When shooting in Auto mode (otherwise known as ETTL or TTL), the flash will choose the power for you.
This means that every time you fire the flash, the flash power can change, making it difficult for you to understand why it sometimes looks good and other times does not. The exposure might be too bright, too dark, or even occasionally just right.
By changing the flash power to Manual, you’ll get consistent power every time and you’ll understand how to use flash more quickly.
You’ll also save a lot of time in post that you’d otherwise spend trying to balance the exposure across a set of photos.
The most important thing to remember when manually setting your flash power is that flash power is generally measured in fractions.
In other words, the lower power has a higher number in the bottom of the fraction.
For example, a 1/64th power setting would produce a less powerful flash than a 1/32nd or 1/16th power setting.