For beginners, the concept of f stops seems like something so intangible and out there that it's diificult to get your head around it at all, let alone realise what it means to your photos. In general, the lower f the number written on the lens, the better the photos you can take with it and the more the lens costs. Or at least that's what you are lead to believe through all the marketing and brand hype that goes on!!!
Beginner Canon users can get their hands on a 50mm 1.8 lens for less than 100 dollars/euros/pounds on Amazon, and it performs pretty darn well. Even if it is a bit cheap and plasticky, it still blows away the standard 17-55mm kit lens for the photo style, creativity and fun you can have with it. It's also the first experience a beginner will get with really shallow depth of field.
Once you are hooked by all that shallow focus, creamy bokeh, and buttery smooth out of focus areas in your photos, you can never go back, and you end up wondering how long it will take you to save up for all of the lenses on offer with the lowest f-stop numbers (and highest prices).
However, a lot of the hype is little more than just that. It's a clever way to get consumers to lust after the most expensive lenses and think that lenses in their regular price range can't compare. However, for a beginner or budget minded photographer, you don't often need the most expensive lenses. Here are some points in favour of buying cheaper lenses and holding out on the expensive ones until you are a retired millionaire:
1) The lower the fstop number the lens is capable of, the more it costs. If you can barely afford it anyway, then you may end up getting a lens without image stabilisation. This can turn out to be a false economy. In most conditions, a lens with a stabiliser will help get you more shots without blur from camera shake, than one without no matter how high the fstop is.
2) You don't need a lens with a very low f number to get a photo with a very shallow depth of field. You can get photos with a shallow depth of field and much less background distraction by using a lens with a longer focal length. A 50mm 1.4 lens focused at 3m away has a depth of field of 2cm. A 250mm f5.6 lens focused at 3m away also has a depth of field of 2cm. The image produced will obviously look very different, but this just goes to show that depth of field is a combination of focal length (lens zoomability) and the f number.
3) Using low f numbers all the time isn't always good. The depth of field can be so shallow that only a person's eyelashes might be in focus. Any slight movement can throw the subject out of focus entirely. Portrait photographers often use f8 and flash when taking pictures in a studio to make sure everything in the picture is sharp. I am pretty sure every modern DSLR lens is capable of f8.
4) Other factors are also important. Most photographers want the low f number lenses because it gives them 'more light'. That means they can hold the camera for taking photos, take photos with a faster shutter speed and get the shots they want when it gets darker. However, in my experience when I find that the light is fading eough for me to want to use a lower f number, it is already too late. I always want to change what I am doing - use a tripod, flashes or crank up the ISO a little. If you start using lower f numbers, you start reducing the depth of field and have potential to get more out of focus shots anyway.
5) You can forget about low f numbers on a sunny day. Remember there is a general f16 rule for sunny days. On a sunny summer day, you're not going to be able to use a low f stop number unless you are in some serious shade, like underground. If it's sunny, leave your prime 1.4 in your bag!