What is wrong with this lens

There is a Canon 75-300 f/4-5.6 III EF Lens for 199.99...why is it so cheap and why when they add USM to the title change the price to 279.99? I am obviously extremely new to all of this. Help, Help, Help! I just need an additional lens to the kit lens 18-55 IS that came with my XSI. I take pictures of the kids (4 of them 10- 21months) at sports, the house, acting, school programs, etc. HELP!

Short answer:
Get the EF-S 55-250 IS.

Agree. I have had both. 55-250 IS complements the 18-55 IS without a gap.

Agreed, the 55-250 IS is a better choice, I've heard not so good things about the 75-300.

USM stands for "ultrasonic motor" which means it focuses faster and is more accurate. Generally adds a good chunk to the price.

55-250 is my first choice for a budget telephoto. It has image stabilization and it is pretty sharp.

Second options would be Tamron 55-200 Di (if you are really broke) or Sigma 70-300 APO (if you want an extra bit of reach and/or 1:2 macro focusing).

Check the reviews on that lens, it doesn't get very high marks. The reason it is so cheap, is that you get what you pay for.

I agree with getting the 55-250 but if you plan to use it for indoor you will an external flash but I think that will be too much for you as your still learning the camera. The 55-250mm is cheap for what it offers (IQ/IS/light weight) and will pair nicely with your kit lens.

You won't be sorry if you get the 55-250mm. For outdoor sports it is a great lens (for the money). The focus is very good. The IS makes a difference.

It is a fun lens. As someone else said, if you are going to try to use it in low light, you will be disappointed, but the IS will streach it to lower light than a lens without IS. IMO, it is a daylight, outdoor lens for the most part. I am, however, able to use it in very well lighted indoor situations if the subjects are not moving.

mishymama wrote in post #7471111
There is a Canon 75-300 f/4-5.6 III EF Lens for 199.99...why is it so cheap and why when they add USM to the title change the price to 279.99? I am obviously extremely new to all of this. Help, Help, Help! I just need an additional lens to the kit lens 18-55 IS that came with my XSI. I take pictures of the kids (4 of them 10- 21months) at sports, the house, acting, school programs, etc. HELP!

Its one of Canon's cheapest built and cheapest quality lenses. The USM version isn't a huge improvement in image quality, but the AF motor is a lot faster. The EF-S 55-250IS is the new cheapo king lens, it offers great value with IS, but the image quality is what you would expect for such a cheap lens.

nureality wrote in post #7471959
The EF-S 55-250IS is the new cheapo king lens, it offers great value with IS, but the image quality is what you would expect for such a cheap lens.

That is not my reading of the situation at all. From what I've read, most people are very pleasantly surprised by the excellent image quality this lens offers for its price.