Canon PowerShot SD400 User Guide (en)

Using the Viewfinder
The viewfinder can be used to conserve power by turning the LCD monitor 
off (p. 37) while shooting.
Image Seen in Viewfinder vs. Recorded Image
Usually the recorded image contains more of the scene than is seen in 
the viewfinder. Confirm the actual image size with the LCD monitor.
The image recorded may differ from what is seen in the viewfinder due 
to the distance between the viewfinder and lens (especially with 
close-ups). If you use the viewfinder to take close-up shots, portions of 
the close-up subject may not appear in the recorded image. Use the 
LCD monitor when taking close-up shots (p. 71).
Shows the center of the image
About the Autofocus Function
This camera employs AiAF (artificial intelligence autofocus) technology 
which uses a broad metering field to calculate the focal distance with 
high precision. It delivers a crisp focus even when the photographic 
subject is slightly off-center.
The AiAF function can be turned off so that the autofocus is metered from 
a fixed AF frame at the center of the field (p. 89).