1) Fast Prime: Nikon 35 f/1.8 or Nikon 50 f/1.8 for Nikon, and Canon 35 f/2 or Canon 50 f/1.8 for Canon
These are fixed focal length lenses, otherwise known as a prime lenses that do not zoom. These lenses provide a much larger aperture compared to the kit zoom lens and for that matter any other zoom lens available.

Lets talk about the name of these lenses, for instance the 35 f/1.8. The first "35" number indicates the focal length, which is the angle of view that you see when looking through the camera's view finder. All entry level (below $2,500) DSLR have crop sensors to save cost on sensor fabrication and on the glass used for dedicated crop sensor lenses. Understanding crop sensor focal length is a bit more complicated than on full frame cameras because the smaller sensor essentially only "sees" the center portion of the total image that the lens projects back into the sensor. Nikon cameras are 1.5x crop and Canon's are 1.6x crop, meaning that the 35mm lens projects 35mm but your camera only sees 52.5mm (35mm x 1.5 = 52.5mm for Nikon and 35x1.6=56mm for Canon). So whenever you put a lens on your camera you need to multiple it by 1.5 for Nikon and 1.6 for Canon to understand the focal length that it will produce.

The "normal" focal length is 50mm, which is approximately what the human eye sees without using peripheral vision. Any focal length number smaller than 50mm is considered wide angle, while any focal length number larger than 50mm is considered telephoto. This means that the Nikon 35 f/1.8 would provide a "normal" view of 52.5mm (Nikon) or 56mm (Canon) on your crop sensor camera. So this is a very good lens to get because you do not have to "train" your eye to see wider or tighter shots than normal.

Now lets discuss the second part of the 35 f/1.8 name, the "f/1.8" portion. The number after the "f/" signifies the lens's maximum aperture opening size. The SMALLER the number after the "f/" the LARGER the aperture opening. Do you get that? It is an inverse relationship! The larger the aperture opening the more light the lens lets into the camera and the smaller the aperture number. The more light you let into the camera the faster the shutter speed the camera will produce for any given lighting condition. Can you imagine how this would be helpful in low-light conditions, such as shooting indoors or at night?

Since these large aperture lenses produce faster shutter speeds in low-light conditions they are called "fast" lenses. The main benefit of faster shutter speeds in low-light conditions is that it prevents the normal shaking of your camera and thus causing the subject in your image to look blurry. Faster shutter speeds also prevent the subject from blurring as a result of their motion.

In addition to speeding up the camera's shutter speed, large apertures also provide shallower depth-of-field, which is the distance between in-focus and out-of-focus portions of the image. The larger the aperture the smaller the depth-of-field and the more the in-focus subject pops out from the out-of-focus background. This is a highly desirable look for portraits.

So why would you purchase a 35 f/1.8 or any of the fast prime lenses listed above? For all the reasons we just reviewed: 1) it captures images in low-light conditions without camera shake or subject motion blur, and 2) when taking portraits it renders a beautifully blurred background to highlight your sharp and in-focus subject.

A quick note about Nikon's affordable 50 f/1.8 fast prime. Like Canon's 50 f/1.8 this lens only costs around $100 but unlike Canon's version, Nikon's version cannot be used on their entry level and affordable DSLRs. You need a D80, D90, D7000, D200, D300 or D300s to use this lens, and unfortunately none of those camera bodies are affordable. Nikon cut their costs by leaving out an in-body AF motor, which means you have to get the more expensive 35 f/1.8 or the much, much more expensive 50 f/1.4. It is unfortunate that Nikon did this but luckily the 35 f/1.8 is a great lens, even at twice the price.

2) General Purpose Fast Zoom: Tamron 17-50 f/2.8 or Sigma 17-50 f/2.8 OS for Nikon or Canon, and Canon 17-55 f/2.8 IS only for Canon only
So now that you understand how to interpret lens names, what can you tell me about these lenses? First that the focal length is not fixed but has a range from 17mm to 50/55mm, and to understand what this range will look like on your camera you need to multiple by 1.5 or 1.6 (17mm x 1.5 = 25.5mm and 50mm x 1.5 = 75mm), so for example the Tamron or Sigma lenses "see" from 25.5mm to 75mm on a Nikon body. That is a very useful range in that you see quite a bit wider than the normal 50mm and quite a bit tighter than the normal 50mm. This zoom range is similar to the kit lens, so you should have a good idea of the what that zoom range "looks" like.

