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Thread: 4-5.6?

  1. #1

    Default 4-5.6?

    Hi I got a newbie question. What does it mean when my len's is a 4-5.6? I know it's the aperture but someone please explain in detail for me.

    Sometimes I am able to go to the biggest aperture at 4 but sometimes I can only go to 5.6 during manual mode. Am I missing something here?

  2. #2

    Default Re: 4-5.6?

    This kind of range is usually expressed for zoom lenses. The lower f-number (here, f4)is for the lower focal length (wide-angle side of the zoom). But as the focal length increases the amount of light incident drops and it has a higher f-number like f5.6 (1 stop higher) in this case.

    So, basically you wont be able to go to f4 on the higher focal length side, unless you have a fixed-aperture lens like 24-70mm/f2.8L or 24-105mm/f4 L, which tend to be more expensive.
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  3. #3

  4. #4

    Default Re: 4-5.6?

    The f/stop is usually denoted with the "/" sign for a reason. "/" means divide, and "f" means focal length.

    First of all, all the following discussion relates to 35mm film cameras, which will probably be the same as FF cameras these days. APS-C lenses probably have different sized aperture, but the concept is the same though the maths is different.

    f/stop tells you the size of the aperture opening when you are shooting. For example, if you have a 50mm lens opened at f/4, the aperture size is 50mm/4 = 12.5mm. But nobody really cares for the actual size of the opening because it is the stop number that is more conceptually related to the exposure effect. That is, if you have a longer lens, you need a larger actual aperture size to expose the same amount as a shorter lens. So, 100mm at f/4 is opened to 25mm, but 50mm f/4 is opened to 12.5mm, but the picture will appear similarly exposed.

    So, if you have an expensive lens that has a constant max f/stop at f/4 for example, the aperture size actually increases as you zoom to tele. So, these lenses are usually thick and fat, so that they can open wider to accomodate this.

    If you have a cheaper prosumer lens, there is a limit to how wide the aperture can open. Once it hits the limit, the f/stop starts to increase... sometimes all the way to f/6.3. But bear in mind... the aperture size is actually constant.

    Get it?
    Last edited by nottipiglet; 12th May 2009 at 03:59 PM.

  5. #5
    Senior Member giantcanopy's Avatar
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    Default Re: 4-5.6?

    Quote Originally Posted by nottipiglet View Post
    So, if you have an expensive lens that has a constant max f/stop at f/4 for example, the aperture size actually increases as you zoom to tele. So, these lenses are usually thick and fat, so that they can open wider to accomodate this.
    Get it?
    I do not think the aperture actually increases at the telephoto end. So why not the manufacturer maintains the larger aperture size at the longer end so the shorter focal length has an aperture of less than 4 ?

    Most of the newer constant aperture zooms manipulate the movement of lens elements to vary the magnification and effective focal length. Think of it as magnification towards the longer end. But with very good optics the IQ degradation is insignificant.

    * there might be some old zooms that operates otherwise

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    Last edited by giantcanopy; 12th May 2009 at 04:08 PM.

  6. #6

    Default Re: 4-5.6?

    Quote Originally Posted by Atomant669 View Post
    Hi I got a newbie question. What does it mean when my len's is a 4-5.6? I know it's the aperture but someone please explain in detail for me.

    Sometimes I am able to go to the biggest aperture at 4 but sometimes I can only go to 5.6 during manual mode. Am I missing something here?
    Ok the aperture determines how much light enters the lens and in a way influences your shutter speed. the smaller the number, the faster your shutter speed, the narrower your depth of field and the more background blur or bokeh you're gonna get.

    your lens is probably a zoom lens; at the lower focal lengths (fully zoomed out) it will have a maximum aperture of f4, but as you zoom in to the longest focal length it gets limited to f5.6 hence the inability for you to go lower than 5.6 even in manual mode, you're at the mechanical limit of the lens.

  7. #7
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    Default Re: 4-5.6?

    Quote Originally Posted by Atomant669 View Post
    Hi I got a newbie question. What does it mean when my len's is a 4-5.6? I know it's the aperture but someone please explain in detail for me.

    Sometimes I am able to go to the biggest aperture at 4 but sometimes I can only go to 5.6 during manual mode. Am I missing something here?
    I think you're referring to the marking on the lens that reads something like "f 1:4-5.6", and I think you probably have a zoom lens.

