What exactly is f-stop?

Question

i have heard 1 stop, 2 stop always wondered. I know my f ranges from 2.5 to 8 or so.... is stop same as step?

Answer 1

An f/ stop is the ratio between the focal length of the lens and the diameter of the opening of the lens. Since it is an inverse proportion, the higher the number of the f/ stop, the smaller the opening in the lens.

F/ stops are important for two reasons.

The first is that the f/ stop is one of the two compnents used in figuring an exposure. The other is shutter speed. Each of the standardized f/ stops (i.e. f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, etc.) decreases the amount of light passing through the lens by 1/2. F/2 lets in half as much light as f/1.4. F/8 lets in half as much light as f/5.6. As you decrease the amount of light, you need to increase the length of the expsure.

If you have a proper exposure at f/5.6 at 1/125 of a second, you could increase you lens opening (or aperature) to f/4 at 1/250 of a second and get the same exposure. F/8 at 1/60 of a second would also give you the same exposure

This is important to know because of the other aspect of the f/ stop -- depth of field. Depth of field refers to the amount of the picture that is in focus. The smaller the f/ stop (of the larger the lens opening) the narrower the depth of field. Conversely, the higher the f/ stop, the wider the depth of field.

If you focus on an object using f/2.8 most everything in front of and behind that object will be out of focus. If you decrease your lens opening to f/11 or f/16, you will see that most of the objects in front and behind the central object are now in focus. This is called stopping down your lens. If you want the full frame in focus, you need a smaller lens opening, or a higher f/ stop. If you want a narrow range of focus to emphasize the central object in the frame, you want a wider lens opening, or a lower f/ stop. To keep the proper exposure, you then need to adjust the shutter speed.

If you want a more detailed explanation check out: http://www.uscoles.com/fstop.htm

I disagree with those who say there is no need to understand the relationship between f/ stops and shutter speeds. If you are serious about photography, knowing how to manipulate your exposure will help you create some truly wonderful pictures. There are times when you want to intentionally overexpose or underexpose a picture to achieve a specific effect. There are other times when you want to adjust the depth of field to emphasize or demphasize certain parts of your picture. Its what separates good photographers from people who only take snap shots.

Answer 2

It's like Nihl said. Here's the summary:
2.5 = big lens opening = short shutter time and blurry background.
8 = small lens opening = long shutter time and more will be in focus.
The full f-stops are: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, and 64.
For every f-stop that you move to the right, you have to double the shutter time.

Answer 3

The problem is your focusing on the things in life that don't really matter. When I was a kid I had hopes and dreams. We all did. But over time, the daily grind gets in the way and you miss the things that really matter, even though they are right in front of you, staring you in the face. I think the next time you should ask yourself "Am I on the right track here?". I don't mean to be rude but people like you I really pity. So maybe you could use the few brain cells you have and take advantage of the knowledge I have given you now. Good luck.

Answer 4

Technically, f-stop is derived by dividing the focal length of a lens by the diameter of the lens opening. In other words, the ration between the focal length of the lens and the apparent size of the lens opening as viewed from the front.

Back when SLR film cameras were the height of technology, f-stop was one of the main factors in determining the proper exposure of the film. By about 30 years ago, most cameras had automatic settings where they would calculate the best exposure settings for you -- usually you just told it "still-life", "action", "scenery" and it would pick the best combination for you. This carries over into the fancier digital cameras today.

"F-stop" is also the name of a photography magazine.

Answer 5

Dragonfly gave you a very good answer. Back when the F-stop and Shutter speed where used together to focus your pictures. No need to use it anymore.

Answer 6

what in the world are you talking about? :)