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The manual flash mode maybe the most basic of flash modes around used for extreme macro, but it is also the most useful one that's around.

Most flashes come with manual modes nowadays; both my Pentax AF360FGZ and AF540FGZ flashes have it; so does the AF160, but the Metz-58 AF-2 excels with a range down to 1/256 power in 1/3 stop increments. The newest Pentax DSLR also offers manual onboard flash, although its successor the K-3ii opts for a GPS unit over an onboard flash instead.

What is a Manual Flash Mode?

the Metz-58 AF-2 excels with a range down to 1/256 power in 1/3 stop increments

The manual flash mode is a flash setting that lets the photographer decide the power output of the flash when it pops. It is typically set on a scale that halves, ie 1/1 power, 1/2 power, 1/4 power, 1/8 power, 1/16 power and so on. The better the flash the more options you have and the broader the range of the options.

The Sharper Image

Flash Duration

Flashes on manual settings have an extremely short duration: this freezes motion and makes for sharp extreme macro. For example, the Canon 580Ex:

Flash Power	Flash Duration
1/1 power	1/1000 second
1/2 power	1/2000 second
1/4 power	1/4000 second
1/8 power	1/9000 second
1/16 power	1/15000 second
1/32 power	1/21000 second
1/64 power	1/30000 second
1/128 power	1/35000 second

For extreme macro, using your flash at a low manual power setting has the immediate benefit that it freezes motion and therefore renders your photograph sharp. It gives you none of the fuzzyness that you might have using continuous lighting on an unstable platform, because the flash pop is so fast that you are effectively using a fast shutter speed.

Although the flash sync speed dictated by your camera maybe 1/250 or 1/180, if all your light comes from the pop of the manual flash, and the duration of your flash is 1/20,000 of a second, then for all intents and purposes your exposure is actually 1/20,000 of a second, the 1/250 or 1/180 doesn't matter. That's happy days.

Of course, if it's very bright out and you need a shutter speed faster than the flash sync speed then you'll need to look at HSS, but that's another story.

Battery Conservation

Using a flash with manual settings at a low power has plenty of other advantages, not the least of which is battery conservation. A flash being used at 1/32 has a much longer battery life than a flash doing pre and main flashes for PTTL. And longer battery life is always welcome. Not only that, but sticking to a low manual power fraction also gives you a nice fast flash recharge time, which is the huge advantage that an external flash unit gives you over using the onboard one. Means you can use bracketing and fast bursts, so you get more keepers.

Rear Sync

One thing to keep an eye on if purchasing a flash unit is to check that the flash unit you intend to buy can do both manual settings and rear sync together without requiring the use of *TTL using the optics of your choice. It is exceptionally annoying, for example, that a Pentax K-7 and Metz 58AF-II can do these together, but for some inexplicable reason the Pentax K-5, the successor to the K-7, cannot. But it's a great relief that the K-3ii behaves in this regard, and lets you have manual rear sync flash.

Consistency

When you're using your flash for an extreme macro stack, lighting consistency is your friend. Although electronic flashes can show some inexplicable inconsistency variation between shots, using the flash at the same manual setting minimises any potential variation between flash pops. Consistency is good.

Flexibility

check that the flash unit you intend to buy can do both manual settings and rear sync together

Manual flash doesn't require the communication between a camera and the flash that PTTL requires. Therefore, it is perfectly possible to use manual flash with PC sync, adding flexibility to the potential flash setups that you can use. The other way of using it would be a cheap eBay radio trigger. This is the way I use manual flash on my K-3ii for some fill in butterfly photography, works a treat.

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