Aperture numbers

Discussion in 'Photography Beginners' Forum' started by Tom3, Nov 29, 2008.

  1. Tom3

    Tom3 TPF Noob!

    Joined:
    Nov 28, 2008
    Messages:
    77
    Likes Received:
    0
    Can others edit my Photos:
    Photos OK to edit
    Hi,

    I'm having an issue to understand something:

    I have two lenses, one is an AF-S Nikkor 18-70 lens, and the aperture listed on the lens is 1:3.5-4.5

    However, the camera leaves me choices out of the 3.5-4.5 range.

    What is the deal? what really happens when I select an aperture out of this range?

    Also, another question, which might show a fundamental misunderstanding from my end: why aren't the blades by the sensor in the camera instead of being in the lens?
     
  2. laurah

    laurah TPF Noob!

    Joined:
    Dec 14, 2007
    Messages:
    28
    Likes Received:
    0
    Location:
    MA
    Can others edit my Photos:
    Photos OK to edit
    The aperture numbers listed are the max aperture that can be used at each end of your zoom range, i.e. at 18mm your max aperture is 3.5 and at 70mm your max aperture is 4.5. Using apertures smaller than these (larger number) is fine, the lens is just not capable of going to a larger aperture.
     
  3. Ptyler22

    Ptyler22 TPF Noob!

    Joined:
    May 23, 2008
    Messages:
    789
    Likes Received:
    0
    Location:
    Massachusetts
    Can others edit my Photos:
    Photos NOT OK to edit
    Ya, laurah pretty much summed it up.
    The smaller the number, the bigger the opening. Meaning more light can get through the lens, with a bigger aperture, and you can have faster shutter speeds. You can select bigger numbers (smaller opening) but you will have to use a slower shutter speed, you might want to do that for more depth of field or if it's really sunny, I am not good at explaining things, but I am sure you will get more responses from people that can explain it better.
     
  4. Tom3

    Tom3 TPF Noob!

    Joined:
    Nov 28, 2008
    Messages:
    77
    Likes Received:
    0
    Can others edit my Photos:
    Photos OK to edit
    yes, I understand the concept of aperture, but I did not know these were the max allowed! that sheds some light!

    Now, this brings two questions:
    - My second lens is a 18-200 1:3.5-6.3; If I want to make a shot at 100mm, can I make linear math to find the max aperture or does it follow some more complex curve?

    - Also, if I set the lens at 200mm, the camera will still let me select a wide aperture, like 3.5; does it make its lighting math as if the lens could really do a 3.5 opening?
     
  5. ann

    ann No longer a newbie, moving up!

    Joined:
    May 14, 2007
    Messages:
    4,263
    Likes Received:
    189
    Can others edit my Photos:
    Photos NOT OK to edit
    sat the full 200mm the lens will only allow the maxium of 6.3 f stop.


    set you camera in aperture priority mode. use the widest opening and the widest angle. i.e. your 18mm wide focal length with a 3.4 fstop. watch the lcd on the top of the camera and rack the lens out to it's longest focal length. You will see at which point the aperature begins to shut down finally reaching 6.3 when you hit 200mm.

    this is one big reason some of us don't like these types of lenses and are willing to pay the price for a lens that will maintain the same fstop wide open regardless of focal length.
     
  6. epp_b

    epp_b No longer a newbie, moving up!

    Joined:
    Aug 22, 2008
    Messages:
    2,135
    Likes Received:
    12
    Location:
    True North Cold and Freezing
    Can others edit my Photos:
    Photos OK to edit
    It's not linear. The curve depends on the lens. Use Aperture Priority mode, zoom down to the wide end of your lens, set the aperture as wide as it can be and then zoom back to 100mm. Whatever your camera says is the aperture is the minimum aperture available at 100mm. Same for any other focal length.

    I'm not sure if that's a statement or a mistyped question. At 200mm, your lens will not do f/3.5. It will be something like f/5.6 or f/6.3. You'll have to check using the instructions I gave you above.
     
    Last edited: Nov 29, 2008
  7. Tom3

    Tom3 TPF Noob!

    Joined:
    Nov 28, 2008
    Messages:
    77
    Likes Received:
    0
    Can others edit my Photos:
    Photos OK to edit
    what I meant is that, in manual mode, the camera will let me select 'impossible' choices, such as 3.5 @ 200

    but I'm not sure if the camera is aware of the real value when it shows me if the picture is over/under exposed for example.
     
  8. dhilberg

    dhilberg TPF Noob!

    Joined:
    Jul 26, 2008
    Messages:
    516
    Likes Received:
    0
    Location:
    Washougal, WA
    Can others edit my Photos:
    Photos OK to edit
    Even in manual mode, your aperture choices should still be constrained by the limitations of the lens. In other words, even in manual mode the maximum aperture you can choose on your 18-200mm f/3.5-6.3 @ 200mm should be f/6.3.

    What is the make and model of your camera and lens?
     
  9. Tom3

    Tom3 TPF Noob!

    Joined:
    Nov 28, 2008
    Messages:
    77
    Likes Received:
    0
    Can others edit my Photos:
    Photos OK to edit
    I understand the physical limitations of the lens, but I'm wondering what does the camera do internally with its light calculations:

    If it expects a 1/3.5 but in reality it's only getting 1/6.3, I would guess the math would be messed up.

    I've a Nikon D200 with two lenses: Nikkor 18-70 3.5-4.5 and Sigma 18-200 3.5-6.3

    So, I can set the camera to 3.5, set the lens on 200mm: will the camera still think it gets a 3.5 opening and assume the right amount of light will get in, or will it really know it's only going to get a 6.3 and adjust the light computation accordingly?
     
  10. Jklersy

    Jklersy TPF Noob!

    Joined:
    Nov 20, 2008
    Messages:
    122
    Likes Received:
    0
    Location:
    I live in MN
    Can others edit my Photos:
    Photos OK to edit
    If you can set your camera to f3.5 in aperture priority mode with your lens at 200mm then you might want to get your camera looked at. You should also search online to see what is being said about that camera/lens combo because the camera should know what the limitations of the lens are and account for it always.
     
  11. Ptyler22

    Ptyler22 TPF Noob!

    Joined:
    May 23, 2008
    Messages:
    789
    Likes Received:
    0
    Location:
    Massachusetts
    Can others edit my Photos:
    Photos NOT OK to edit
    I am not totally sure I know what you are trying to say but I might. and the answer is no. Ok so here is a real world test you can do for us to prove what we mean. OK so assuming the d200 has a depth of field preview button, push the depth of field preview button while looking into the lens. Look into the end and then push it while looking into the lens. Then zoom in while still looking into the end of the lens. If you look closely you will see these plates move together toward the center of the lens, making the opening for light smaller. Also what you can do is hold the DOF button down and crank up the aperture all the way to whatever the highest number is you can set the lens to. you will see the actual aperture getting smaller. It's not a calculation, it is a physical hole and the aperture pertains to the size of the hole. I hope this makes sense. If not I can try to explain it better. Or if somebody understands they can try to explain it.
     
  12. Ptyler22

    Ptyler22 TPF Noob!

    Joined:
    May 23, 2008
    Messages:
    789
    Likes Received:
    0
    Location:
    Massachusetts
    Can others edit my Photos:
    Photos NOT OK to edit
    And I missed the last part of your question. Yes, it will know in reality it will only get 6.3 and adjust, because that lens isn't physically possible of 3.5 at 200mm, the camera knows what lens it is and every thing about it, so it can figure out that, and sooo much more
     

Share This Page

Search tags for this page

aperature numbers on a 40d

,

camera aperture math