BetterPhoto Q&A
Category: New Answers

External flash, fill flash..help!!

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March 03, 2008

Flash in bright sun becomes a fill flash. Shadows cast by the sun are generally harsh, luckily they can be filled using reflectors and at close ranges, an electronic flash. The trick to using the flash is to make sure the light from the flash arrives at the subject plane subordinate to the main light which in this case is the sun. In most cases we want the fill to arrive at the subject plane 50% weaker (1 f/stop) for the classic 3:1 lighting ratio and 2 f/stops subordinate for a more contrasty 5:1 ratio.

Perhaps others will give you outstanding and achievable advice. Here comes mine which I and others often labeled as pure gobbledygook. You need to ascertain the flash unit’s guide number. It’s in the booklet. Each ISO has a specific guide number so choose the guide number intended for your ISO setting. We often use guide numbers, they are a tried and true way to calculate aperture setting. When we use them we throw out the automation. Stated another way the guide number is used when we manually set our camera using flash. It goes like this: Once the guide number is known, we arrange the scene and read off from the camera’s focus scale the subject distance. Say the guide number is 100 and the subject-to-flash distance is 12 feet. We divide this distance into the guide number. As an example: 100 divided by 12 = 8.3 we round this value and set the camera to f/8. This method works like a charm for indoor shots.

For outdoor fill we employ this method with a twist. Outdoors we meter the scene or allow the camera’s automation to meter for us. In any event let’s pretend the results are 1/250 second at f/11. Now we divide the guide number by the indicated aperture. For this described circumstance the math: 100 divide 11 = 9. That math tells us the required distance flash-to-subject that will cause the light from the flash to arrive at the subject plane at the same brilliance as the sunlight. That’s nice but we want the flash weaker or the results will be too flat. We multiply this value by 1.4. Thus 9 times 1.4 = 12 1/2. This is the flash-to-subject distant in feet that yields a 3:1 ratio. Multiply again 12 1/2 times 1.4 = 17 feet this will be flash-to-subject for a 5:1 ratio.

Electronic flash is balanced for daylight so daylight is OK I use auto white balance most of the time.

Nobody said my gobbledygook was easy.

Alan Marcus
ammarcus@earthlink.net

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March 03, 2008

If you want to adjust the ambient/flash exposure balance from that chosen by the camera's programming, you can apply FEC (flash exposure compensation) or manual output level (1/1, 1/2, 1/4, ...) on the 430EX, and control the ambient exposure by shooting in M.

In sunlight the 430EX can be set for FP High Speed Sync ("lightning bolt H" symbol) so that the camera is not limited to shutter speeds of 1/200 and longer.

The color temperature of the flash is 5500-6000°K, and is close enough to daylight to not normally need any special white balance. The ambient light outdoors is going to range from 5200°K in full sun, 6000°K when overcast, and 7000°K in full shade.

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March 04, 2008

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March 04, 2008

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March 04, 2008