Best Lens for a Canon EOS 40d/50d for a Beginner

I myself own the 50d  and the best standard lens for this camera is the canon EF-S 17-55 F/2.8 USM IS. It's not cheap and it's not light, but if you're serious about this hobby that's the zoom to get. Tamron and sigma has more affordable versions of this lens, but they can't match it's performance.

If you don't mind the distortions and fringing, the Canon EF-S 15-85mm f/3.5-5.6 USM IS is also a good choice, but it's not as fast as the 17-55 (wich is more expensive and heavier).

I used to own the kit lens (18-55) when I had the 350d, and it was fine for family shootouts, but trust me, you'll soon crave for a better lens as soon as you start to shoot with the 50d.

The 50mm 1.8 is also great for a beginner, because it's sharp,  fast  and very cheap. 

I'd stay away from long zooms, unless you rather have commodity instead of maximal quality.

My final advice: try different lenses for yourself and start off with the best  you can get on the 2nd hand market. If you buy wisely you can sell it again for about the same amount you bought.

First off, great idea to pick up a 40D or 50D.  These are both great cameras and you'll be able to get amazing deals on them both (and their accessories like batteries, battery grip, etc).

I'd suggest you start with the basic kit lens (18-55mm f/3.5-5.6) or if you want a little more zoom, look at something like an 18-135mm. These are both very affordable and you'll be able to find them second hand.

Also, if you don't mind switching lenses then I recommend the 55-250mm f/4-5.6 for telephoto shots.

If you want to have pictures where the background is out of focus then you'll need something like the 50mm f/1.8

Step up to the 60D which in my opinion beats the other two because its newer and offers a bit more for not that much more money

May I suggest something different? Both cameras are discontinued, so unless you are looking for a second hand body either will be a good choice, but you can find good deals on new T3i, T3, or even the new T4i.

As for the lens, go for the EF-S 18-135mm. It will give you more coverage than the 18-55  (although many people like having a 50mm around, and the 50mm f2.8 is quite cheap). Give yourself some time to explore the lens, see what it can do for you and just have fun with it. Later on choosing another lens will be easier according to what you like to do, like macro, or portrait, or landscapes, etc. I strongly sugest you give yourself time and when you decide to get a second lens go for the best you can possible get. I know good lens are not exactly cheap, but keep inmind they will be with you for a long time and you will be using them when the time comes to upgrade the camera.

As example, my old XT came with the original EF-S 18-55, not the best lens to be honest, but it was a great tool to start with (there was no EF-S18-135 at that time) My second lens was a 70-200mm f4L because I like shooting from some distance, and then as I got interested with macro (and doing some crazy experiments) I got an extension kit (the Kenko one, is great) and I used them together. When the time came to upgrade my body I got the 60D and when I realized I love macro I decided to go full speed and got me a 100mm f2.8L IS.

While I agree, if you want the best "all around" lens for your camera, in the right price range, the best option walking into a store, and picking up a NEW 55-250 is the best choice.

The honest to goodness best lens that I have used for the price, is simply the 50mm 1.8ƒ.  due to it having less glass than most of the other lenses, it is much sharper than almost every lens you will ever use.  It is likely the least expensive lens option you can find brand new, and worth every penny.

Having this lens, takes up nearly no room in your kit bag, and because of the 1.8ƒ, responds better than any zoom lens in low light.

If you don't mind the "BUYING A USED LENS" bit, and are fairly certain you are not getting a damaged lens, you can find on ebay or amazon, a 70-210mm push/pull lens.  These lenses are fantastic, and generally sharper than the USM focus lenses.  and in most situations, will be paired with an EOS film camera for under $100.  I have had the luck of owning an unused one of these, and have since sold it upon buying a 70-200 L lens, and to be honest, it performs almost identically, aside from the fact that the L lens is internally operated for zoom and focus, eliminating the need to re-zoom, after letting the camera hang around my neck.

If you are interested in dropping $450-750 on a single lens, the 70-200 ƒ4 L lens (NON-IS) is easily the best lens you can purchase.

50mm lens is a very powerful piece of glass with any 18-55 or 18-135 mm standard zoom lens

I agree with the 18-55mm standard lens as it is a great starter lens at a very affordable price. Beyond that, you need to simply define your needs. Wildlife photography would dictate a good entry level lens like the 70-300mm telephoto while close up pictures like bees on a flower would be a 100mm macro. The 50mm prime lens is also a great affordable lens that has very good optics for the price.