Sharper images

I'm using a canon 40d with a 18-200 sigma lens. I struggle to get sharp images will it make a difference if i use a canon lens on a canon camera.

The 18-200mm lens will not be as sharp as a Canon zoom or, even better, prime lens. This is because a wide zoom range needs more compromises to be made in its optical construction.

However, one of the main drawbacks of your lens is a fairly limited maximum aperture, especially at the long end. This means shutter speeds will be slower and in all but the brightest light camera shake may be a problem.

Try your camera and lens using a solid tripod and see if the results are sharp then. If they are, then camera shake may indeed be the problem. If not, then there may be a fault with your lens. All lenses should be at least acceptably sharp for smaller prints up to A4 and for viewing on a monitor.

Having said that, the Canon lenses will be better. I'd be inclined to use two zooms to cover the range, both of more modest zoom range. Perhaps something like an 18-55mm and a 50-200mm, depending on what lenses are available from Canon.

Hope that helps!

John