I need a prime lens for landscape photography, and i dont know whether i need a really low f-stop (f1.8 or f2), or if i can get away with a higher f-stop number (f3.5 or f4). Or on the other hand, should I just get a zoom lens ? but i heard they decrease quality and the f-stop number can be higher. thanks :)

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It all depends on how much light you have to work with. 3.5 is fine outdoors in sunlight.

To be honest, you need to get a better understanding of what the aperture setting really does before buying a lens you may or may not need. The short version is that it controls how much light is allowed in during any given exposure. But we can do this via a time value as well. So the long version is that, aperture also causes a change in depth of field. The wider the aperture (smaller f-number) the more shalow the depth of field or in simple terms, less of the area in front of or behind your subject will be in focus. Like wise, a narrow aperture (bigger f-number) provides a greater depth of field and so more of background behind your subject and the foreground in front of your subject will be in focus. When shooting landscapes, more often than not, you'll want greater depth of field but the apparent depth of field increases as the focal length (Z-mm) of the lens gets shorter. This means shooting wider lenses like a 24mm or wider (smaller mm number) lens with smaller aperture or larger f/number settings like f/16 or f/22. This means a slower lens (one with an f-stop number of f/4 or f/5.6) is not a hinderance to shooting landscapes in most cases. It is also important to note that image quality is not necessarily directly related to the maximum aperture of a lens. For example, look at Canon's 70-200mm f/4L zoom versus their 70-200mm f/2.8L zoom. With both lenses set to the same aperture, you'll be hard pressed to see any difference in image quality. In many cases, all a fast lens (small aperture number) like an f/1.4 prime or f/2.8 zoom, gets you is slightly greater flexibility to shoot in low-light or the ability to create a dramatic seperation of your subject from its background. So putting it all together, you need to figure out what you want to do in shooting landscapes because both wide lenses and telephoto lenses are commonly used. If you are shooting a consumer grade DSLR and you want wide spaces to look like wide open spaces, you want a shorter lens. With the primes, that means 20mm and shorter focal lengths. Be warned that these lenses can get quite expensive. And among zoom lenses, it will probably mean a 10-24mm or similar zoom lens. If on the other hand you want to shoot sunsets for example and you want the sun to appear large in the scene, you will find yourself looking at telephoto and super-telephoto lenses...Again, these can get quite expensive. But, you'll probably want to start with lenses that offer focal lengths of 200mm or greater.

For landscape photography, you want the highest depth of field you can get away with (higher depth of field means more things in focus, from background to foreground), which means you will want the highest f-stop number possible. Depending on the lighting conditions, you may need a tripod, since a higher f-stop setting means less light coming into the lens). Some people nowadays claim that modern zoom lenses give image quality as good or nearly as good as prime lenses. I can't say for sure (I don't have a modern prime to compare), but I can honestly say that in the past zoom lenses were very inferior to prime lenses. I'm guessing they still are, at least for film photography.

The price of the lens will tell you also the length. If you want to do macro close-ups, too, you will find the lenses tend to be slower.

Almost all the landscape photography I do is with a 21mm lens at f16 and "focused" at 6'-0'' to maximize Depth of Field (DOF) - from 2'-2'' to infinity. I also use a tripod 100% of the time when shooting landscapes. (My lens has a distance scale so its easy for me to set my focus at 6'-0'') You can learn about DOF at http://www.dofmaster.com/dofjs.html You want a low f-stop (f1.4, f1.8) for available light photography.