Aerial Cameras for Photogrammetry and Main Parts of Frame Aerial Cameras

Aerial cameras, by virtue of their application, have certain special characteristic as compared to normal cameras.
Important of Aerial Cameras :
Lens remains focused to infinity
Bigger format
Aerial Cameras for Photogrammetry

Arrangement for the operation of the camera forms a moving platform, to neutralise the effects of  aircraft motion and vibration.
High resolution lensed to overcome the atmospheric effects to some extents
Arrangement for exposures at predetermined intervals.
Main parts of frame aerial cameras

The three basic components or assemblies of a frame aerial camera as shown in the generalised cross section are
The magazine
The camera body
The lens cone assembles

The camera magazine houses the reels which hold exposed and unexposed film, and it also contain the film advancing and film flattening mechanisms. Film flattening is very important in aerial cameras, for if the film flattening mechanisms. Film flattening is very important in aerial cameras, for if the film should be buckled during exposure, image positions on the resulting photographs would be incorrect.

The camera body is a one piece casting which usually housed the drive mechanism. The drive mechanism operates the camera through its cycle; the cycle consisting of
Advancing the film
Flattening the film
Cocking the stutter and
Tripping the shutter

The lens cone assembly contains a number of parts and serves functions. Contained within this assembly are the lens, filter, shutter and diaphragm. The camera lens is the most important (and most expensive) part of an aerial camera. It gathers light rays from the object ape and brings them to focus in the focal plane behind the lens. Lenses used in aerial cameras are highly corrected compound lenses consisting of several elements.

The filter serves three purposes:
It reduces the effect of atmospheric haze
It helps provide uniform light distribution over the entire format
It protects the lens from damage and dust

The shutter and diaphragm together regulate the length of time a given amount of light is allowed to pass through the lens to make the exposure. The shutter controls the length of time that light is permitted to pass through the lens.

Focal Plane and Fiducial Marks
The focal plane of an aerial camera is the plane in which all-incident light rays are brought to focus. In aerial photography object distances are great with respect to image distances. Aerial cameras therefore have their focus fixed for infinite object distances. This is done by setting the focal plane as exactly as possible at a distance equal to the focal length behind the rear nodal point of the camera lens. The focal plane is defined by the Upper surface of the focal-plane frame. This is the surface upon which the film emulsion rests when an exposure os made.

Camera fiducial marks are usually 4 or 8 in number, and they are situated in the middle of the sides of the focal plane opening, in its corner, or in both locations. These marks are exposed onto the negative when the picture is taken.

Fiducial marks serve several important functions. Lines joining opposite marks intersect at a point called the centre of collimation, and aerial cameras are carefully manufactured so that this occurs very close to the principal point. The principal point defined as the point in the focal plane where a line from the rear nodal point of the camera lens,  perpendicular to the focal plane. intersects the focal plane.

The shutter speeds of aerial cameras typically range from about 1/100 to 1/1000 sec Shutters are designed to operate to operate efficiently so that they open instantaneously, remain open the required time, and then instantaneously close, thereby providing uniform light to all parts of the focal plane.
There are a number of different types of camera shutters. Those used in aerial cameras are generally classified as either between the lens shutters of focal plane shutters. Between the lens shutters are most commonly used in mapping camera lens.

Cameras have been perfected to compensate for image motion, which occurs during the time that the shutter is open. Image motion compensation (IMC) is usually accomplished by moving the film slightly across the focal plane during exposure, in the direction of, and at a rate just equal to, the rate of image movement.

Types of Aerial Cameras:
Cameras are often classified according to their angular field of view. Angular field of view, as illustrated in the following figure.
Is the angle α subtended at the rear nodal point of the camera of the lens by the diagonal ‘-d’ of the picture format [The most common frame or format size of aerial mapping cameras in 9 in (23cm) square]. Classification according to angular field of view are
Normal angle (up to 75 degrees)
Wide angle (75 to 100 degrees)
Super wide angle (greater than 100)
Angular field of view calculated as follows.