Benji Mast’s Photoblog

Warning post contains levels of technical detail like never before on this blog

Thanks Elliot for the question on technique for my Luminance. Heh, this is going to abbreviated. I just lost about half a page of carefully thought-out technique. I think I hit the keyboard shortcut for “Back” and lost the entire post.

A little bit of jargon that should be defined before we begin.

stop: refers to light and exposing it properly. increasing exposure by one stop doubles the total light recorded by the camera. one stop less of light decreases light amount by one half.

ambient light: all the light besides the flash

In order to understand how to balance flash and ambient light, an important concept needs to be grasped. That is that the duration of the flash bulb actually being “on” is a small fraction of the actually exposure. So shutter speed makes no difference in flash exposure. The sequence of events surround the taking of a photo using a flash goes something like this.

1. Shutter opens
2. Flash fires quickly
3. Remaining exposure time passes
4. Shutter closes

So lets get a working scenario here. We’re shooting “Sara” outside under the shade of a tree so there is less ambient light hitting her than what is in the background. For learning purposes we will assume there is no ambient light hitting her. To start out with we meter for the background and figure out what the camera wants to expose the background /ambient for. Let’s say it wants to expose for 1/30 5.6. Now lets say our flash is set at 1/4 power, on or off camera, about 10 feet away. So we set it to “M” for manual exposure and set it to 1/60 5.6 because we want to underexpose the background by 1 stop to make Sara lighter than background and “pop”. Sara’s face is too dark. Now we have a number of options to brighten her face by 1 stop, increase the power of the flash to 1/2 power, decrease the distance between subject and flash, and move the flash 5 feet closer, or open up the aperture. So in order to keep things complicated we will open up the aperture and pop off a frame at 1/60 4.0. Now ambient is back a “properly exposed” according to the camera and we wanted underexpose it a stop. Decrease the shutter speed to 1/120 or 1/125th, and keep the aperture at 4.0. And Sara’s face is where we want it exposure-wise, but by this time her smile has worn out and you’re left with a nice technical photo.
In review, all things remaining the same, the difference between 1/125th 4.0 and 1/60 5.6 is that the flash with be one stop brighter.
So on the Luminance picture I shot my maximum shutter speed I can use with a flash 1/200 to make so my flash wouldn’t have to work as hard. Then adjusted aperture until I got the proper exposure on the flower.

I hope all this makes sense and while I acknowledge that it may be a bit over some of my readers’ heads, I hope that those who do understand it, will correct my mistakes and ask questions to clarify what I have said.

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