I've got an understanding of apparent light size. I know what happens when I change the distance between my light source, my subject and my background. And I've moved through diffuse and direct reflections. Now it's time to get into some of the practical aspects of flash photography, and there's no place better to start than balancing ambient light with flash.
Some starting points
Just so we're all on the same page, I'm working with both my camera and flash in manual mode. I've learned to work this way because I've found that this allows me to control the final outcome of my images as opposed to giving up some of the creative decisions to TTL, aperture priority, or shutter priority modes.
As for the flash, I typically use a shoot-through umbrella to soften the light. If your working along, I suggest using an umbrella or some other type of diffuser, as simple as a white shower curtain if you don't have anything else. If you're using bare flash, you'll have to knock the power back from settings that I recommend.
So what do I recommend? Well, after doing this for awhile I've found that starting out at 1/4th or 1/8th power, with my flash fired through an umbrella somewhere between two to four feet from my subject is a good starting point. I've never had a flash meter, and learned to do this stuff by shooting, looking at the image, and making adjustments as needed. I can tell you with some degree of certainty that if you start out at an 1/8th of power, and then set your camera's shutter, ISO and aperture correctly, you'll be in the ballpark for a good image. So what about those camera settings. Well, that's where we're going now.
At the heart of this balance concept is the idea that there are two exposures occurring every time you press the shutter release, the ambient exposure, and the flash exposure. If we take a step back and just think about exposure in general, we know there are three variables at play that affect the outcome: shutter speed, aperture and ISO. Whenever we make a photograph, we adjust each of these independently to get the proper exposure for a given scene.
Now, back the the two exposure thing. When a flash fires, it occurs instantly, and it doesn't really matter how fast or slow you have your shutter set so long as you're at or below your camera's max-sync speed, typically 1/250th or 1/200th of a second. That being the case, our flash exposure can be controlled by tinkering with two of the variables mentioned above, aperture and ISO. Change the shutter speed and you'll see no change to your flash exposure.
Then there's your ambient exposure. Well, we're already familiar with how we affect ambient exposures, we do that all the time by monkeying around with all three variables, shutter speed, aperture and ISO.
So, in the simplest of scenarios, if we snap a shot at say ISO 200, f5.6, 1/125th of a second, fire a flash to light our subject, and you get a resulting image in which the subject is too bright by one stop but the rest of the image looks okay, you know you've got to alter the flash exposure but keep the ambient exposure the same to produce a decent photograph.
Let's handle the subject first. Since speeding up the shutter won't do anything to darken our subject, which is being lit by flash, we need to close down our aperture by a stop to f8. But wait, changing the aperture affects both the ambient and the flash exposures, so now that our subject is perfectly exposed but our background is one stop under exposed. Well, that's not a problem, because we can move our shutter speed around to alter the ambient exposure without affecting our flash exposure. So, by dialing the shutter down a stop to 1/60th, we've brightened the ambient lit part of the image but haven't changed the flash exposure at all, resulting in balanced light. Easy enough, right? Actually, maybe some real world examples will help, and so I've followed an exercise from the Strobist website for this post.
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