Flash Friday: Know Your 'Full Stops' for Aperture, Shutter Speed, ISO, and Flash Power
"Full stop," what does that mean? Well, as a photographer who wants to take control of your camera, it's a concept you need to try to understand. And as one who also wants to take more control of using your hot shoe flash off camera creatively and effectively, you definitely should try to understand full stops.
It is easy to grasp that when you change your shutter speed from 1/250 to 1/125 you have doubled the amount of time your shutter remains open (1/125 is a slower shutter speed than 1/250) and, conversely, when you increase it from 1/250 to 1/500, you cut the amount of time your shutter remains open in half. Simple math seems to make that fairly obvious. So, the change of a shutter speed that halves or doubles the speed is referred to as a "full stop." The term 'stop' comes from the days of film cameras where when one increased or decreased a setting, one moved a mechanical control to the next 'click' or 'stop' on the adjustment dial. Unlike digital cameras, one often only could make full stop changes. There was not always the ability to make intermittent stops between full stops as there is now with modern digital cameras, where most digital cameras today will allow one to even choose between 1/3 and 1/2 stops!
We refer to full stops when we make ISO and aperture changes, as well. Though somewhat of an oversimplification, ISO controls the camera sensor's sensitivity to light. The lower the ISO, the less sensitive and the higher the ISO, the more sensitive the sensor is. Changing one's ISO from 200 to 400 doubles the sensitivity level and is, therefore, referred to as a "full stop" increase in ISO.
The same is true with aperture values, too. An aperture value is, typically, expressed like this: f/8. The "f" refers to the focal length of the lens and the actual aperture value, i.e., the actual numerical value of the circumference of the lens opening, is the result of dividing the selected aperture number (in the above example, 8) into the current focal length of the lens in use. That's a lot of math and, thank goodness, you don't have to worry about it. What you as a photographer do need to understand, though, is that as that number gets smaller the aperture opening (the circumference) gets larger and, conversely, as it gets larger i.e., f/11, f/16, etc., the aperture opening gets smaller. Now, with aperture values, the math is not as obvious, as an aperture opening of f/11 is twice as large (double again!) as one of f/16. And an aperture opening of f/8 is twice as large as one of f/11. Therefore, to cut an aperture of f/8 a full stop to reduce the light hitting the sensor, you change it to f/11.
Now, as you add flash to the equation, you will learn that you control how much light your flash unit splashes onto the scene by changing the power level of your flash. Working in Manual flash mode, as I recommended in a recent post (http://www.whboozimages.com/), you will be able to adjust these power settings which will start at full power, usually indicated on the flash unit's LCD with value 1/1. How you actually do that varies somewhat from flash model to flash model, so refer to your flash's guide or manual. Most hot shoe flashes, even the less expensive ones, will have power settings in the range from 1/1 to 1/8. The "full stop" power settings in between will look like this: 1/1 - 1/2 - 1/4 - 1/8. Again, you see the parallel with your other exposure controls since a power setting change from 1/2 to 1/1 is clearly doubling the power output and is, thus, a "full stop" on the flash's power scale. More expensive flashes will usually give you control all the way down to 1/128 power, giving you a broad range of control of your hot shoe flash.
Download the image attached to this post, print it, and keep it in your camera bag until the full stops for all the exposure, that is, "light," controls on your camera are second nature for you.
Check out my photo classes and workshop page: http://www.infotor.com/photoclasses! Classes for Fall 2012 available for review and registration.



