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.
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