focal distance (original) (raw)
Author: the photonics expert (RP)
Acronyms: FFD, BFD
Definition: the distance between an objective and its focal point
general opticsRelated: objectivesfocal length
Page views in 12 months: 328
DOI: 10.61835/6as Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
Content quality and neutrality are maintained according to our editorial policy.
What is a Front Focal Distance?
The front focal distance of an objective is the distance between the front surface of the objective (the vertex point of the first lens) and its front focal point. The back focal distance is the distance between the vertex of the last lens and the back focal point.
Focal distances should not be (but are sometimes) confused with focal lengths. The latter are measured from the principal points and are the same on both sides if one has the same refractive index on both sides. In contrast, the front and back focal distances can be quite different, even if there is air on both sides.
Frequently Asked Questions
What is the front focal distance of a lens?
The front focal distance of a lens or objective is the distance from its front surface (the vertex of the first optical element) to its front focal point.
How does the front focal distance differ from the focal length?
The front focal distance is measured from the front surface vertex of the lens system, while the focal length is measured from the corresponding principal point. These two quantities are generally not equal.
Are the front and back focal distances of a lens the same?
No, the front and back focal distances can be very different. This is unlike the front and back focal lengths, which are identical if the refractive index is the same on both sides of the lens.
Questions and Comments from Users
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.
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.
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.