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More Questions About On-Camera Flash

Thanks for your reply. What I am still struggling with (maybe I'm just technically challenged) is how a person can gauge the flash to function as either fill or the dominant light source. My desire to understand this is motivated by our first assignment in shooting direct flash and whether to make it fill or a key light. As an illustration of my question: If I metered the ambient light outside on a sunny afternoon to be f/16 and a shutter speed of 1/100th of a second how do I know how much flash is required to put out the appropriate power of light to accommodate the given meter reading for fill flash, or if I so desired, for enough output to become the main light source and overpower the ambient light? I understand the theory of assessing the need for fill verses key...what I am having trouble wrapping my head around is how to technically figure out how to adjust the flash to become fill or key, based off of my ambient light meter reading. Sorry if I seem thick headed about this. Thanks for your help, Paul.

Michael

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January 07, 2010

We fill shadows from the camera's perspective.

In the studio, we generally place a key light high to simulate afternoon sun. Likely, the key light will cast harsh shadows so we counter with a fill. The fill is placed to fill shadows from the camera's perspective. Therefore, a flash mounted on the camera fulfils this requirement.

The problem is how to adjust the flash for the fill task?

We want the fill to be subordinate to the key. If the fill is set in error brighter than the key, then the fill becomes the key.

If the fill plays on the subject with the same intensity as the key than the lighting ratio is flat, as it measures 2:1.

If the fill is one f/stop subordinate (50%) as compared to the key, then the lighting ratio is 3:1. This is considered the ideal ratio for portraiture.

If the fill is two f/stops subordinate (25%) as compared to the key, then the lighting ratio is 5:1, a bit mescaline (contrasty).

If the fill is three stops subordinate to the key, then the lighting ratio 9:1, quite contrasty, considered theatrical (very contrasty).

How to adjust: Best to use a flash meter. If the ambient light measures f/16 then expose at f/16 but we adjust the power of flash to f/11 for 1 stop - adjust to f/8 for 2 stops - adjust to f/5.6 for 3 stops. Keep in mind the fill is always set subordinate (more feeble) than the key.

If it is not possible to adjust the flash intensity then we use distance as the adjustment. The light from a flash falls off significantly with increased distance. Measure flash-to-subject distance and multiply by 1.4. This math yields a revised flash-to-subject distance that reduces light energy at the subject plane by 1 f/stop (50%).

All flash units have a guide number. This is a published value for your flash based on the ISO of the media. If you can't find the guide number place the flash at 10 feet and shoot a series at every aperture. Choose the best exposure of the series. Say f/11 was best then multiply aperture used by distance thus 11 x 10 = 110 this is the guide number for your flash at the current ISO setting.

Once we know the guide number we estimate flash-to-subject distance and divide. Example flash is 20 feet from subject then 110 ÷ 20= 5.6 (rounded). We set the aperture at f/5.6. The above math works for flash only but we can work it backwards to calculate how it performs as a fill.

Meter reading is f/16 for sunlight exposure. Thus 110 ÷ 16 = 6.8. This value is the distance in feet that will cause the flash to arrive at the subject plane at the same intensity as the sunlight. But we want the flash to be subordinate so we back it up thus we multiply this distance by 1.4 thus 6.8 x 1.4 = 9.5. This is the distance in feet for 50% or 1 f/stop subordinate fill. Multiply again 9.5 x 1.4 = 13.3 feet this is distance for 25% or 2 f/stop subordinate. Multiply again 13.3 x 1.4 = 18 feet for 25% or 3f/stops subordinate.

Sorry for the gobbledygook, don't dismiss this method, likely someone with more smarts will give you a simpler way.

Best of luck!

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January 08, 2010