I need another lens.  Can anyone recommend a zoom lense for a canon 20D that focuses really sharp?  I'm willing to spend, but I do not take pictures in low light, or sports shots, so it doesn't have to be a fast lens.

on wide angle I would recommend 17-40 f/4.0 L (my copy is sharp from corner to corner) and telephoto 70-200 f4.0 L nice lens if you don't need 2.8

Check out fredmiranda.com buy/sell forum - I have seen a poster selling mint copies of these two lens for $520 for 17-40 and $475 for 70-200.

So you need wide angle lens and telephoto lens - what are you planning to shoot with it, as those have different applications.
It's not the wedding photographer and lanscape photog. will use same lens, altough they both use wide angle.

Tomas

I do child portraiture.  My previous zoom was perfect, just not sharp enough.  I don't think I have time to keep switching lenses.  What else is out there?

You just narrowed it down to about 60 lenses that will fit into this description.
Canon or other brand?
Budget up to? as we can easily go to thousands.
Small kids? seniors would still fit into child category. Heck, even I will. lol
Newborns?
Studio? How big? Lights used?
On location? At the house or outdoors?
and so on, more info you provide, more likely you'll get recommended the right lens.
Tomas

LOL, I keep doing this.   (Lack of sleep.)  Okay, my budget could go up to $1,000, but if there's a really really good one...well, I'll step over my budget by another few hundred.  Again, I don't need a fast lens.

I basically do newborns to 13 year olds.  I mainly do 3 year olds.

I have a large basement, I have two softbox strobes.  I only shoot in my studio, but will be doing outdoors in the summer.  Um, what did I leave out?

Basicaly it rules out 17-40  too short and gap to 70-200 is to big.
Unless you have PJ style, I'd personaly skip 70-200.
I think you should be able to do everything with one lens, since you don't want to switch them - however this would defeat what you paid for - abilito to use other lens.
Anyway, 24-70 f/2.8 L is probably your best bet. I myself would pair it with something faster anyway. Think that one plus 85mm f/1.8 (as the 1.2 version costs arm and leg), or maybe 135 f/2 , but I'm not sure about the cost, probably skyhigh to I'm afraid.

Anyway, fast lens is NOT only for low light shooting!
People are shooting over 1/2000 sec. to be able to shoot open wide in broad daylight, to blur the background. Indeed, it will be blurred anyway, but if you look at outdoor shot at 1.8, it makes persons almost 3d ish, whereas f4 already has dof large enough to eliminate this.

Tomas

You sure don't want background in focus. Nobody does. And there are very few cameras having flash sync higher that 250.
I was talking about outdoor shots. DOF is most powerfull gun one can have outside.

You just use http://www.dofmaster.com/dofjs.html to get idea about dof, distance for shooting and background, and adjust your lights output.

Tomas

Raizy:

Another reason I don't want a fast lense:  My sync speed is 250, and I shoot at 125, because indoors I want everything in focus. 

Raizy, you really need to do a little reading.  I say that kindly.  I highly recommend the following book for beginners:

A fast lens will not keep you from shooting at 1/250 or 1/125.  You can set an f/2.8 lens from f/2.8 to f/22 (or f/32) in some cases.  And shutter speed has nothing to do with depth of field.

Tomas has probably recommended the best lens for your situation.  If you truly do not think you will ever need f/2.8 then the 24-105 f/4 L IS might be a good solution as well, giving you a little more reach, IS but with a slower lens.  But there is so much you do not understand that is leading you to make decisions based on faulty information.

For instance, a lens that can go to f/2.8 will be sharper at f/4 (stopped down) than a lens that can only go to f/4.  You are at the outer limits of the f/4 lens, while you are well within the abilities of the f/2.8 lens.  I also find it interesting that you never want to defocus the background.  Most studio photographers have f/1.4-f/1.8 primes in their kit for that very reason.  I'm not saying you should have every picture with the background defocused, but to think you will never need or want to do that seems like another decision based on lack of knowledge. 

I apologize upfront for any offense you take at this post.  It really isn't intended to be offensive, but you really should invest $50 in books before investing $1,500 in lenses and I get the feeling that you might be rushing to conclusions without fully knowing the equipment. 

