My Photo Blog

What Is An f/stop?

For a lot of people starting out in photography, the f/stop has been a point of confusion. Because as the “f” number grows larger, the amount of light going through the lens decreases. On the surface it seems like an oxymoron, but as you start to understand it, it does make sense. For example, we will look at a 50mm f/1 lens. 50mm is the focal length of the lens and f/1 is it’s speed, which translates, the maximum aperture of the lens is 50mm in diameter, same as it’s focal length. Now when we stop the lens down to f/2, the light is now traveling through a smaller opening, decreasing the amount of light and increasing the exposure time, as showing in the diagram below. If you could take apart your lens and trace the f/2 opening two times side by side, this gives you an example of what f/2 means. In essence, each time you stop down, you are increasing the focal length of the lens. Lenses that have low numbers such f/1, to f/2.8 are considered fast, verses lenses with the widest opening being f/5.6, or smaller. So, what’s the point? Why can’t we just work with the lens at it’s widest aperture and cut out the confusion? It’s all about controlling exposure and depth of field in our photograph. As we enlarge, or reduce the f/stop, we change the volume of light traveling to the film plane. Too much light will over expose our image and not enough light will under expose our image. A creative function of the f/stop is also to control depth of field. The photograph of the flags in the beginning of this article was taken with a 145mm lens set to f/2.8, giving the image a shallow depth of field. The city scape below was taken with a 35mm lens set to f/8 creating what appears to be an endless depth of field.Below are two additional examples taken with a 50mm f/1.4 lens. In the first image the aperture is set to f/1.4 and as you can see the depth of field is extremely shallow. The only part in focus is the middle slice of garlic. The second version is set to f/16 and the depth of field is greatly increased.



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