There are four main functions of the lens. The first and most obvious is The second function of the lens is controlling the quantity of light or while the shutter speed controls how long the film will be exposed to light, the aperture controls how much light it will be exposed to. Aperture is measured in F-stops Some common f-stops are f22, f16, f11, f8, f5.6, f4, f2.8, f2, f1.4 conveniently, these are also the same f-stops on the lens in the enlarger. The lower the number, the bigger the aperture is An f-stop number is determined through a mathematical equation of the focal length the diameter of the aperture. Like shutter speeds, apertures run on the principle of doubling and halving. So, a standard aperture will be half as big as the one to one side of it and twice as big as the one to the other side of it. Some lenses, like some shutters have half stops, allowing more of a range of shutter and aperture but confusing this hole doubling/halving thing even more. For example, assume you have a 50mm lens and a 25mm aperture, you are using an f-stop of F2. Since this hole is half the size of the lens, you can imagine that f2 is a pretty big aperture. The next function of the lens is to control the area and angle of view. This is determined by the focal length. Focal Length = The distance measured in mm from the film plane to the nodal (or mid) point of the lens. The longer the focal length, the narrower the angle of view. ie: a 28mm lens is wider than a 135mm lens. A 50mm lens is an average or normal size lens. Anything larger will get you closer but give you a smaller angle while anything smaller will allow you to get closer with a wider angle. The final function of the lens is controlling Depth of Field refers to the zone of focus or the distance between the first and last thing in focus in your frame and is determined by three things... The aperture the smaller the aperture, the greater the Depth of Field The focus distance the greater the distance from the camera to the subject, the greater the Depth of Field. ie: 15 feet is a better Depth of Field than 5 feet. The Focal Length of the lens the longer the focal length, the smaller the depth of field. Aperture Point of Focus Focal Lenth How do you determine what your depth of field is when you are taking a picture? Measure it! Some cameras have a depth of field scale right on the lens. The relationship between Aperture and Shutter Speed is Reciprocal So when we shoot our project, we bracket images like so ImageApertureShutter