In a previous episode, we explained the three things affecting exposure are:
1) aperture; 2) shutter speed; and 3) ISO.
And to establish your exposure, you use the Sunny 16/Basic Daylight Exposure rule. Set your f-stop at f/16, and and then simply matching your shutter speed to your ISO number. For example, f/16 with an ISO of 200, so your shutter speed is 1/200th of a second.
But what if you need a certain f-stop, shutter speed or ISO? From your correct Sunny 16/Basic Daylight Exposure, if you change one setting, then the exposure is off and you must change another setting to compensate. It is like balancing a scale. If you adjust the aperture and it changes the exposure, you have to select either the shutter speed or the ISO to make up for the difference.
For example, if the Sunny 16/BDE exposure is f/16 at 1/125th and we change the aperture to f/11, we opened up one stop (meaning we are letting in twice the amount of light). We must then cut the amount of light hitting the sensor by ½ using the shutter speed or the ISO to equalize the exposure. So, in the case of adjusting the shutter speed, the exposure changes from 1/125th to 1/250th.
Here’s a tip we regularly share in the workshops: Count the clicks! On our DSLR cameras, however many clicks you move, say, the aperture dial in one direction, then the shutter speed control must be moved in the other direction the same amount of clicks.
But here is an important observation! The image exposure would be absolutely the same but images would appear very different.
When you change your aperture and shutter speeds, aside from changing exposure, you are also changing to visual affects that are byproducts of the aperture and shutter speed: depth of field (connected to aperture) and motion blur (connected to shutter speed.
ISO also comes into play as a control that you can adjust instead of aperture or shutter speed. And sometimes you just can’t change your aperture or shutter speed. Say you need a certain depth of field or you need to freeze action … your only choice is to change your ISO.
The combination of these three controls, an exposure triangle, allows you the most control over your image.