The more power, the better.
However, the size and price increase as the power increases.
With lots of power, your flash will go farther.
And, you can bounce the flash off of a ceiling and still use a small aperture.
The power is measured by a specification called the guide number.
The larger the guide number, the more power.
When comparing guide numbers, don't compare apples to oranges.
Make sure they have the same ISO (usually 100) and distance.
A versatile flash can be aimed at any point on a ceiling or wall.
For example, if you're standing on a chair doing a group shot, the camera is tilted downward.
You want to still be able to aim the flash at the ceiling.
Usually, the optimum lighting from your flash is when you're bouncing the light off of a ceiling.
However, because the light is from overhead, people's eyes will be dark.
Many flashes have a small white reflector that you can pull up to use.
This "kicker card" reflects some of the light into the peoples eyes.
A few flashes have a second flash on the front for this purpose.
Avoid flashes that use expensive batteries, such as the CR123A lithium batteries.
Consider purchasing a flash that can be used off of the camera with a cord.
With autofocus and today's light-weight cameras, you can hold both the flash and the camera.
You can aim the flash anywhere, and can create shadows where you want them.
For example, you could photograph a grandmother with the flash close to the camera.
This will to conceal wrinkles.
When photographing the latest granddaughter, you could move the flash above and to the left.
The granddaughter's face will have shadows, giving volume to her face.
Make sure the flash is compatible with your camera.
Check your camera instruction manual or the flash manufacturer's website.
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Bob Atkins Canon
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You can use a forum to gain insight into which flash to buy.
Ask a question, and do a search, about the flash you're about to buy.
Phoenix (slave flash D91-BZS)