What does F stop have to do with lense speed

Discussion in 'Photography Beginners' Forum' started by Bryce, Jul 23, 2010.

  1. Bryce

    Bryce TPF Noob!

    Joined:
    Jul 15, 2010
    Messages:
    188
    Likes Received:
    0
    Location:
    Florida
    Can others edit my Photos:
    Photos OK to edit
    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.
     
  2. Petraio Prime

    Petraio Prime TPF Noob!

    Joined:
    May 28, 2010
    Messages:
    1,217
    Likes Received:
    0
    Location:
    Ohio
    Can others edit my Photos:
    Photos NOT OK to edit
    The f-stop used controls both depth of field and exposure intensity. The lens focal length also has an effect on depth of field.
     
  3. JamesMason

    JamesMason TPF Noob!

    Joined:
    Aug 20, 2009
    Messages:
    532
    Likes Received:
    0
    Location:
    Nottingham
    Can others edit my Photos:
    Photos NOT OK to edit
    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
     
  4. Neil S.

    Neil S. TPF Noob!

    Joined:
    Jul 16, 2010
    Messages:
    1,128
    Likes Received:
    21
    Location:
    Japan
    Can others edit my Photos:
    Photos NOT OK to edit
    I thought Speed was that movie with Keanu Reeves....
     
  5. Petraio Prime

    Petraio Prime TPF Noob!

    Joined:
    May 28, 2010
    Messages:
    1,217
    Likes Received:
    0
    Location:
    Ohio
    Can others edit my Photos:
    Photos NOT OK to edit
    I think you meant maximum aperture.
     
  6. OrionsByte

    OrionsByte No longer a newbie, moving up!

    Joined:
    Jul 6, 2010
    Messages:
    1,500
    Likes Received:
    261
    Location:
    N. California
    Can others edit my Photos:
    Photos OK to edit
    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.
     
  7. Bryce

    Bryce TPF Noob!

    Joined:
    Jul 15, 2010
    Messages:
    188
    Likes Received:
    0
    Location:
    Florida
    Can others edit my Photos:
    Photos OK to edit
    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?
     
  8. Neil S.

    Neil S. TPF Noob!

    Joined:
    Jul 16, 2010
    Messages:
    1,128
    Likes Received:
    21
    Location:
    Japan
    Can others edit my Photos:
    Photos NOT OK to edit
    Ya you tell em Petraio.

    Now wheres my popcorn.....
     
  9. kundalini

    kundalini Been spending a lot of time on here!

    Joined:
    Jul 18, 2007
    Messages:
    13,601
    Likes Received:
    1,929
    Location:
    State of Confusion
    Can others edit my Photos:
    Photos NOT OK to edit
    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?
     
  10. Gaerek

    Gaerek TPF Noob!

    Joined:
    May 2, 2009
    Messages:
    1,341
    Likes Received:
    98
    Location:
    Sitka, AK
    Can others edit my Photos:
    Photos OK to edit
    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.
     
  11. Bryce

    Bryce TPF Noob!

    Joined:
    Jul 15, 2010
    Messages:
    188
    Likes Received:
    0
    Location:
    Florida
    Can others edit my Photos:
    Photos OK to edit
    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.
     
  12. Neil S.

    Neil S. TPF Noob!

    Joined:
    Jul 16, 2010
    Messages:
    1,128
    Likes Received:
    21
    Location:
    Japan
    Can others edit my Photos:
    Photos NOT OK to edit
    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.
     

Share This Page

Search tags for this page
aperture size ratio
,
effective aperture
,
focal length and aperture
,
foto lense shape
,
physical aperture
,

what does f stop do

,

what does fstop do

,

what does lower f stop do

,
what does the f stop do
,
what does wrong f stop do