Shallow – as in depth of field! One of the features / benefits / consequences of a wide aperture lens is the shallow depth of field as you open up the lens. Depth of field is simply how much of the picture is in focus. This picture was taken at f/2 with an f/1.8 EF 50mm lens. Basically only her right eye is in focus and the rest of her face is progressively blurry.
Why is the depth of field narrow? Lens design is very complex and lots room to get lost in. But the most basic explanation is the light going through the center of the lens follows a different path than the light going through the edges. The edge light has a different focus point than the center light. So when you close the aperture (stop down the lens) all the light goes through the same point and most everything is in focus.
But… everything in focus is not always good for a portrait. I want her face to be the focus of the picture, pun intended. I don’t care about the stuff behind her. So open aperture also gives the picture a single focus point.
Canon EF 50mm f/1.8 | Canon XSi | 1/100 sec at f/2.0
