Max Apperture and Max. Shutter Speed Confusion-HELP

Discussion in 'Digital Photography' started by bhaskar, Jul 15, 2003.

  1. bhaskar

    bhaskar Guest

    Hi,
    Many zoom lenses have a specification like
    f/2.8-4.0
    What does the range mean ?
    I know that one is max. aperture at wide angle end and
    another at tele end. But why do zoom lenses have two diff
    values ?
    Also when you say max. aperture is say e.g. f/2.8
    Does is mean that aperture can be opened max up to f/2.8
    i.e. f/22 to f/2.8 or that it can be closed only up f/2.8
    i.e f/2.0 to f/2.8 ?

    Also when max. shutter speed for a perticular lens/apperture is 1/1000 of a sec .
    Does it mean the shutter can't close any faster than 1/1000 of a sec or it
    can't close any slower than 1/1000th of a sec ?

    Many books are sometimes confusing on these terminologies, so i was hoping
    some pro can clear this up for me.
    thanks
    bhaskar
     
    bhaskar, Jul 15, 2003
    #1
    1. Advertisements

  2. For focal plane shutter/interchangeable lens cameras, your statement is
    nearly correct in all cases.

    However, many cameras (film or digital) with leaf shutters site the
    shutter in the lens coupled with or even implemented as a part of the
    aperture mechanism. There are some limitations in some designs which
    limit the available shutter speed range at particular apertures because
    of this ... For instance, the top shutter speed may require an aperture
    of f/5.6 or smaller in many cases.

    Godfrey
     
    Godfrey DiGiorgi, Jul 15, 2003
    #2
    1. Advertisements

  3. bhaskar

    Jim Townsend Guest

    The 'f' rating of a lens is the ratio between the length and width.

    A fixed lens that has a focal length of 50 millimeters and a maximum aperture
    of 50 millimeters would have a 1:1 ratio.. It would be a: 50mm f/1.0 lens.

    A fixed lens that has a focal length of 50 millimeters and a maximum aperture
    of 25 millimeters would have a 2:1 ratio.. It would be a 50mm f/2.0 lens.

    A fixed 50mm f/4 lens would have a maximum aperture of 12.5mm (You determine
    this by dividing 50 by 4).

    OK.. now..

    Zoom lenses get their 'zoom' by changing the distance between the individual
    lenses and the film or imaging device. Or in other words, changing the focal
    length.

    If you have a lens designed to slide the elements back and forth between a
    distance of 50mm and 100mm, you have a 50-100mm lens.

    Now lets assume this lens can only open to an absolute maximum of 50mm because
    of it's physical size.. (eg: A 20mm wide tube can't give you a 50mm wide
    aperture).

    With this lens, you will have a 1:1 ratio with the zoom set to 50mm, but when
    you extend the lenses to 100mm, you now have a 100mm lens and the same 50mm
    aperture. The ratio changes to 2:1. In this case, this hypothetical lens
    would be rated as: 50-100mm at f/1.0-2.0

    The above is simplified a bit.. Modern compound lenses are a bit more
    complicated, but they are all based on the above fundimentals. You'll always
    see that the higher 'f' number relates to the longest focal lenght.

    If your'e wondering if you can you make a lens that varies the focal length
    *and* aperture at the same time to keep everything constant.. (ie 28-125mm
    f/2.8).. Yes.. You can. There are such lenses, but they are EXPENSIVE.. So
    the common people have to put up with zoom lenses with varying 'f' ratings :)
     
    Jim Townsend, Jul 15, 2003
    #3
  4. bhaskar

    Matti Vuori Guest

    No, they don't. They can just buy an Olympus C-4000.
     
    Matti Vuori, Jul 15, 2003
    #4
  5. When the lens is zoomed in and the aperture is set to its maximum, the
    camera lens will let through less light than when the lens is set to
    wide-angle as the light will have to travel a longer distance from the
    camera's lens to the film. As the aperture indicates the amount of light
    that is passed to the lens it is also true that the value of the aperture
    will differ when using different zoom values. The actual opening of the
    aperture ring will stay the same, but the value differs.

    The amount of light passed to the film affects the depth of the photo. To
    get a larger range into focus you'll use a higher f-value while a lower
    f-value will result in a smaller part of the picture (in front and behind of
    your object) being in focus.

    The shutter speed either freezes the object or let you add movement to your
    photo by using a slower shutter speed.

