Small strobe talk and the two exposures.

The lighting talk went very well on Thursday. In the interim we were overrun by a whopping storm here in Indy, and we had a fair bit of cleanup to do this weekend. (We had twelve to fourteen inches of water in our basement.)

One of the points I wanted to bring up again here concerns adjusting the ambient exposure separate from the flash exposure. This seemed to be a little confusing, I know it was for me. After the talk I thought of a better way of explaining it. Indulge me a little, this could get hokey:

Imagine a black room with only a door for entry; outside it’s a bright day. If you open the door for 1/30th of a second you let in 1/30th’s worth of light. If you open it for 1/60th you get half as much.

Now imagine you’re a burst of light from a flash (I warned you.) The door opens for 1/60th and you walk in. The room was lit with 1/60th’s worth of outdoor light and a… yous worth of flash light. If you open the door for 1/30th and walk in you get twice as much outdoor light but still the same amount of flash light.

This all make sense so far?

Now, the door is the kind from a barn, where the bottom half opens separate from the top. You open the top half for 1/30th of a second and get half as much ambient light as your previous 1/30th shot. Because the door is only open half way you also get half as much of the flash burst as well.

So, if you change the shutter speed you’ll get the same flash exposure, but a different ambient exposure. If you change the aperture you change both the ambient and flash light that hits the sensor (or film). Changing the flash power of course will change the amount of flash light and changing the ISO will change how sensitive the sensor is to both exposures. Did this last line just push you over? :)

Thanks to everyone who came on Thursday; I had a really great time teaching. I bought a sheet of 1/2 CTO and cut it into flash size pieces, I’ve got a few more if anyone needs it.

p.s. My little explanation is obviously a little over simplified. There are things like maximum sync speed to think about; in most cameras it’s something like 1/250th or 1/500th. There are ways to cheat on some cameras. I suggest you read up on Strobist for the details.