what does the f/2.8 or f/4L or f/1.8 mean in canon lenses?

Discussion in 'Canon EOS' started by einav leshetz, Jun 18, 2006.

  1. I'm sorry if this is a silly question but I'm trying to understand the digital
    camera world..

    I'm planning on getting the 24-105mm f/4L Canon lenses. However I'm trying to
    understand what the f/4L stands for. does it stand for the widest f-stop? and
    what does L mean? - therefore why is the 4L series better than the 2.8 -
    because as if you'd want to shoot under such a low aperture- I figure there's
    something I'm not getting here...


  2. Yes. The f4 is the smallest f number (largest aperture).

    In general f2.8 is "better" than the f4 though it all depends on the lens. The f2.8 lenses tend to be heavier and bulkier than their f4 counterparts.

    The 24-105/4L IS is the only full frame standard L zoom with IS. I would argue that its 2.8 counterpart, the 24-70/2.8, is slightly better. Slightly faster for low light, shallow DOF for portraiture, and almost no vignetting at the wide end. It lacks IS and a little reach.
  3. To dispose of the L first, this indicates that is one of Canon's top notch professional
    lenses, built for optical and mechanical excellence, rather than to meet a consumer or
    'prosumer' price point. Amateurs are allowed to buy them, though!

    The number indicates the maximum aperture of a lens. Oddly, the smaller the f number
    the bigger the max aperture (the f number is actually the ratio between the focal length of
    the lens and the diameter of the elements in it - thus a 50mm lens with 50mm diameter
    glass in it would be f1.0, with 25mm glass would be f2, with 18mm glass would be f2.8,

    Practically speaking a bigger aperture means the lens captures more light. This means that
    you can - if you wish - use faster shutter speeds or slower film (or sensor) speeds in poor
    light for better results. Another upside/downside of shooting at large apertures is that the
    front-to-back area of a picture which is acceptable sharp is much shallower, allowing you
    to shoot - for example - a portrait against a distracting background by throwing the
    background completely out of focus while keeping the subject razor sharp. There are
    many other uses for differential focus like this.

    Within the lens there is an aperture control mechanism (a diaphragm) which can restrict
    the size of the aperture, thus changing the f number from shot to shot. Typically whatever
    the maximum aperture of the lens you choose, you will be able to control the shooting
    aperture all the way down to f16 or f22 (that would be an aperture size of 2.2mm on a
    50mm lens, for example.

    Hope it helps - there are many photography 101 sites around the web which will explain
    (and illustrate) all this much better than I can.
  4. Ken's answer is substantially correct.

    However when you look at a lens you will see it is not the size of the elements that determine the maximum f-stop. Otherwise a 17-40 would have elements of size 1 cm whereas the front element is slightly over 6cm. You often here it explained that the f-stop is the ratio is between the focal length and the size of the aperture. If you diassemble a lens and look at the physical aperture you will typically find the hole is much smaller than it should be (typically it buried deep within the lens to make the blades lighter). To a first approximation what matters is the size of the aperture when viewed through the front element. Especially with wide angle retrofocus designs you will find very large elements even when the lens is not particularly fast.
  5. The lens I referred to is the 17-40/4L. It has maximum focal length 40mm and maximum aperture f4. The simple calculation shows that the largest aperture should be 10mm at focal length 40mm and a massive 4.25mm at focal length 10mm. Actually the maximum aperture of the diaphragm is likely fixed but the apparent size changes since the actual diaphragm sits behind many of the elements and changes apparent size when you zoom.
  6. You often "hear" it explained ....

    I can spell and I can type. Apparently I can't do both simultaneously.
  7. the 24-70mm 2.8 has an ability to shoot with a larger aperture and has less problems with vignetting. Typically, trying to cover too much ground with one lens is a bad thing. It becomes physically difficult to create a quality lens, trying to be all things to all people. I would go for the 24-70 as it's the same price, but a much better lens.

    Look up in Wikipedia "aperture" and the L is the series of lens, IE Luxury :) Basically, the larger the number (f/22) the smaller the opening and the more area that's in focus and the inverse for the smaller number (f/1.8)
  8. Thanks for agreeing with my post. I am not sure I would characterize the 24-70/2.8 as much better than the 24-105/4. The 24-105/4 does have fairly hefty vignetting at the widest setting when wide open but is otherwise optically very good.

    For a walk around lens I think the extra reach and IS probably makes the 24-105 a better lens. However for portraiture I think the 24-70/2.8 is a better bet. You could however get the 24-105/4L for an everyday lens and primes for portraiture.

    My everyday lens set is based around f4 max lenses.
  9. thanks for your time and explinations.
    I do understand aperture and shutter ... I just dont understand 'the mechanics' of lenses. I was initially going to buy a 17-40 F/4L lens but was told to get the 24-105 because I'll get better coverage in the long term. I want a lense that I can take when I go backpacking but that also suits semi-professional work. I'm trying to do my research as im a couple of weeks from purchasing...

    I'm a medium format 60mm lense girl myself...
  10. Nav wrote:
    "I'm a medium format 60mm lense girl myself..."

    There is a thread on the medium format digest - "any bad lenses?".

    Hardly any bad MF lenses....you buy the focal length you need and they are pretty much excellent.

    These small format camera makers seem to have 3-4 tiers of products, so it is confusing when one starts to look for very good equipment. Then even at a high price point, people disagree with how good these L lenses are.
  11. the L means you pay a Lot of money for a Luxury item and your wallet feels Lonely as a result :)

    I shoot at f4 and 2.8 all the time. great for portraits. I'm looking at getting the 85mm 1.2 and - you guessed it - I would plan to shoot at 1.2 95% of the time.
  12. Thomas that is so true, I feel so overwhelmed with digital lenses.

    I've been thinking though, I really like the 60mm medium format feel so I'm thinking of getting the 17-40 f/4L lenses instead.. to get the wider feel.. I'm wanting to suit my 30D so it will be more like a 27-50 once on the body. I havent heard anything bad about the 17-40... has anyone?
  13. Hi Nav,

    If you can live with the slower f/4 aperture of the 17-40, it is an excellent lens. I use it on
    my 1DS bodies, and also frequently on my 20D it gives a good workable range of
    27-64. In lower light situations with your 30D you can push your ISO to 800 to make up
    for the slower f/4 of the 17-40. Newer digitals seem to be handling noise better at higher
    ISOs, and that's a good thing!

    Take care,


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