Exposure, Histograms and Flashmeters
Of necessity, using studio flash requires the use of manual aperture and exposure times
. Automatic means of setting these parameters is not practical for any number of reasons. For most studio uses, the exposure time will be set a click or two below the camera’s maximum sync speed – typically about 1/125 second. The camera exposure time does not affect the amount of exposure from studio flash. >> See also: Syncing to the Camera - Sync Speeds
The standard method of determining proper aperture settings is via the use of a flashmeter
. If you know what aperture setting
you would like to use for a particular shot, you can test-fire each light in your system while metering its output with the flashmeter. You can then adjust the power level of each light to cause the meter to read the desired f-stop
. Normally, you would do this with an incident light flashmeter pointed toward the camera lens (though some photographers prefer to point the flashmeter toward the light they are adjusting).
Remember, the output of multiple lights will add to one another to produce an overall light level somewhat higher than the output of any individual light. Therefore, if you wanted to achieve an exposure aperture of, say, f8 you would probably want to set your main light first to produce 5/10f to 1f below f8 to leave room for the other lights.
Once you have established the settings you want for the individual lights, you would then take a flashmeter reading of all lights, with the incident mode flashmeter pointed toward the lens. You will likely not get exactly the f8 you were shooting for. Say you get f8 + 5/10. In this case, the easiest thing to do is to simply set the camera aperture to f8 + 5/10 (or the nearest click to that value). If you are picky, you could go to each light unit and drop it by 5/10f to get the overall exposure down to exactly f8.
In setting the balance between various lights in the system, you can either rely on predetermined ratios of main, fill, background, etc., or you can compose the desired balance by eye if you have accurate modeling lamps. >> See also: the "Modeling Lamps" section of Studio Flash Basics
However, with modern digital cameras there is a second step that is highly recommended before you take you pictures...
Your pro or semi-pro camera probably has the ability to display Histograms
of your shots. The histogram gives you a visual map of exactly what range of dark and light values are being exposed. Of particular importance with digital cameras are the highlights.
If any important part of the scene produces highlights brighter than the upper limit of the histogram, these highlights will be burnt out in the exposure and all highlight detail in that region will be irretrievably lost in the exposure. So, after you have made flashmeter readings, you should always look at the histograms to make sure nothing goes off the right hand portion of the scale. If it does, forget what the flashmeter says and adjust your camera aperture upward until the brightest parts of the picture are within the range of the histogram.
The converse is also true.
If your histogram indicates your highlights are well below the right side of the scale, it’s probably wise to reduce your aperture number or increase the power of your lights to bring the right hand portions of the histogram near the right end of the scale. This will maximize the dynamic range of your exposure to take full advantage of the camera.
For the very best results, you should normally set the ISO of your camera to the lowest possible number (typically ISO100) and shoot your important pictures in Camera RAW Mode so you can make final adjustments to color, exposure, contrast, sharpness, etc. in post-processing, using Adobe Photoshop or similar programs.
Setting Exposure Without a Flashmeter
If you don’t have a flashmeter or don’t choose to use one, you can also achieve good exposures solely by using the camera histograms. Simply set your camera to the aperture you desire and do some test shots, adjusting the power levels for the desired ratio of light and shadow and as required to bring the histogram into the proper range as described above.