When I turn my camera on a message says lens error 16 and shuts off, if i want to take a picture. If I put the camera in play mode to review pictures its says lens error 21 but lets me look at the pictures.

See http://www.fixya.com/support/r13011856-lens_error

Take a look at http://www.fixya.com/support/r13011856-lens_error

This is probably THE single most common failure among digital cameras. There's a halfway chance of fixing it yourself, described here: http://camerarepair.blogspot.com/2007/12/fixing-lens-error-on-digital-camera.html

A technician would need to replace the main board.

I had the exact same problem. Since my camera is out of warranty, I decided to try to fix it myself; figuring the cost of repairing it would be close to the cost of a new, better camera. If yours is still under warranty, let Canon fix it for the cost of shipping.

I unscrewed all of the screws and carefully pried open the back of the camera. Part of it will come apart easily, but for the part that wouldn't, I slid a razorblade into the crack between the pieces and used it to separate the pieces.

I was shocked by what I found. The "mechanism" for switching between picture and review mode, appears to consist of a tiny black piece of something (doesn't look like metal, but I assume it's conductive). The physical switch that you operate on the back of the camera, appears to push this away from the circuit board. When that happens, the camera goes into review mode.

I assume that over time, this flimsy piece of whatever begins to wear out and eventually won't return to the circuit board. I slipped a small screwdriver under this piece and bent it back towards the circuit board.

That did the trick, The camera now functions as it's supposed to with one exception. Now, when I go into review mode, the lens won't retract (although it will when I power the camera down). Personally, I'm fine with that, because the lens going in and out is a huge waste of battery.

One thing I learned from this is that when you leave your camera inactive for long periods of time, make sure it's in picture mode, as this should reduce the strain on the flexible "switch".

This is probably THE most common failure among digital cameras. There's a halfway chance of fixing it yourself, described here: http://camerarepair.blogspot.com/2007/12/fixing-lens-error-on-digital-camera.html

Lens assembly is defective in your camera. If you know about camera, open it and inspect.

Low battery maybe? Try holding camera firmly in one hand, with camera on and in record mode, and strike in on its edge firmly on the heal of your other hand. This technique is often sucessful with temporary camer 'quirks' like freezing, lens not fully retracting, etc.

Have you fully charged the new batteries

This sounds like what started happening with my camera a few weeks ago. I found that turning the LCD screen off during picture-taking allows me to take pictures. I then have to go into review mode to look at the pictures I took using the LCD screen. Definitely not a great fix. I'm trying to locate a firmware upgrade that may solve it.