F/2.8-5.6

[frankie_frank1]

<p>How are the f/ numbers calculated?</p>

[Matt Laur]

<p>Here's <strong><a href="http://en.wikipedia.org/wiki/F-stop">some reading</a></strong>.</p>

[franklin_polk]

<p>Simple answer: focal length divided by front element size.<br>

More detail: It actually is the size of the aperture <em>as viewed through the front of the lens</em> ; i.e. effective aperture. Effective aperture is not nessasarily related to front element size. In addition, marked aperture is only true at infinity. At distances closer than that, it decreases.</p>

[jcuknz]

<p>Something which Wikipedia doesn't mention which was apparent when I was a young tog [c.WWII] was the different standards between Anglo-American and European Continental equipment in the 'standard' [ as Wikipedia calls it] of ie. f/4, f/5.6,f/8 was found on Anglo-American lens while on continental cameras it was common to find f/4.5,f/6.3, f/9. I guess with the upheaval and devastation of WWII the Europeans re-tooled to Anglo-American standards perhaps?<br>

It is also somewhat amusing to see f/5 giving a nicely blurred background, obviously taken wit a large format camera rather than a digital camera with very short focal length lens :-) </p>

[godfrey]

<p>There were simply two commonly used f/stop series at one point or another:<br>

f/3.5, 4.5, 6.3, 9, 13 ... when most lenses tended to be slower. <br>

f/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 ... in the 35mm and later era when lenses tend to be faster. </p>

<p>It had little to do with Anglo-American vs European Continental standards. Frankly, most of the faster generation lenses have originated from Japan since at least the late 1950s, but there's plenty of crossover between US, Europe and Japan on common aperture series. </p>

<p>BTW: it is not the front element diameter that is used to calculate f/number, it's the dimension of the lens' iris or smallest diameter element in the case of lenses without an iris (also commonly referred to as the "lens opening"):</p>

<p>f/number = focal length divided by lens opening (in like units, typically millimeters)</p>

<p>Godfrey</p>

[jcuknz]

<p>I think Godfrey that I am maybe speaking from an earlier era than you and from cameras I owned if was definitely a continental/anglo difference in style ... though cameras made for the world market such as Leica and Contax could have used the Anglo system. I'm not a collector but it was one of the 'facts of life' I learned or mis learned :-) very early on. I'm thinking of lens for LF cameras , various folding cameras, and the Reflex Korelle I had for a time which must have dated from pre-WWII. They were old and 'affordable' when I had them in the fifties as a student. I've a 'feeling' that Rollei originally came out with the continental systm though had changed by the time I bought my Model T in the sixties. More affluent students had Rollei's in my early days :-)</p>