Lens Basics
Lenses are foundational pieces of equipment in every aspect of photography. While they’re far more complicated to manufacture than they are to use, learning how they work in practice often takes a lot of time and experience. Before I can go on to talk about which specific lenses are well suited for architectural photography, I want to summarize the basics of what lenses are and what they do. This will also cover some important terminology.
A simple lens is a single piece of transparent glass where at least one of the surfaces is curved. Lenses, as we refer to them, are compound lenses made from individual elements that share a common optical axis. A lens barrel houses and protects this compound design. The function of a lens is to gather in light and bend it to converge onto an aperture. Light exits the lens through its rear nodal point and is then projected onto a recording medium. If all goes well, this produces an image of a subject in sharp focus.
The aperture is a circular opening that allows light to pass through the lens. On any lens made in the last century, the diameter of the aperture is variable. The opening is adjusted with a ring located on the lens barrel. The aperture value is written as an f-number, representing the diameter of the opening as a fraction of the lens’ focal length. For example, the diameter of the aperture f/8 on a 50mm lens is 50mm ÷ 8 = 6.25mm. On a 150mm lens, f/8 will be three times larger. However, knowing what the number means is less important than understanding the role of the aperture in practice. Its basic function is to provide control over the volume of light entering the camera in tandem with a shutter which controls the duration. It is also used to alter depth of field while also influencing image sharpness.
Illustration of variable aperture on a 75mm Rodenstock Grandagon-N lens. From left: f/5.6, f/13, f/45.
The rear nodal point is another important element of the lens. It is the position within a compound lens, usually near the aperture, where light rays cross each other. It's important to know where this point is on a given lens for advanced applications, such as complex view camera movements and multi-image panoramic stitching. It is also important because the distance between the rear nodal point and the sensor (or film) plane constitutes the focal length of the lens.
A common focal length for 35mm format cameras is 50mm. These numbers are both notated in millimeters but they mean different things. 35mm refers to the dimensions of the film or sensor, which is usually 36x24mm. It should be called 36mm format. The focal length of the lens, 50mm in this instance, is the distance between the rear nodal point and the sensor. Converting from metric to imperial gives us 1.97 inches. It's an obsolete naming convention, but we could round up and call this a 2-inch lens.
The position of the rear nodal point places a lens into one of three categories: standard, telephoto, or retrofocus. On a standard lens, this point is either at or very close to the aperture. On a telephoto lens, the rear nodal point is located in front of the aperture. This means that the rear element of a telephoto lens is closer to the sensor plane than indicated by the focal number. A retrofocus lens is an inverted telephoto, where the rear nodal point is located behind the aperture, and the rear element is farther from the sensor plane than the focal number.
The focal length of a lens is fixed regardless of the physical dimensions of the recording medium. A 50mm lens is always that, no matter what it’s mounted to. However, the same focal length lens will have a different angle of view if the size of the sensor changes. Angle of view is the amount of coverage in front of the camera to each side of the optical axis. There are three groupings when referring to angle of view: normal, wide, and long. Normal lenses have an angle of view corresponding to human vision, an angle of about 50 degrees. Wide lenses have a larger angle of view, 65 degrees and more. Long lenses have a smaller angle of view, usually 35 degrees or less.
What determines the focal length of a normal lens on a particular camera is the diagonal length of its sensor. For example, this measurement on a 36x24mm sensor is 47mm, meaning that a lens with a focal length of 47mm or thereabouts is a normal lens in the context of the 35mm format. A lens with a shorter focal length, 28mm for example, is considered wide. Anything with a longer focal length, say 135mm, is considered long. Change the size of the sensor, and these relationships change. The Panasonic Lumix G9, a micro four thirds format DSLM camera, has a 17x13mm sensor. The diagonal is 21.4mm, meaning that any lens with a focal length of 22mm or more will be a long lens on this camera.
This is general information and applies to the whole of photography. There is so much more to talk about with lenses. I haven’t even looked at lens imperfections, like chromatic aberration or field curvature. And field curvature will come up again! Nor have I covered the intricacies of lens design, most of which is beyond my pay grade. But it’s best to stop here, and then get into greater detail when necessary as more advanced problems arise. The next stop is a look at perspective control lenses for architectural photography.