The big difference between these fast zoom lenses and the kit lens is signified by the "f/2.8" part of the lens name. This means that the lens has a maximum aperture of f/2.8. Now this is a smaller or slower aperture as compared to the f/1.8 primes as discussed above but it is still considerably larger and faster than the kit zoom lens. With this lens you get the best of both worlds, the versatility of a zoom with the faster aperture for less camera shake and better portraits. You shouldn't write off the Fast Primes option because they perform even better in low-light and offer even more subject pop from shallow depth-of-field.

With Canon's 17-55 f/2.8 IS you get the best general purpose zoom currently made for either a crop sensor camera or a full frame camera! It is insanely sharp throughout the zoom and aperture range, plus has image stabilization, a feature I will discuss in the next section. This lens isn't cheap but it offers uncompromising quality.

3) General Purpose Daylight Zoom: Nikon 16-85 f/3.5-5.6 VR for Nikon, and Canon 15-85 f/3.5-5.6 IS for Canon
Okay here's last lens upgrade type. Now what can you tell me about these lenses, why would you want this lens over the others? The main reason is that these lenses offer an extended zoom range. This allows you to get "closer" to your subject while making the "view" much tighter. Now what's the obvious trade-off of this increased zoom range? The "f/f3.5-5.6" is a much larger number, which means you have less light available for faster shutter speeds that reduces camera shake and subject motion blur. We will talk about how Nikon's "VR" and Canon's "IS" eliminates the camera shake problem in a bit. Back to the aperture f/3.5-5.6 discussion. The other thing you will notice is that the aperture designation f/3.5-5.6 isn't just a single number but a range, similar to the zoom range. This means that the maximum possible aperture changes or reduces as you zoom. So at the long 85mm end of the zoom range the maximum aperture is only f/5.6, which is a small opening and resulting in slow shutter speeds.

Now lets discuss what the "VR" and "IS" designation means. VR stands for "Vibration Reduction" and IS stands for "Image Stabilization", a very useful feature that eliminates camera shake that results form slow shutter speeds. The VR/IS feature does nothing to help with subject motion blur that may occur from slower shutter speeds. One other fact that I would like to introduce, is that the longer the focal length the more camera shake will effect the photo. Think about this, it makes perfect sense. If you magnify an image in a microscope very small movements of the slide or slide contents result in huge movements in your magnified view. Right? Same principle here. The rule-of-thumb is that you need a shutter speed at least equal to the focal length, but double is preferable. Of course you need to use the crop sensor focal length, such as 85mm x 1.5 = 127.5mm, so at this focal length you need at least 1/127 second to be free of camera shake, that is without VR/IS. With VR/IS you can cut it in half or more! This means that VR/IS compensate for the smaller lens apertures when shooting still subjects, such as landscapes.

So why would you get this slower f/3.5-5.6 lens in the first place? The intended purpose of this lens is for outdoor and daytime photography. This is the best hiking and travel lens, especially when couple with any of the Fast Primes list above in option 1 for low-light and evening photography.

Final Recommendations
There's nothing wrong with having all three lenses and even more for that matter! However, budgets are limited so you probably only want to purchase and use one or two lenses:

Option 1: Keep your current kit lens for general outdoor/daylight use and purchase the a Fast Prime from Option 1 above for low-light and portrait use. This is the lowest cost option and allows you to upgrade your kit lens in the future. Keep in mind that the 18-55 kit lens performs fairly well at f/6.3 to f/8, especially Canon's latest version that optically out performs more expensive EF-S lenses in some criteria.

Option 2: You could replace your current kit lens with a General Purpose Fast Zoom from option 2 above for improvements in both general outdoor/daylight use and low-light/portrait use. Go with this option if you don't want to switch lenses and you don't mind having less than the ideal set up for each of the two types of photography discussed in this posting.

Option 3: You could replace your current kit lens with a General Purpose Daylight Zoom from option 3 above for improvements in general outdoor/daylight use and purchase a Fast Prime from Option 1 above for low-light and portrait use. This option will cost the most but provide you with the best performance for the two types of