F-number and NA

Note: this box searches only for keywords in the titles of articles, and for acronyms. For full-text searches on the whole website, use our search page.

A common early method of range finding was to use pulses of a single frequency or tone. This is the well-known “ping” from submarine movies. In the early days of radar during the battle of Britain, for example, radar would be sent in pulses that swept across a circular area. In between pulses, the radar would listen for the echoes from distant objects and those echoes would be translated into blips on a cathode ray tube display. Many modern radars still mimic these old displays despite using vastly more advanced technology.

F numberwelding

These explain quite comprehensively a wide range of aspects, not only physical principles of operation, but also various practical issues.

1.2M views. Principal Research Scientist at Georgia Tech. The Infinite Universe (2020). andersenuniverse.com; https://timandersen.substack.com/

The f-number of a lens is directly related to the maximum angles of output rays as obtained for parallel input rays: the tangent of that maximum angle is half the inverse f-number.

Camera f numberchart

Most photographic objectives contain a diaphragm (an optical aperture) of variable diameter. It is common not to directly specify the used aperture diameter, but instead the f-number. This is defined as the ratio of focal length and the diameter of the entrance pupil. Specifications are often done in the format f/N, where N is the f-number. For example, f/5.6 means that the entrance pupil diameter is the focal length divided by 5.6. The notation f/# is also common.

Some objectives offer only relatively large f-number values because image aberrations could not be properly compensated for lower values. Unfortunately, that limits their light gathering power, which for distant objects is determined by the f-number. Particularly for close objects, the light gathering power can be reduced substantially. That aspect is relevant for macro photography, where exposure times have to be increased accordingly.

f-number calculator

The entrance pupil is the aperture stop as seen from the object side. It may not be identical to the physical aperture if there are lenses between the entrance and the aperture.

Camera f numberexplained

Normally, the f-number of a photographic objective can be changed in certain steps, with typical values like 2, 2.8, 4, 5.6, 8, 11, 16 and 22, progressing roughly such that each step (“going up one stop”) reduces the aperture area by a factor of 2, which has two consequences:

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

When imaging an object which is not at infinity, the output ray angles are smaller. One can define the working f-number based on that for given imaging conditions; it is correspondingly larger than the f-number.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

F-stop vs aperture

f-number formula

These single tones had to be relatively short in duration. Consider that if two objects, such as airplanes, were close together, the echoes from the two would overlap one another. This would make it look as though the aircraft were one large object instead of two.

For photography of small objects over short distances with a substantial magnification (macro photography), the image brightness is substantially lower than one might expect from the f-number. It depends on the working f-number, which is larger (see above).

F-stop chart

In any active ranging system, whether it be radar, lidar, or sonar, a signal is broadcast from a transmitter and reflected off of objects in the far field. The return signal is then received, usually at the same site but sometimes elsewhere, and processed.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.

Generally, the term lens speed is often used in the context of photographic objectives. Lenses with low f-number, which therefore allow for relatively short exposure times, are often called fast lenses, while those with high f-number are slow. The lens speed usually refers to the minimum possible f-number of an objective.

Pulse compression, sometimes called matched filtering, is a technique for extracting information about a signal based on knowing what that signal is. Frequently used in radar, sonar, and photonics, pulse compression is particularly useful in suppressing noise and improving the accuracy of range finding.

Note: the article keyword search field and some other of the site's functionality would require Javascript, which however is turned off in your browser.