    How to use this:
    1. Set the film's ISO value ( or ISO indicator when using a digital camera)
    to the desired setting. ISO 100 for daylight conditions to ISO 1600 for near
    pich dark conditions)
    2. Leave the camera on auto and let the camera focus and decide the
    approppriate apperture and shutter speed. Keep the zoom on the zoom you'll
    want to use when making the photo.
    3. Every f/stop indicates a doubling of the amount of light that is passed
    through. If you want to double the shutter speed from lets say 1/250 to
    1/500 you'll also have to double the amount of light, f/8 should then be set
    to f/5.6

    Also remember that a teleconverter will also convert the aperture because
    the light will have to travel further. With a 2x converter a f/4 will result
    in a f/8.

    Sorry Martin, but you'll have to read more up to date books before awaiting
    a fee. There is ample digital camera's on the market with more modes than P
    (even SLR digital camera's are out).
    Wide angle lenses does not neccesarily mean a faster lens. The maximum
    aperture (value determined by the parts used inside the lens) determines the
    speed of the lens. A 18mm f/2 is faster than a 18mm f/4.0.
    It is also not cheaper to make a lens with a 'fixed maximum aperture' as the
    opening of the aperture will allways stay the same, the amount of light and
    thus also the value of the aperture differs with different zoom values.

    Werner Spreeuwenberg
     
    Werner Spreeuwenberg, Jul 15, 2003
    #5
  6. bhaskar

    Jim Townsend Guest

    So the C-4000 keeps its 2.8 aperture over the entire optical zoom range..

    OK fair enough. I was thinking of old SLR lens theory.. I guess constant
    ratio lenses aren't expensive on digital cameras.

    Any other models do this ?
     
    Jim Townsend, Jul 15, 2003
    #6
  7. Sorry Martin, but you'll have to read more up to date books before
    awaiting
    Thanks for the heads up, but I sold a Fuji S2 on Saturday; doesn't mean the
    OP has one. In fact, the OP really didn't state much about his/her own
    camera, or even if he/she had one. There are still a number or low-end
    compacts that don't have an option to go into full-manual exposure control.
    Big sellers like the Optio S, Ixus II and Dimage XT all lack A and S modes,
    and I applaud Canon for releasing budget compacts that have this option
    clearly advertised on a top dial. It's a big selling point with the 10D
    buyers who realise they can't carry their LUSMIS everywhere, but still want
    to buy Canon.
     
    Martin Francis, Jul 15, 2003
    #7
  8. bhaskar

    STIG Guest

    Also remember that a teleconverter will also convert the aperture because
    This is true for film converters but is it true for converters designed for
    digital cameras? On my Nikon c4500 with the 2x teleconverter installed I
    notice no loss in light.

    Rob.
     
    STIG, Jul 16, 2003
    #8
  9. bhaskar

    MarkH Guest

    No, you need to square the number for it to make sense:
    F1 -> 1
    F1.4 -> 2
    F2 -> 4
    F2.8 -> 8
    F4 -> 16
    F5.6 -> 32
    F8 -> 64
    etc

    Each step represents 1 stop, halving the light as the number gets smaller.
     
    MarkH, Jul 16, 2003
    #9
  10. bhaskar

    STIG Guest

    Yes, you're right, don't know what I was thinking :0(

    Rob.
     
    STIG, Jul 16, 2003
    #10
  11. bhaskar

    Don Stauffer Guest

    Zoom lenses have a relative aperture that varies with focal length,
    because some element of glass has a diameter that fixes the maximum
    aperture diameter. However, the focal length varies depending on the
    relative axial spacing of the elements. Since the f/# is the ratio of
    focal length to aperture, the result is that it varies with focal
    length. While may be possible to make a zoom lens that has a fixed
    aperture stop, most do not.
     
    Don Stauffer, Jul 16, 2003
    #11
  12. The important distinction isn't film/digital, it's whether the converter
    installs in front of or behind the main lens. Non-SLR cameras are
    restricted to the former, while SLRs normally use the latter.

    A 2X converter installed in front of the main lens may not change the
    aperture at all. A 2X converter installed between lens and body will
    always lose 2 stops of light, because the focal length has been doubled
    but the front element of the combination is the same size it was before.

    Dave
     
    Dave Martindale, Jul 16, 2003
    #12
  13. No, it goes by sqrt(2) ~ 1.41

    Bye, Dragan


    --
    Dragan Cvetkovic,

    To be or not to be is true. G. Boole No it isn't. L. E. J. Brouwer

    !!! Sender/From address is bogus. Use reply-to one !!!
     
    Dragan Cvetkovic, Jul 22, 2003
    #13
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.