I never understood what the f really stands for in the f-stop values, like f/1.8. Wikipedia explains it in various articles, but I still find it very confusing. What I understand, though, is that it has something to do with focal length.
Should I assume, therefore, that bigger focal lengths capture more light? For example, should 85 mm f/1.8 lens capture a lot more light than for example 24 mm f/1.8?
References to back up my confusion:
The article about aperture goes on to claim:
The amount of light captured by a lens is proportional to the area of the aperture, equal to:
Where f is focal length and N is the f-number.
The "f-number" is mentioned here. But the article about F-number claims:
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter.
This seems very recursive. Why does the aperture article refers to both the focal length and the f-number, when the latter article claims that f-number already carries the property of focal length?
What's going on here?
Answer
The f-number is in use to express how much light a lens can capture, so the 85mm f/1.8 and 24mm f/1.8 can capture the same amount. Here, f is the focal length, and f/1.8 means that maximum aperture diameter is 47.2mm in first example and 13.3mm in second.
What you have to consider here is that the 85mm lens has a much narrower field of view, therefore it has to gather the same amount of light from a much smaller area - to compensate the narrow view, aperture has to be bigger.
How much the aperture has to be bigger is linearly correlated to focal length. We could say that a 24mm lens with 13.3mm aperture can gather as much light as a 85mm lens with 47.2mm aperture, but talking about the F-number makes this much easier to notice.
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