What does F stop have to do with lense speed

[Bryce]

I keep hearing about lenses being fast because of being an f 1.2 or 1.4. I have purchased a 2.8 and a 1.8 and to me 1.2 and 1.4 make no sense to me.
When I see pictures taken at those numbers the part at the lense is clear then the rest goes out of focus without. To me you should have some transition. Example: A picture of a cat I saw taken at 1.8 had a clear face and the cat was sitting pretty. The body and the background are just as blurry as can be. I would rather see a transition or at least the face and body in focus and the background out of focus. I have to crank down to f3 for starters and f11 on the other end and rarely 1.8 unless I am at infinity. I keep being told I should have went with the 1.4 that is way more expensive so I would be pay way more for a lense I would crank down anyway. 1.4 is only faster if you use 1.4. I almost never would.

What I have figured is that if you have to crank down the f stop anyway then would a wider filter size be a faster lense since the wider lense lets in more light and allowing you to make shutter speeds faster. If you can crank the shutter speed faster with a lower f stop and wider lense then why does f stop have anything to do with speed? The wider the f stop the less is in focus. Wider lense makes more sense to me as far a speed goes. Am I not understandin somthing about this? Because at this point lower f stops than f 2 seem like money in the garbage to me. Help me out on this one.

 

[Petraio Prime]

I keep hearing about lenses being fast because of being an f 1.2 or 1.4. I have purchased a 2.8 and a 1.8 and to me 1.2 and 1.4 make no sense to me.
When I see pictures taken at those numbers the part at the lense is clear then the rest goes out of focus without. To me you should have some transition. Example: A picture of a cat I saw taken at 1.8 had a clear face and the cat was sitting pretty. The body and the background are just as blurry as can be. I would rather see a transition or at least the face and body in focus and the background out of focus. I have to crank down to f3 for starters and f11 on the other end and rarely 1.8 unless I am at infinity. I keep being told I should have went with the 1.4 that is way more expensive so I would be pay way more for a lense I would crank down anyway. 1.4 is only faster if you use 1.4. I almost never would.

What I have figured is that if you have to crank down the f stop anyway then would a wider filter size be a faster lense since the wider lense lets in more light and allowing you to make shutter speeds faster. If you can crank the shutter speed faster with a lower f stop and wider lense then why does f stop have anything to do with speed? The wider the f stop the less is in focus. Wider lense makes more sense to me as far a speed goes. Am I not understandin somthing about this? Because at this point lower f stops than f 2 seem like money in the garbage to me. Help me out on this one.

The f-stop used controls both depth of field and exposure intensity. The lens focal length also has an effect on depth of field.

 

[JamesMason]

Speed Simply refers tho the size of minimum aperture. Think of it as the speed that it can let light in. For example

F1.2 is a large aperture and lets lots on light in (within the time the shutters open). Fast lens

F4.5 is a smaller aperture and does not let as much light in(within the time the shutters open). Slow Lens

 

[Neil S.]

Speed Simply refers tho the size of minimum aperture. Think of it as the speed that it can let light in. For example

F1.2 is a large aperture and lets lots on light in (within the time the shutters open). Fast lens

F4.5 is a smaller aperture and does not let as much light in(within the time the shutters open). Slow Lens

I thought Speed was that movie with Keanu Reeves....

 

[Petraio Prime]

Speed Simply refers tho the size of minimum aperture. Think of it as the speed that it can let light in. For example

F1.2 is a large aperture and lets lots on light in (within the time the shutters open). Fast lens

F4.5 is a smaller aperture and does not let as much light in(within the time the shutters open). Slow Lens

I think you meant maximum aperture.

 

[OrionsByte]

Well, first of all, don't confuse the filter size with the width of the aperture - they're not the same thing. When talking about aperture, it's always a ratio, not a precise measurement. A lens at f/4 is going to let in the same amount of light whether it's got a 52mm filter size or an 85mm filter size. Put differently, it's not the size of the lens that matters, it's the ratio of its aperture (which is the smallest element in the lens, which may not be the opening at the end) to its focal length.

A lot of photography is about trade-offs. An increase in aperture (smaller f/stops) will let more light in, but will decrease the depth of field. Faster shutter speeds will stop action in its tracks, but requires more light by either a wide aperture (and thus a shallower DOF) or higher ISO. Learning that "exposure triangle" is vital to learning photography.

 

[Bryce]

Speed Simply refers tho the size of minimum aperture. Think of it as the speed that it can let light in. For example

F1.2 is a large aperture and lets lots on light in (within the time the shutters open). Fast lens

F4.5 is a smaller aperture and does not let as much light in(within the time the shutters open). Slow Lens

But if the 1.2 has a low depth of field and you have to crank it to 4.5 to get a good shot then how is the 1.2 any better than the 4.5. I would think a 67mm filter size 4.5 would be better than a 52mm filter 1.2. Because you have increased depth of field in a 67 4.5 and the 67 lets in more light. What picture would you take with a depth of field reduced to 1.2?

 

[Neil S.]

