lenses please explain i am confused

Discussion in 'Beyond the Basics' started by miskin83, Dec 4, 2005.

  1. miskin83

    miskin83 TPF Noob!

    Joined:
    Dec 4, 2005
    Messages:
    24
    Likes Received:
    1
    Location:
    Hull
    Hi I am new could you answer me a question Please If i buy a lens for example 24-70mm F3.5-5.6 what does the f mean?? Does it mean i can i only use the lenses between f3.5-5.6, or can i use them at differnet apertures Please Help i am confused Many thanks
     
  2. doenoe

    doenoe TPF Noob!

    Joined:
    Mar 4, 2005
    Messages:
    4,267
    Likes Received:
    23
    Location:
    Hoorn, The Netherlands
    Can others edit my Photos:
    Photos NOT OK to edit
    if i got this correct, it means this: if you got the lens on 24mm, the smallest F-number you can use is 3.5. If the lens is set to 70mm, the smallest f-number will be 5.6.
    Something like that anyways, but im sure there will be someone more knowledgable then me to come and make it for sure.
     
  3. 'Daniel'

    'Daniel' TPF Noob!

    Joined:
    Dec 30, 2004
    Messages:
    1,507
    Likes Received:
    14
    Location:
    Manchester, UK
    Yes that's what it means. If yuo set to the largest aperture at 24 mm it will be 3.5; if you then zoom in to 70mm then the aperture will close to 5.6.
     
  4. Hertz van Rental

    Hertz van Rental TPF Noob!

    Joined:
    Nov 8, 2004
    Messages:
    16,728
    Likes Received:
    45
    Location:
    Where am I now?
    When you zoom the aperture does not change in size, it's just the f-number that changes.
    This is because the f-number is given by the focal length of the lens divided by the aperture diameter.
    For example:
    Aperture diameter = 10mm
    At a focal length of 80mm = 80/10 = f8
    At a focal length of 40mm = 40/10 = f4

    What actually happens is that the light intensity reaching the film/sensor changes due to the Inverse Square Law so the change in f-number compensates for this.
     
  5. amoki

    amoki TPF Noob!

    Joined:
    Mar 7, 2005
    Messages:
    91
    Likes Received:
    0
    Location:
    Hamilton, New Zealand
    The F-number refers to an aperture, and the number on a lense typically means the maximum aperture the lense can have (in this case, the "biggest" hole your lense can have) for reasons beyond me, but it probably has something to do with the lense's construction. That's why people are willing to pay good money for lense with the widest aperture. E.g, the famous Canon "L"-series lense with its constant F2.8 aperture... :p

    In this case, the widest "hole" you can get at 24mm focal length from the lense that you'd mentioned is F3.5. . At maximum zoom (70mm), its F-5.6. Not too bad, but certainly not the kind of lense you would want to stay with if you're a proffesional photographer

    Having said that, you can always stop down from the widest aperture of a lense to the smallest aperture (F22?) at any given focal length unless your camera is a point-and-shoot. Digital p-o-s will have vastly limited aperture, I believe.

    All F number are the same on all cameras. The F number if a standardized unit.

    If there is any terminology you do not understand stroll to the uppermost thread in this part of the forum. After all, we're all about helping. Amen? :lmao:

    Hertz - what you're saying is useful, but I think it's way too advance for him. Let him learn step by step before bringing in the big guns :D
     
  6. Hertz van Rental

    Hertz van Rental TPF Noob!

    Joined:
    Nov 8, 2004
    Messages:
    16,728
    Likes Received:
    45
    Location:
    Where am I now?
    I actually thought that what I said answered his question simply and succinctly, but what do I know? ;)
     
  7. miskin83

    miskin83 TPF Noob!

    Joined:
    Dec 4, 2005
    Messages:
    24
    Likes Received:
    1
    Location:
    Hull
    Thanks For all your answers

    you have all helped

    Miskin83
     
  8. stingray

    stingray TPF Noob!

    Joined:
    Oct 28, 2005
    Messages:
    179
    Likes Received:
    0
    yeah i thought your reply was fine hertz... and i'm, no pro. i must go have a look at this inverse squares rule though...
     
  9. Hertz van Rental

    Hertz van Rental TPF Noob!

    Joined:
    Nov 8, 2004
    Messages:
    16,728
    Likes Received:
    45
    Location:
    Where am I now?
    Inverse Square Law
    Light intensity is inversely proportional to the distance from the source.

    Basically saying that the further you move away from the light the darker things get. But there is a set ratio so you can work it out.
    Think of Geometry.
    Think of a pyramid.
    If you have a pyramid (or cone) of a set height the base has a certain base area.
    If you double the height of the pyramid the base area increases by a factor of 4.
    So too with light.
    If you have a light at a known distance from a surface it gives a certain level of illumination.
    Move the light twice (2x) the distance away and the illumination level on the surface decreases by a factor of 4 (2 stops).
    It's fundamental to virtually all aspects of Photography - but a very simple principle.
     
  10. PlasticSpanner

    PlasticSpanner TPF Noob!

    Joined:
    May 1, 2005
    Messages:
    4,125
    Likes Received:
    51
    Location:
    Cheshire, England
    Can others edit my Photos:
    Photos NOT OK to edit
    A pyramid is the best way of explaining ISL I've ever seen! :thumbup:
     
  11. Hertz van Rental

    Hertz van Rental TPF Noob!

    Joined:
    Nov 8, 2004
    Messages:
    16,728
    Likes Received:
    45
    Location:
    Where am I now?
    That's because it's the same Geometric law that governs both, if you think about it.
    Most people miss it.
    Some of the strange stuff you learnt in Maths in school does have a use...
     
  12. LaFoto

    LaFoto Just Corinna in real life Staff Member Supporting Member

    Joined:
    Feb 1, 2004
    Messages:
    34,814
    Likes Received:
    814
    Location:
    Lower Saxony, Germany
    Can others edit my Photos:
    Photos NOT OK to edit
    Hey. Shhh.
    Silly question: what does the letter "f" stand for in "f-stop" then? (I know all the rest, but in German it is called "Blende" and... well... this question is for my English knowledge rather than anything else).
     

Share This Page