BetterPhoto Q&A
Category: Problems with Photo Equipment - Tips & Tricks

Main and fill

I have to shot students in a class room in september and I would like to understand the following thing:

If my main is 11 and my fill is 8,2, how I calculate the avaerage in the aim to set my F stop? I don't understand the principle.

Somebody told me that at the total is suppose to be 18,,,

thanks

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August 12, 2007

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August 14, 2007

First a note about fractional f/stops.
The f/stop values are relevant for setting the lens aperture. We adjusts lens aperture to control how much light energy will play on the light sensitive film or chip during exposure. Additionally we use them to control depth-of-field. The f/number system can be daunting. We need this method because it provides a universal method to set the camera. You see, all cameras positioned to the same f/number will pass the same amount of light energy. This is true regardless of lens size or type. It’s kina a cross platform value. We need a universal lens setting method otherwise confusion. Now the f/number set (numbers) is: 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 (these are the full f/stops not fractional ones). This number set affords the ability to make changes via a twofold increment. Going right we reduce light energy (exposure) 50% per figure, going left increase 100% per increment. In most cases we don’t need the ability to adjust in a finer increment. The point I am trying to make, working with fractional f/values like 8.2 contributes to confusion. The tiny difference between 8.2 and 8 is minuscule. I am recommending, you should stick to the numbers in the set above, for now. OK to round to the nearest f/stop for your exposure settings.

Light shines on the subject from multiple sources. We need to take all into account when be set our camera. The main is usually set high and off to the side. The fill is placed near the camera lens (we are filling shadows from the camera’s prospective). The main and fill are accumulative. Say the main contributes 1000 watts and the fill 500 watts. Now both shine on the frontal areas of the subject. Thus the light energy will be 1500 watts where both commingle. The main does not reach everywhere. The nose and hollows of the face are in shadow (from the main). The fill on the other hand does reach into these shadows where the main can’t reach. Thus the shadows receive only 500 watts. The lighting can be view as a proportion. The value is 1500 frontal and 500 shadows. This is written as a ratio thus 1500:500. It is weird to express a ratio without reducing it. We look for a common denominator and reduce it to 3:1. This is the best lighting ratio to use for a portrait rendering.

How to set your camera:
If using a gray card, place card adjacent to the subject, square with the camera, use as the metering target. Meter with both main and fill on and set you camera as indicated.

If using an incident light meter, measure from the subject’s position, pointing the meter at the camera. Set the camera as indicated.

If using the subjects skin as a target meter with both main and fill on. This technique often results in too dark a skin tone on the final rendering. Best add 1 f/stop more exposure.

If the lighting ratio is 3:1, turn off the main and meter the face with only the fill on. Set camera as indicated. Turn on main, compose and shoot. This technique grant the necessary 1 f/stop increase.

In all cases final skin tone is determined by exposure. Bracket for best results.

Alan Marcus (dispenses marginal technical advice)
ammarcus@earthlink.net

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August 16, 2007

Maybe this is a trick question?
Take it light ;>)
M.

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August 17, 2007