Figure 1: Distortion comparison between wide and telephoto cameras
In the photo on the left above, the distortion of the subject has occurred due to taking a close up shot with the main, wide camera. This distortion arises due to the wide camera’s short shooting distance for a close-up shot. Since the human face’s 3D shape is projected onto the 2D image sensor, the center of the face is extended compared to the face’s peripheral because of the relative distance from camera. This is called “extension distortion.” Shorter shooting distances increase the relative distance difference of facial 3D shapes, so the shorter the shooting distance, the greater the extension distortion. This explains why the wide camera is suitable for taking pictures of people against a landscape background (like at a tourist spot), but not for images in which people are the main subjects. On the other hand, a telephoto camera makes it more suitable for portrait photography.
Another reason telephoto cameras excel at portrait photography is the out-of-focus effect produced by the focal length. When all other variables are equal, the longer the focal length, the shallower the depth of field. In other words, the background around the subject will be blurred and out of focus, resulting in a photo that is more focused on the subject. So, while a wide camera is great for capturing both people and backgrounds in a tourist spot, a telephoto camera is much better for scenes that focus on the people themselves.
The Limitations of Telephoto Cameras Until Now
Currently, telephoto cameras are most often used in high-end smartphones. The optical format of image sensors for wide cameras in this segment ranges from 1/1.4" to 1", with image quality matching their size and features, such as high dynamic range (HDR) and high-speed shooting. In comparison, the optical format of telephoto image sensors is relatively small, ranging from 1/2.7" to 1/2" (besides a few exceptions). From a user perspective, such difference in specification means that telephoto cameras offer different image quality, color and HDR experiences than wide cameras. Telephoto cameras are often used for tracking subjects from a distance, such as in stadiums or concert halls, and users might find it difficult to capture photos and videos at the level of quality they want.
Comparison of Wide and Telephoto Sensors