Another confusing idea tied to these aperture values is that it would seem logical that going from f/4 to f/2 would let in twice as much light. However, that is not the case. Going from f/4 to f/2.4 lets in twice as much light, and then going from f/2.4 to f/2 lets in twice that amount. So, in reality, going from f/4 to f/2 lets in four times the light!
While cameras and lenses can be set to various f/stop values that are not shown on our chart, these represent the change of one full stop. So, a value of 3.5 (as many kit lenses have for a maximum aperture value) is somewhere between f/2.4 and f/4. the distance from f /3.5 to f/4 is not a full stop. If, however, you realize that your meter is showing that you are 1 stop under exposed at f/8, you will know that moving the aperture to f/5.6 will set your camera for proper exposure. Using the chart above with the meter readings from your camera will help you to understand what is happening. I understand that most DSLRs today have the information readily available, usually just by looking through the view finder. Simply line the marker up with the “0” on your scale, and your exposure will be correct. However, if you are like me, it is not good enough to just know what is correct. I want to know why the settings are correct! By understanding the relationship between each of these numbers, one can know what aperture changes to make without simply running through the trial and error process. Knowing what to do makes the process of changing settings on the fly much easier!
To illustrate these principles, we set up a little experiment to visualize the different light transmission based on the aperture set. Each of these photos were made with my Canon EF 50mm f/1.8 II Camera Lens, set at 1/40 sec shutter speed, and ISO 400. We set the first aperture to f/2 and then progressed one full stop at a time through f/11. Published below are the results through f/8 (the last photo with any discernible detail).
Jacob found this exercise to be quite helpful in understanding the concept of aperture. He could see the distinct difference as the light was cut into half for every full stop. Principles that he was struggling with suddenly became clear to him. While he will still have trouble remembering the list of f/stops, how the change of aperture impacts his exposure will stick with him going forward!
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