The zoom and aperture of a lens

Discussion in 'Beyond the Basics' started by dhawald3, Mar 25, 2008.

  1. dhawald3

    dhawald3 TPF Noob!

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    I am just learning photography as a hobby on my new
    Fuji finepix s9600.

    Here is my doubt

    When I am in A mode and
    I set the aperture to f2.8 and then zoom in the lens the aperture becomes f4.9

    Is this same as setting the aperture to f4.9 and then zooming in to get f4.9??

    will I get the same thing in the two cases??
     
  2. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    It sounds like the lens is a variable aperture lens. f/2.8-f/4.9. At certain focal lengths, the minimum aperture changes.
     
  3. JimmyO

    JimmyO TPF Noob!

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    ^^^ yup

    The more telephoto a lense it is to more expenisive and harder it becomes to keep the aperture low. This camera is not SLR but the an example is kind of why they 70-200mm f/2.8 costs over a thousand dollars more than the 70-300mm f/4-5.6, even though its less zoom.
     
  4. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Some lenses are 'constant aperture' and have the same maximum throughout the zoom range. These are usually the higher end lenses. The cheaper zoom lenses usually have a 'variable' max aperture, so it gets smaller as you zoom.

    To understand this better, you need to understand that the F-number is actually a ratio of the size of the aperture to the focal length. F2.8 is 1:2.8...but when you zoom out, you change the focal length and the ratio changes to 1:4.9...which is F4.9.
     
  5. dhawald3

    dhawald3 TPF Noob!

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    Ya that's all fine
    but I haven't got my answer yet

    What I need to know is that will there be any difference in the two methods
    mentioned above.
     
  6. Socrates

    Socrates TPF Noob!

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    No. These are two different situations.

    First, be advised that large "f-numbers" mean small apertures and verse visa.

    When you are zoomed in and set the aperture to f/2.8, that aperture is larger than what that lens can do when it is zoomed out. When you zoom out, the camera will tell the lens to open to f/2.8 and the lens will do the best that it can, which apparently is f/4.9 in your case.

    On the other hand, if you set the aperture to f/4.9, that aperture is possible at all zoom positions. The camera will tell the lens to open to f/4.9 and the lens will comply.
     
  7. passerby

    passerby TPF Noob!

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    Your f2.8 applies for the widest focal length. When you slowly zooming in into the telephoto side you should notice of the aperture dropping somewhere. Naturally if you set the aperture at f4.9 at the widest end than it is constant f4.9 all the way.
    That is assuming if I understand your query correctly.

    Happy shooting
     
  8. dhawald3

    dhawald3 TPF Noob!

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    ya that's what I ment

    so that means there won't be any diffrence in the end result of the images
     
  9. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    If the final settings are the same, it doesn't matter how you got there. The resulting image will be the same.
     
  10. christopher walrath

    christopher walrath No longer a newbie, moving up!

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    O.K. The f-stop number is a ratio btween a lens' focal length and its maximum aperture diameter.

    Av = F / d

    Av - Aperture ratio (f-stop number)
    F - Lens Focal length
    d - diameter of maximum aperture opening

    Take a 50mm lens. With the aperture stopped all the way up, the maximum aperture diameter is 25mm across.

    AV = 50 / 25

    AV = 2

    f/2

    The same lens but with a maximum aperture diameter of 18mm.

    Av = 50 / 18

    Av = 2.8

    On a zoom, assuming a maximum aperture diameter of about 10mm, the minumum focal length would be about 28mm if the minimum aperture was f/2.8. The maximum focal length would be around 50mm. The aperture diameter remains the same but the focal length increases thus reducing the aperture.
     
  11. Helen B

    Helen B TPF Noob!

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    "On a zoom, assuming a maximum aperture diameter of about 10mm, the minumum focal length would be about 28mm if the minimum aperture was f/2.8. The maximum focal length would be around 50mm. The aperture diameter remains the same but the focal length increases thus reducing the aperture."

    (I think you mean ...if the maximum aperture was f/2.8 at the end of the first sentence.)

    Chris,

    You have to be careful with zooms because the 'aperture' diameter (actually the entrance pupil diameter) usually changes as you zoom, it does not stay the same.

    Best,
    Helen
     

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