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I just bought a 25mm 1.8 lens. It seems I can set the f stop to higher than 1.8? Are all primes like

Discussion in 'Olympus Cameras' started by mootxico, Dec 18, 2014.

  1. mootxico

    mootxico Mu-43 Rookie

    Dec 6, 2014
    Still a newbie here. Been taking pictures with my e-pl1 for 3 years using kit lens, recently decided to upgrade to a e-pl7 and bought a 25mm F1.8 recently.

    It appears I can increase the F to higher than 1.8? Isn't the prime lens supposed to be only fixed focal length?
  2. emorgan451

    emorgan451 Mu-43 Veteran

    The F number is the aperture not focal length. Your lens is fixed at a 25mm focal length.
    • Like Like x 1
  3. MarkRyan

    MarkRyan Instagram: @MRSallee Subscribing Member

    May 3, 2013
    Focal length determines field of view. Aperture determines how much light passes through the lens.

    A prime lens cannot change its field of view, but it can certainly change how much light passes through.
  4. pellicle

    pellicle Mu-43 Hall of Famer Subscribing Member

    Feb 10, 2010
    Killarney, OzTrailEYa

    Question is confusing. Yes F can go "higher" in the selection of bigger numbers. These represent smaller aperture diameters.

    You then mention the focal length so I'm confused as to what you are asking. Perhaps its simply that you have confused the two numbers. Some lenses have f also written for focal length. So your lens has a fixed focal length of 25mm but you can of course alter the aperture from fully open (f1.8) through to having a very small aperture like f22

    Try looking at the front of the lens from the front when you take a shot. You will see the aperture stop down (if you have set it smaller, like say f22)

    Hope that helps
  5. mootxico

    mootxico Mu-43 Rookie

    Dec 6, 2014
    Ah, sorry. I'm such a scatterbrain

    I meant by the aperture (the f1.8 thing).

    Is there a difference between using my 25mm prime at f3.5, and using my kit lens (14-42 1:3.5-5.6) zoomed in a little at 25mm, at f3.5 too?
  6. Vivalo

    Vivalo Olympus loser Subscribing Member

    Nov 16, 2010
    You can only open your aperture to largest setting of f3.5 at 14mm with your 14-42mm zoom. At 42mm you can only open the aperture to f5.6. So it is not possible to open the aperture to f3.5 at 25mm with 14-42 lens. But if you set your 25mm to f5.6 and your 14-42mm to 25mm and f5.6 there shouldn't be much of a difference.
  7. shnitz

    shnitz Mu-43 All-Pro

    You are correct, theoretically 25mm and f/3.5 is an identical photo, regardless of which lens you use. In the real world, the 25mm has fewer optical deficiencies (vignetting, aberration, softness at the edges, etc).
  8. JNB

    JNB Mu-43 Veteran

    Dec 11, 2014
    Nova Scotia, Canada
    The widest aperture available on your zoom at 25mm is about f/4.4. If you set your 25mm prime lens to the same aperture, the picture would be substantially the same. The 25mm prime will be a bit sharper.

    The advantage of the fixed-focal-length 25mm "prime" is that you can use a wider aperture (up to f/1.8). A) you are letting in more light with a wider aperture, and therefore you can use a faster shutter speed and/or lower ISO for the same exposure; B) your "depth of field" decreases with a wider aperture, and a shallower depth of field can help to isolate the subject from the background.

    So, the "prime" gives you some low-light and creative options that you can't achieve with the zoom. However, when you have lots of light, and you would like a wider depth of field, you can adjust the aperture of the prime to something smaller (f/5.6 for a landscape, as an example).
  9. Jfrader

    Jfrader Guest

    As others have said, the prime at f/4.5 and the zoom set at 25 mm and f/4.5 will give approximately the same image. The differences are in the greater sharpness of the prime, the wider aperture (lower number) available and the ability to use that lower aperture to blur out the background more than with the zoom. When you hear photographers talk about "bokeh," that is a result of using the widest possible aperture to get that nice, creamy, out-of-focus background..

    These are primarily artistic differences which will allow you to achieve somewhat better images once you understand those differences. In effect, you trade off the greater versatility of the zoom for greater sharpness and image quality with the prime. The choice of which to use comes down to that. Sometimes you will want that zoom's versatility more than the wider aperture and shallower depth of field. Sometimes you will want greater artistic control and be willing to give up the zoom's ability to alter composition at the twist of a wrist. Both come in handy at different times.
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