Speed Simply refers tho the size of minimum aperture. Think of it as the speed that it can let light in. For example

F1.2 is a large aperture and lets lots on light in (within the time the shutters open). Fast lens

F4.5 is a smaller aperture and does not let as much light in(within the time the shutters open). Slow Lens

I think you meant maximum aperture.

Ya you tell em Petraio.

Now wheres my popcorn.....

 

[kundalini]

... to crank down the f stop ....

I think you mean 'open up', have a larger apeture (smaller number). When you 'stop down' the lens, you are making the apeture smaller.

Each full f/stop either lets in twice the amount of light or halves the amount of light. Think of your f/4.5 lens as a garden hose. An f/2.8 would be a fireman's hose and a f/1.2 would be the water mains outside your house. Which would fill your kiddie pool faster?

 

[Gaerek]

I think, and I could be totally wrong here, but lens speed refers to the fact that with a fast lens, you can use a faster shutter speed for the same amount of light as a slower lens.

For example:

Let's say proper exposure can be found on a slow lens at ISO 100, f/3.5, and 1/60sec shutter.

Now, let's bring out the fast lens, say, an f/1.4. The new lens, in the same situation can, will have an exposure of ISO 100, f/1.4, and a 1/400sec shutter.

Now, of course, there's other reasons to have a fast lens, but if I'm not mistaken, this is where the term 'fast lens' came from.

 

[Bryce]

Well, first of all, don't confuse the filter size with the width of the aperture - they're not the same thing. When talking about aperture, it's always a ratio, not a precise measurement. A lens at f/4 is going to let in the same amount of light whether it's got a 52mm filter size or an 85mm filter size. Put differently, it's not the size of the lens that matters, it's the ratio of its aperture (which is the smallest element in the lens, which may not be the opening at the end) to its focal length.

A lot of photography is about trade-offs. An increase in aperture (smaller f/stops) will let more light in, but will decrease the depth of field. Faster shutter speeds will stop action in its tracks, but requires more light by either a wide aperture (and thus a shallower DOF) or higher ISO. Learning that "exposure triangle" is vital to learning photography.

I don't understand why a wider filter would not give more available light. I have not gotten any wide ones yet but this is what I am thinking about for future purchases so I know what to buy.

 

[Neil S.]

Speed Simply refers tho the size of minimum aperture. Think of it as the speed that it can let light in. For example

F1.2 is a large aperture and lets lots on light in (within the time the shutters open). Fast lens

F4.5 is a smaller aperture and does not let as much light in(within the time the shutters open). Slow Lens

But if the 1.2 has a low depth of field and you have to crank it to 4.5 to get a good shot then how is the 1.2 any better than the 4.5. I would think a 67mm filter size 4.5 would be better than a 52mm filter 1.2. Because you have increased depth of field in a 67 4.5 and the 67 lets in more light. What picture would you take with a depth of field reduced to 1.2?

You could say the 1.2 is "better" because it has the option of going much wider. This of course lets in more light, and opens up new creative possibilities.

You may not choose to, but at least you can.

 

[tirediron]

"Fast" lenses, those with physically large such as f1.4, 1.2 and the holy grail, Leicha's f0.95 are special purposes lenses. They are really meant to help you acheive one or both of two purposes: To let in large amounts of light so that you can use higher shutter speeds in dim conditions (for instance an f1.4 lens will allow you to use a shutter speed as much as four times faster than many 'kit' lenses; this could be the difference between having to shoot at 1/15 of a second and 1/60th).

The second purpose is to permit very shallow depths of field as you've already alluded to. This is most often desirable in portrait work, where you may want the face in sharp focus and the rest of the subject softly focused, and the background completely out of focus.

Depth of field is also a function of focal length. At any given aperture the shorter the focal length (wider angle lens) the greater the depth of field a lens will have. Check out this on-line depth of field calculator and plug in different numbers to get a better sense of how much of a scene will be in focus based on lens choice, focal length, and aperture.

 

[OrionsByte]

Well, first of all, don't confuse the filter size with the width of the aperture - they're not the same thing. When talking about aperture, it's always a ratio, not a precise measurement. A lens at f/4 is going to let in the same amount of light whether it's got a 52mm filter size or an 85mm filter size. Put differently, it's not the size of the lens that matters, it's the ratio of its aperture (which is the smallest element in the lens, which may not be the opening at the end) to its focal length.

A lot of photography is about trade-offs. An increase in aperture (smaller f/stops) will let more light in, but will decrease the depth of field. Faster shutter speeds will stop action in its tracks, but requires more light by either a wide aperture (and thus a shallower DOF) or higher ISO. Learning that "exposure triangle" is vital to learning photography.

I don't understand why a wider filter would not give more available light. I have not gotten any wide ones yet but this is what I am thinking about for future purchases so I know what to buy.

Because a lens is made up of more than just the front glass. When you look the specifications for a lens it will usually tell you how many elements are in the lens. You can think of them like a stack of magnifying glasses. The aperture of a lens would be the width of the smallest element in that stack. You may have a big ol' piece of glass on very the top, but depending on how the lens is manufactured, it may have to pass through several smaller elements before it hits your sensor.

 

[edouble]

This will help you Bryce Understand what is Aperture in photographic basic