I have a Canon 650D with 50mm budget lens. When I take photos they end up looking different than when I preview them on the LCD display. Usually they are darker and there is less bokeh.
Is this normal? Because I see little difference between the LCD and final photo with the kit lens. Are there any settings I need to adjust on the 650D for the new 50mm lens?
If I am understanding you correctly, it sounds like you are taking a photo with the aperture set to less than the smallest f/ number possible for the lens. When you use a smaller aperture (larger f/number) then the image is darker and the depth of field is bigger (resulting in a sharper background).
When you look through the viewfinder, the aperture is kept wide open so that you can see the most light. It only stops down when you actually take the photo or when you use a depth of field preview button if your camera has it; on your 650D, this is the small circular button just beneath the lens unlock button.
The image may also be darker if you are using too fast of a shutter speed.
There are two main reasons why the viewfinder image may look different from the result:
You are using a small aperture when taking the photo, but when you look through the viewfinder or on the LCD display the aperture is always wide open to let in as much light as possible.
You are using a large aperture when takting the photo, but the viewfinder limits the light so that you see the result that you would get with a smaller aperture.
For the first case, there is a button next to the lens, that lets you stop down to the actual aperture when looking through the viewfinder. That will show you the correct DOF and focus.
For the second case, there isn't anything to do. The camera just isn't able to show a preview of how the image will look at large apertures. You can read more about it in this question.