What measurement is necessary to determine the focal length of a lens?

To determine the focal length of a lens, understanding the characteristics of the lens itself is crucial. The focal length is defined as the distance from the lens to the image sensor when the subject is in focus. This relates very closely to the specifications of the lens, such as its size and design. The electronic iris lens size is particularly important because it directly influences the lens's ability to gather light and focus images correctly.

A lens with a specific diameter will have a particular focal length that allows it to focus light from subjects at various distances effectively. Therefore, knowing the size of the lens helps to establish the lens's optical properties and its focal length characteristics. In practice, the electronic iris lens size gives insights into how the lens will behave with varying lighting conditions, further connecting its physical attributes to its focal length.

Distance from the subject does contribute to understanding how the lens will project an image, but it does not inherently define the lens's focal length. Similarly, camera resolution and lumens output pertain to image quality and brightness rather than to the fundamental optics of the lens itself. Hence, the electronic iris lens size is the most relevant measurement for determining the focal length.