mgo

Mathematically you're right f-number = focal length / effective lens opening diameter. But from a photographer's POV the other way round is more interesting, since you set the f-number on the camera: effective lens opening = focal length / f-number. You often see the f-number denoted as "f/8" or somthing like this. This reminds of the equation above and means that the eff. opening of the lens equals to 1/8th of the focal length. The standard sequence of f-numbers (1.4 - 2 - 2.8 - 4 - 5.6 ... ) was designed to increase or decrease the amount light passing through the lens by a factor of 2 depending on whether you open or close the aperture. So for example opening the aperture from 5.6 to 4 doubles the amount of light that passes through the lens. This "normalization" of the lens opening enables us to measure the exposure and to properly set the camera and lens independend of the focal length. It also allows us to setup strobe and camera to properly expose the image manually. Look into the manual of your strobe and find the guide number. This "magic" number is defined as gn = distance * f-stop That makes it easy to find the correct f-stop for your shot: Estimate the distance between your camera and your subject and divide the gn by that value, there you have the f-stop you need. That's how the exposure table on the strobe has been computed and it's why Mike's method works! Happy bubbles and good images!

Buying a new PC running windows 7 (or switching to it) might make you face the same problem. On a Mac you may be able to solve this by using a Windows emulation software. I'm using two macbooks and a DELL PC running Windows XP. In general the Mac is really somewhat more comfortable and easier to use. Software installation/removal simply means copying programs to or deleting them from the Applications folder, connecting to networks (Ethernet, WLAN), attaching external hardware and general adminstration tasks can be done with the help of easy to use and easy to find programs. If you need to use Microsoft office then exchanging data between a company PC and your home Mac is just a matter of copy the files and continuing your work. From my personal experience most programs run more smoothly on a Mac than on a PC (assuming computers with comparable power and similarly equipped). That's why using a Mac is simply more fun. You see: my vote is for the Mac! Regards!