The simple definition of f stop is it's the number that refers to how wide-open the camera lens is set.
A smaller number like f/4 means the lens opening is relatively big and a big f/stop number like f/11 means the lens opening is fairly small.
A deeper understanding of the definition of f stop can be found by looking at the other ways you can describe f/stop.
F-stop is also referred to as "f-number", "f-ratio," "relative aperture," or "focal ratio."
That's because it is a number, it does represent a ratio, and it is relative. It's actually a fraction too. This all seems complicated, but I am just poking a little fun at the use of our crazy language.
It's a number that is the ratio of the lens opening compared to the focal length of the lens.
As an example, if the diameter of the lens opening is 25mm and it's a 50mm lens, then it's 50/25 or f-2.
The word stop in the term f-stop was the physical stopping points where lens diaphragms engaged at set increments, such as f-2 f-2.8 f4.0 f5.6 f-8. That was back in the old days when lens were mechanically and not electronically controlled.
Now diaphragms are step-less.
There are blades within the lens.
Usually more blades give a smoother bokeh.
They are more accurate and are expressed digitally such as f-4.7 or f-8.3.
How the f-stop affects your photograph is is really what matters, not the actual number itself. Exposure, depth of field, and sometimes image quality are effected by what f stop you choose to use.
Each doubling of the f-stop number equates to only 1/4 of the amount of light reaching your sensor. So changing your f-stop from f-4 to f-8 reduces the size of the lens opening to only allow 1/4 the amount of light in.
Depending on the automatic mode your camera is set on, it will change the shutter speed or ISO setting to compensate so your exposure will still be correct. If you are on manual, it is your job to make the settings appropriate for the correct exposure.
Here is more on the topic of of proper photo exposure and the exposure triangle.
Image quality is fairly constant through the range of what f-stops you use with modern lenses, but there are issues, usually at both extremes of the f stop range.
At the biggest f-stop numbers (the wide-open apertures) image sharpness is sometimes compromised a bit. At the highest f-stop numbers ( the smallest lens openings), diffraction is usually more apparent.
For the casual photographer, these lens shortcomings are barely noticeable. They do exist though and on an image-quality basis only, the best results are often obtained by closing down two or three f numbers from wide-open lens settings.
This is the big kahuna. A good understanding of depth of field related to what your f-stop is separates the advanced and pro photographers from the amateur ones.
Using smaller f numbers will result in more depth of field (a larger range of objects in your photograph in good focus). Using large f numbers results in shallow depth of field. It's not always desirable to have a large depth of field and I've written several articles on this topic because of its importance in photography:
Here is a table showing the sequence of common, single f-stop numbers. Note that each subsequent step upward results in HALF the amount of light passing through the lens for any given shutter speed.
The technical definition of f stop can simple be remembered as the size of the lens opening compared to the size of the lens. The important things to know about what is f stop are its effects on depth of field, exposure, and image quality.
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