Common photography myths (7)

Myth #6: Flash range is increased when using positive flash exposure compensation

September 5th, 2009 - 08:35:23 PM:

Actually, the opposite is true. But first things first:

An electronic flash basically is a flash bulb connected to a capacitor. The batteries of the flash unit load the capacitor, and the camera closes and opens the circuit between capacitor and bulb. With TTL-OTF metering, the camera closes the circuit at the beginning of the exposure, and when it detects that proper exposure is reached, it opens the circuit, cutting off the flash. “Proper exposure” here means that the sensors have detected a total amount of light that causes a mid-toned image on the given film.

It's obvious that the load of the capacitor and therefore the burn duration of the flash bulb is limited. When the capacitor is exhausted before proper exposure is reached, the image is considered underexposed by the camera. Objects farther away from the flash receive less light from it, so to properly expose objects farther away, the flash has to burn longer. This distance is limited because the load of the capacitor and therefore the burn duration is limited. We call this maximum distance “flash range”. Objects at this distance or closer can be properly exposed by this flash unit with the given film sensitivity and lens aperture.

Some people think they can “juice up” the flash and extend the flash flash range by dialing in a positive flash exposure compensation on their camera. But when you understand that the flash works as explained above, you also understand that turning a knob can't increase the maximum load of the capacitor and therefore also doesn't increase the maximum burn time of the flash bulb and doesn't increase flash range. It would be really neat if you could save a lot of money that way, but it doesn't work.

What flash exposure compensation really does is move the cut-off point. With negative flash exposure compensation you tell the camera to cut off the flash earlier than normal, and with positive flash exposure compensation you tell it to cut off the flash later than normal. The main purpose is to compensate for subjects that are not mid-toned, so that flash exposure doesn't result in images that are mid-toned when the subject is not. So when the subject is too far away and the capacitor is exhausted before the cut-off point, it doesn't help at all to move the cut-off point even farther away.

But didn't I say that doing this even reduces flash range? How is that possible?

When using positive flash compensation, you basically tell the camera that proper exposure is not reached with the normal amount of light, but with more. Since you can not output enough light to reach that level for objects at maximum flash range, the image is underexposed for the camera. You have to move these objects closer to reach the level given to the camera, thus effectively reducing flash range for the given level.

Sorry, there are no cheap tricks to increase flash range. You have to either use a stronger flash with a larger capacitor, use a faster film or shoot at wider apertures.

[Real-life modern flash systems are a lot more complex. It would have been too complicated to explain it all here. But these systems still have a limited load of the capacitor. So even with the latest whiz-bang flash system, you can not increase flash range easily.]

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#1: Comment posted by Pranav on January 31st, 2012 - 11:53:56 AM:

Hi,
Myth#6:-
Last line:-
"You have to either use a stronger flash with a larger capacitor, use a faster film or shoot at wider apertures."

I am not able to understand how shooting at wider aperture or faster film(i guess you mean shorter exposure time)can increase the flash range?

Michael Hohner answers:

Flash power is measured as Guide Number, which is calculated as aperture number multiplied with distance at ISO 100. So for the same GN, using a larger aperture (smaller aperture number) results in a greater flash distance. When you use a larger aperture, more light enters the camera, so objects at greater distance can be illuminated less by the flash and still be considered exposed correctly.

The same is true for increased film/sensor sensitivity. Objects at greater distance can be illuminated less by the flash and still be considered exposed correctly, because the higher sensitivity compensates.

Exposure time is irrelevant for regular flash and low ambient light.

Also see the Flash Compendium for more details.

#2: Comment posted by Pranav on February 2nd, 2012 - 05:40:19 AM:

Hi,
With increased sensitivity of film or sensor you mean higher ISO number?

Michael Hohner answers:

Yes.