Posted by M.Kumar at 31 July , 2009
F-Stop Definition
F-Stop is a unit to express how much light is entering a camera lens. Example f/2, f/4, f/8, etc.
Explanation
Assuming this post is for beginners I am briefing out the dependent terminologies: Aperture: Just like our eyes, camera lens has an opening that allows light to enter into the camera. This opening is termed as aperture. Focal Length: Distance between the image sensor and optical center of camera lens. When you buy a lens for your camera, it will have a number on it, something like 18mm, 55mm, 105mm, etc. These numbers are called as focal length. 
Letter “F” in F-Stop is called as focal length. Focal length divided by aperture determines the F-Stop.
Calculating F-Stop
For a 200mm lens with an aperture 50mm: F Number = Focal Length/ Aperture = 200/50 = 4 Finally denoted as F/4
For a 200mm lens with an aperture 25mm (decreasing the opening): F Number = Focal Length/ Aperture = 200/25 = 8 Finally denoted as F/8
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Focal Length
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Aperture
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F-Stop
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Conclusion
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200 mm
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50 mm
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F/4
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Bigger lens opening results in smaller F-Stops More light enters when F-Stop is small
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200 mm
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25 mm
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F/8
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Smaller lens opening results in bigger F-Stops Less light enters when F-Stop is large
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How does this affect your photography?
Change in f number has a very high impact on the image captured. You can actually blur out the unwanted object from your scene and still be able to get a sharp focus of your subject. This impact of changing f number is basically termed as “Depth of Field”. Pictures captured with a low f-number will tend to have your subject in sharp focus while leaving rest of the scene out of focus. This is often used in portrait photography or macro photography where the subject is given importance than the background. Following is a range of pictures of same object with different f-Numbers; see how a change in f-number has an impact over the projected scene:
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f-Number
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Image
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Observation
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- Lens opening is wide, allowing more light to travel to the image sensor resulting in a well exposed scene.
- Bright colors.
- Blurred background with sharp subject. (Shallow Depth of Field)
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- Lens opening is less than the above due to which less light is allowed to travel in resulting in slightly less exposed scene than above.
- Color intensity is getting decreased due to less light.
- Background is less blurred than above.
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- Lens opening is less than the above due to which less light is allowed to travel in resulting in slightly less exposed scene than above.
- Color intensity is getting decreased due to less light.
- Background is less blurred than above.
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- Lens opening is less than the above due to which less light is allowed to travel in resulting in slightly less exposed scene than above.
- Color intensity is getting decreased due to less light.
- Background is less blurred than above.
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- Lens opening is less than the above due to which less light is allowed to travel in resulting in slightly less exposed scene than above.
- Color intensity is getting decreased due to less light.
- Background is less blurred than above.
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- Lens opening is less than the above due to which less light is allowed to travel in resulting in slightly less exposed scene than above.
- Color intensity is has decreased due to less light traveling to the sensor.
- Background is getting sharp now. (Narrow Depth of Field)
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Conclusion
With an increase in f-Number the lens opening decreases; allowing less light to travel to the image sensor. Also with an increase in f-Number narrow depth of field is achieved.
Please leave a comment so as to let me know that this post is at least helping some one 
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Tags: Aperture, Aperture Priority, Choosing DSLR, Depth Of Field, DOF, DSLR, f-number, f-stops, f/11, f/2.8, f/4, f/8, Focal Length, Image Sensor, Lens, narrow depth of field, Photography, Shallow Depth-of-field
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