4.8.12. Focal length

Distance from the optical centre of the lens to the focal plane, when the camera is focussed at infinity, is called the focal length of camera lens.

Focal length is a crucial term in photography, particularly in photogrammetry. It refers to the distance from the center of the camera lens to the point where the lens focuses light to create a clear image. This distance is significant because it affects how much of the scene will be captured in the photograph and how far into the distance objects will remain in sharp focus when the camera is set to focus at infinity.

Think of a camera lens as a magnifying glass. The distance from the lens of the magnifying glass to where it brings an object into clear view is similar to the focal length. If you hold the magnifying glass too close or too far away, you won't see a clear image. Similarly, adjusting the focal length lets photographers determine their field of view and focus on distant objects.

Its value is known, as it is supplied by the manufacturer of camera.

The focal length of a camera not only helps in focusing the image but is also integral to determining the photograph's scale. The scale of an aerial photograph is a ratio that illustrates the relationship between the distance on the photograph and the actual distance on the ground. A longer focal length generally results in a larger scale because it captures details of smaller areas, making objects look more magnified. Conversely, a shorter focal length leads to a smaller scale, allowing a broader view of a scene but with less detail.

Imagine using a pair of binoculars. When you adjust the lens to a longer focal length, the objects you are viewing appear larger and you can see more detail, just like a camera zooming in on a landscape. This means you can create a more detailed image (or a larger scale photograph) of a specific area.

The scale of a photograph varies at different points depending on their elevations.

In photogrammetry, the elevation of the ground affects the scale of the photograph taken. This variability occurs because the scale changes when photographing undulating terrain. For example, when photographing a hilly area, photographs taken from higher elevations above MSL will yield a different scale compared to those taken from lower elevations. This variation is essential for accurately interpreting the photograph since the scale may not be constant throughout the image.

Consider a topographic map of a mountainous region. When looking at parts of the map at different altitudes, the scale can represent different levels of detail because the hills and valleys vary significantly. If you were to take a photograph of these areas, the same principle applies; each elevation presents a different scale similar to viewing the world at ground level versus from a mountain peak.

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