With any camera you can take several different kinds of aerial photographs. The type chosen depends on the goals of the photographer. Vertical airphotos are taken at a vertical viewing angle from the ground and oblique airphotos are taken at any other viewing angle to the ground. Low-oblique airphotos show only the surface of the ground, whereas high-oblique airphotos include the surface, the horizon, and a portion of the sky.

Framing Cameras

A framing camera is a camera that captures the entire scene with each exposure--just like a typical household camera.

The distance between the lens and the film at the back is called the focal length. The shutter and the diaphragm are usually located directly behind the lens. The shutter controls the length of time the film is exposed to light, and the diaphragm controls how much light reaches the film while the shutter is open. The aperture controls the size of the diaphragm--the larger the diaphragm, the more light will reach the film. Together, all of these parts control the film exposure.

Film exposure is important because if there is too much light, the photograph will be too light to analyze. On the camera, the aperture control chooses the appropriate F/stop, which will increase or decrease the aperture. The F/stops are determined by the focal length and the lens diameter of the camera.

F/stop = (focal length)/(lens diameter)

Example F/stops are: F/1, F/1.4, F/2, F/2.8. The larger the number, the smaller the aperture, and the less the film exposure. If the shutter speed is increased as the aperture is decreased, then the film exposure will remain the same.

Mapping Cameras

Mapping cameras are usually used for professional mapping, such as for the USGS topoquads. They are specially designed to acquire vertical airphotos with high resolution and minimal distortion. Below is a diagram of an Apollo mapping camera.

The following features decrease distortion:

1. The film is vacuum-sealed to a metal plate at the time of exposure so that any wrinkles in the film are eliminated. After exposure, the seal is released and the film is advanced.
2. Since the camera is moving at the time of exposure, there is image motion compensation (IMC). IMC is usually applied by having the film move very slightly (in reverse motion to that of the plane) at the same speed of the relative ground motion.
3. The lens is set back in the camera and is surrounded by a sort of cone that eliminates all light except that coming from the targeted scene. The lens itself is compound and has reference, or fiducial, marks drawn on it. These marks help with the interpretation of the photograph.

Reconnaissance Cameras

These cameras are similar to mapping cameras except they do not have features to reduce distortion. Therefore they are usually cheaper and used for interpretation instead of mapping. There are 2 kinds of reconnaissance cameras--single-lens and multiband.

Single-lens cameras use 35 or 70 mm film and can be used to take oblique or vertical airphotos. The multiband cameras separate incoming light into different parts (visible vs. IR). This can be done in 2 ways: one camera with multiple lenses, or a multiple-camera array.

Multiple-lens cameras have been developed with up to 9 lenses, but the most common is with 4. Each lens is covered with a filter so that it only captures a certain wavelength of light. The multiple-camera array consists of 2 or more single-lens cameras (usually 4) mounted on a base. Each camera has a different filter, and the array is usually set up with a trigger that will open all of the shutters simultaneously.

Avery, T.E. and G.L. Berlin. 1992. Fundamentals of Remote Sensing and Airphoto Interpretation. 5^th ed. Upper Saddle River, Prentice-Hall, Inc.