    What it this simply means that the smallest f number (or largest aperture) for the shortest range of the zoom is f/4, and f/5.6 for the furthest end of the zoom.

    For some people who like to shoot at constant f-stop, this is a real pain.

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  8. #8

    Default Re: 4-5.6?

    Hi Thanks for all your responses.

    So unless I manually set it to the smallest at F/22, F/ 4-5.6 means that the widest it'll go would be f/4 at 17mm and f/5.6 at 85mm?

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    Member thenomad's Avatar
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    Default Re: 4-5.6?

    the lens has a maximum aperture of f/4 at its widest zoom, and will become smaller (higher f-stop number) as you zoom to its furthest length, becoming f/5.6

    i think the physical size of the aperture increases as you zoom further. e.g. a 18-55 lens, usually with f/3.5-5.6, has an aperture size of 18/3.5 = 5.14 at 18mm, and 55/5.6 = 9.82

    you can check this by looking at the front of the lens while zooming in. the aperture size does increase as you do this

  10. #10

    Default Re: 4-5.6?

    Quote Originally Posted by Atomant669 View Post
    Hi Thanks for all your responses.

    So unless I manually set it to the smallest at F/22, F/ 4-5.6 means that the widest it'll go would be f/4 at 17mm and f/5.6 at 85mm?
    Yes the largest aperture opening (smallest f/stop number) you can get from your lens is f/4 @ 17mm and f/5.6 @ 85mm

  11. #11
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    Default Re: 4-5.6?

    Quote Originally Posted by Atomant669 View Post
    Hi Thanks for all your responses.

    So unless I manually set it to the smallest at F/22, F/ 4-5.6 means that the widest it'll go would be f/4 at 17mm and f/5.6 at 85mm?
    yes, you are correct

    there's also the same limitation with the smallest aperture, e.g. at widest you can only go to f/22, but at furthest may be able to go up to f/32, depending on the lens

  12. #12

    Default Re: 4-5.6?

    Quote Originally Posted by thenomad View Post
    yes, you are correct

    there's also the same limitation with the smallest aperture, e.g. at widest you can only go to f/22, but at furthest may be able to go up to f/32, depending on the lens
    So if I get an f/2.8L 70-200mm lens, the widest it will go would be throughout the range from 70-200mm at f/2.8? What about the smallest aperture? f/32 throughout? Thanks.

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    Default Re: 4-5.6?

    Quote Originally Posted by Atomant669 View Post
    So if I get an f/2.8L 70-200mm lens, the widest it will go would be throughout the range from 70-200mm at f/2.8? What about the smallest aperture? f/32 throughout? Thanks.
    yes. you can also check the lens' specs from their respective manufacturers

    normally the widest aperture is of much more interest than the smallest one though

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    Default Re: 4-5.6?

    Quote Originally Posted by thenomad View Post
    yes. you can also check the lens' specs from their respective manufacturers

    normally the widest aperture is of much more interest than the smallest one though
    Yep.

    Cos for practically any lens, if you stop it down to it's smallest aperture, you're gonna get severe diffraction (i.e- blur images)

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    Default Re: 4-5.6?

    Quote Originally Posted by Atomant669 View Post
    What about the smallest aperture? f/32 throughout? Thanks.
    What do you want to achieve with using f/32? Google a bit about "diffraction". If you want to close aperture to extend exposure time than have a look at ND filters.

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    Default Re: 4-5.6?

    Do remember that even though f/stops are said to be dependent on focal length, where in

    /n, =focal length

    /n on any lens in your SLR system will still give you constant exposure, because all the lenses are actually calibrated to a more universal unit known as T-stops, where T stands for transmission.

    I quote from the Wikipedia article on f/stops:
    Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness.

    Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly (The T in T-stop stands for transmission), but modern cinematographic lenses now usually tend to be factory-calibrated in T-stops.

    T-stops measure the amount of light transmitted through the lens in practice, and are equivalent in light transmission to the f-stop of an ideal lens with 100% transmission. Since all lenses absorb some quantity of light, the T-number of any given aperture on a lens will always be greater than the f-number.

    In recent years, advances in lens technology and film exposure latitude have reduced the importance of t-stop values. So, F-stops are for focal ratio, T-stops are for transmission.
    In other words, T-stops are a more accurate and universal benchmark. It's just that because everyone is so used to calling them /stops, we still refer to our aperture reading as /stops.

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