F-Number Confusion

Discussion in 'Photography Beginners' Forum' started by Silvius Brabo, Jun 7, 2010.

  1. Silvius Brabo

    Silvius Brabo TPF Noob!

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    Pretty new to photography, been looking around for some film cameras, a little confused about the F-number jargon.

    Does the f-number change automatically whilst zooming? And can you change the f-number when not zooming?

    Thanks for the help, much appreciated.
     
  2. csgrafix

    csgrafix TPF Noob!

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    Depends on wether you have a fixed lens or not. On a fixed lens the f-stop will not change automatically if your in a manual mode such as M or Apeture priority. Variable lens will close up (higher f-stop as you zoom in) which will be your lowest f-stop that your lens can do at that focal distance. You would be able to higher your f-stop but it will have its lowest f-stop.
     
  3. cfphoto

    cfphoto TPF Noob!

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    CSGrafix pretty much covered it.

    One of the easiest ways to determine if a zoom lens is fixed or not is that there will be (2) 'f' numbers beside the size of the lens. Indicating the apeture range, for example a 70-200mm 3.5/5.6 lens will adjust the f/stop automatically as you zoom in or out, a 70-200mm 2.8 lens will have a constant aperture throught the zoom length.

    Hope that helps.
     
  4. benlonghair

    benlonghair TPF Noob!

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    To expand on what CSGrafix said, your F stop is a function of lens length. It refers to the length of the lens (for example 200mm) divided by the number of the aperture (for example 2.8). In this case the aperture would be 200mm/2.8 = 71.4mm aperture.

    Each full stop (for example 2.8 to 4) is doubling or halving the light entering the camera. Most cameras are set up with either half or third stop increments.

    The higher the f/stop (or smaller the aperture) the larger your depth of field will be.

    Hope this helps.
     
  5. KmH

    KmH Helping photographers learn to fish Supporting Member

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    The f-number is actually a fraction as benlonghair mentioned, because it is a ratio of the focal length to the size of the lens opening (aperture).

    In other words you can substitute a 1 for the f, so f/4 becomes 1/4 meaning the lens opening is 1/4 the diameter of the focal length of the lens. With a zoom lens that focal length is variable.

    So, you have a variable aperture 18 mm - 55 mm zoom lens.

    Typically at 18 mm the widest the lens can open is f/3.5 and the widest it can open at 55 mm is f/5.6.

    If you set the aperture in the camera to f/3.5 while at 18mm and then zoom the lens towards 55 mm, the aperture value will change automatically until it is at f/5.6 when you reach 55mm.

    Here is the trick though, f/5.6 at 55 mm is a physically larger lens opening (9.82 mm) than f/3.5 is at 18 mm (5.14 mm). Do the math.

    I'll bet most people visualize the lens opening getting smaller when they zoom a variable aperture lens from 18 mm - f/3.5 to 55 mm - f/5.6. The lens opening actually gets more than 2 times larger, even though the focal ratio get's smaller. 1/5.6 is a smaller number than 1/3.5.
     
  6. digital photo coach

    digital photo coach TPF Noob!

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    Seems like f-numbers mess up everyone when they start out. If only they had defined the number as ratio of aperture to focal length! then big numbers would mean big apertures.
    I tell the rest of the story and the mystery behind numbers like f-2.8, f-5.6, f-6.3, f-7.1, etc. here: 21 April 2011 | thedigitalphotocoach.com

    By the way, some zoom lenses do not change aperature at different focal lengths. The lens designation will tell you what to expect. A 28-70mm 2.8 will have constant 2.8 aperture; a 28-70mm 2.8/3.5 will have 2.8 at 28mm and 3.5 when zoomed to 70mm.
     
  7. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    To further confuse the OP, if you have, say an 18-55 f/3/5-5.6 zoom, once you stop the lens down to at least f/5.6, then the aperture will stay the same throughout the zoom range. If you set it to 5.6, then it will be 5.6 no matter where you zoom the lens to. Only when you open the aperture up to larger than f/5.6 will there be any change to the aperture across the zoom range.
     
  8. Helen B

    Helen B TPF Noob!

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    Er, Coach, f-number is not focal length/aperture area as you say in your blog, it is focal length / aperture diameter (as others have said). That makes it a 'dimensionless number' - which is very important, otherwise the value would be different for different units. If you measured in inches and square inches it would be one value, if in mm and sq mm it would be another value.

    What is also important if you are looking at zooms is that it is not the physical diameter of the aperture, but the diameter of the image of the aperture when seen from the front of the lens (called the 'entrance pupil', which is also the point of no parallax for stitched panoramas). This explains how the diameter can change as the lens is zoomed without there being any change in the physical diameter of the aperture. "Here is the trick though, f/5.6 at 55 mm is a physically larger lens opening (9.82 mm) than f/3.5 is at 18 mm (5.14 mm). Do the math." It's not necessarily a physical change, but a change in optical magnification of the physical iris caused by the movement of the various parts of the lens as it zooms.

    Best,
    Helen
     

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