Tom, I appreciate your post, and by no means am I offended.  I wish I knew 1/10 as much as you know about cameras.  I was pushed into starting in the business by my family spreading that I was ready.  I took a course, but I knew nothing.  One thing I will defend myself though, is this.  I know I can use a fast lens with a smaller aperture, I just don't feel it's required for studio work.  But if a fast lens is sharper than a slower lens, then I definitely want one.  I will buy the book as well, so thanks for recommending it. 

But here's what I meant about the DOF.  Please tell me where I'm going wrong.  I cannot set my strobes any brighter or I find there are hot spots on my subject's faces.  (My softboxes are not that big.)  So if I set my aperture at f/2.8, or f/4, I need to set my shutter speed much faster than 250 to get correct exposure, and my strobes don't fire if I do.  This is where I connected the shutter speed to my depth of field.    

Raizy:

Tom, I appreciate your post, and by no means am I offended.  I wish I knew 1/10 as much as you know about cameras.  I was pushed into starting in the business by my family spreading that I was ready.  I took a course, but I knew nothing.  One thing I will defend myself though, is this.  I know I can use a fast lens with a smaller aperture, I just don't feel it's required for studio work.  But if a fast lens is sharper than a slower lens, then I definitely want one.  I will buy the book as well, so thanks for recommending it.

I appreciate your attitude, I promise you will be a better photographer for it and you will become better able to make the camera take the pictures that you see ahead of time.  And that is the goal. 

But here's what I meant about the DOF.  Please tell me where I'm going wrong.  I cannot set my strobes any brighter or I find there are hot spots on my subject's faces.  (My softboxes are not that big.)  So if I set my aperture at f/2.8, or f/4, I need to set my shutter speed much faster than 250 to get correct exposure, and my strobes don't fire if I do.  This is where I connected the shutter speed to my depth of field. 

Ok, I'll buy that train of thought.  I don't do studio photography so this is an area that I am not as strong as others here.  Hopefully some of them will jump in and add to what I think that I know.  What I THINK is that your strobes should be adjustable so that you can still shoot at those apertures without overexposing.

Strobes do not have to shoot full power (or at least a good one shouldn't, I know my 580EX flash adjusts power based on a huge variety of things) and that would be the secret to using a fast lens wide open and still being able to fire your strobes and keep the shutter speed below the maximum shutter sync speed.

Again, this is an area that you can learn a lot about from reading and don't forget google is always your friend.  Good luck and hopefully someone will chime in with more specific directions for you on the strobes. 

Here's a link that answers a lot of your questions...

Controlling the Exposure 

I suspect you are shooting in some sort of automatic mode, and that may not be possible in the studio if you want to use your strobes at lower settings.  I didn't read the article to the end, but it appeared to address some of the things you are dealing with. 

This book is suggested by the author of the blog I referenced above:

Master Lighting Guide for Portrait Photographers

I can't vouch for it, but it certainly sounds like a good place for a lot of folks here to start...

I'm wondering if a background light will solve some of my exposure problems.  I can definitely adjust the power of  my strobes, but although the exposure is correct, I really hate the hotspots.  

I read the article, but in a year full of experiments, I don't think he told me anything I didn't already know. 

Raizy, you don't use the wide open apertures with studio lighting.  Most are shot with a f/8 or f/11 apertures.  What Tom is saying is that fast lenses are sharper lenses, which is what you are asking for. 

You still keep your shutter speed at 1/125, and f/? whatever you use.  But, since you want to be able to shoot outdoors, having a fast lens (2.8) will be helpful, esp on cloudy days, or using it in the evening.  Tom is saying that with a maximum aperture of 2.8, even if you don't shoot at that, but do use an ap of f/4, your pic is going to be sharp, because all lenses have sweet spots, and it isn't at it's widest aperture.  So, the image is going to be sharper at f4, than if you are shooting with a lens with f/4 as it's max apert., and you'd bump it up to f/5.6 or 8 to find the sweet spot.  Does this make sense?