing.
Always use Camera Manual Mode with Rear Sync when shooting flash in dim ambient conditions so you can control the ghosting.
Never use Aperture Priority when shooting flash in dim ambient conditions, because you cannot control the ghosting.
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