Editing F-number (section)

== Effects on image sharpness ==
[[Image:Jonquil flowers merged.jpg|thumb|400px|Comparison of {{f/|32}} (top-left half) and {{f/|5}} (bottom-right half)]]
[[File:Blumen im Sommer.jpg|thumb|400px|Shallow focus with a wide open lens]]

[[Depth of field]] increases with f-number, as illustrated in the image here. This means that photographs taken with a low f-number (large aperture) will tend to have subjects at one distance in focus, with the rest of the image (nearer and farther elements) out of focus. This is frequently used for [[nature photography]] and [[portrait photography|portraiture]] because background blur (the aesthetic quality known as '[[bokeh]]') can be aesthetically pleasing and puts the viewer's focus on the main subject in the foreground. The [[depth of field]] of an image produced at a given f-number is dependent on other parameters as well, including the [[focal length]], the subject distance, and the [[film format|format]] of the film or sensor used to capture the image. Depth of field can be described as depending on just angle of view, subject distance, and [[entrance pupil]] diameter (as in [[Moritz von Rohr|von Rohr's method]]). As a result, smaller formats will have a deeper field than larger formats at the same f-number for the same distance of focus and same [[angle of view]] since a smaller format requires a shorter focal length (wider angle lens) to produce the same angle of view, and depth of field increases with shorter focal lengths. Therefore, reduced–depth-of-field effects will require smaller f-numbers (and thus potentially more difficult or complex optics) when using small-format cameras than when using larger-format cameras.

Beyond focus, image sharpness is related to f-number through two different optical effects: [[Optical aberration|aberration]], due to imperfect lens design, and [[diffraction]] which is due to the wave nature of light.<ref>{{cite book | title = Basic Photography | author = Michael John Langford | isbn = 0-240-51592-7 | year = 2000 | publisher = [[Focal Press]] | url-access = registration | url = https://archive.org/details/basicphotography00lang }}</ref> The blur-optimal f-stop varies with the lens design. For modern standard lenses having six or seven elements, the [[sharpness (visual)|sharpest]] image is often obtained around {{f/|5.6}}–{{f/|8}}, while for older standard lenses having only four elements ([[Zeiss Tessar|Tessar formula]]) stopping to {{f/|11}} will give the sharpest image.{{citation needed|date=July 2015}} The larger number of elements in modern lenses allow the designer to compensate for aberrations, allowing the lens to give better pictures at lower f-numbers. At small apertures, depth of field and aberrations are improved, but [[diffraction]] creates more spreading of the light, causing blur.

Light falloff is also sensitive to f-stop. Many wide-angle lenses will show a significant light falloff ([[vignetting]]) at the edges for large apertures.

[[Photojournalist]]s have a saying, "[[f/8 and be there|{{f/|8}} and be there]]", meaning that being on the scene is more important than worrying about technical details. Practically, {{f/|8}} (in 35&nbsp;mm and larger formats) allows adequate depth of field and sufficient lens speed for a decent base exposure in most daylight situations.<ref>{{cite book|last=Levy|first=Michael|title=Selecting and Using Classic Cameras: A User's Guide to Evaluating Features, Condition & Usability of Classic Cameras|publisher=Amherst Media, Inc|year=2001|page=163|isbn=978-1-58428-054-5}}</ref>