Buying a camera lens

Set the lens f/stop to the largest number such as f/22 (smallest diameter). Many cameras with automatic exposure must have the f/stop set to the largest number on the lens, or turn off auto focus do manual focus.

The manual for your Canon EOS Rebel 2000 can be found here:

http://downloads.canon.com/cpr/software/camera/EOS_Rebel_2000_manual.pdf

Check out page 24 for instructions on shooting in fully automatic mode.

Hope that helps,
Bax

Turn the mode dial to the green rectangle for automatic exposure. Set the autofocus switch on the lens to AF for automatic focus.

You did not say which camera body you are using, but you probably have three different auto-focus modes on you camera. You might be using the wrong one.

The modes are as follows:
AF-A Mode: Camera automatically selects single-servo autofocus when AF-A subject is stationary, continuous-servo autofocus when subject is moving. Shutter can only be released if camera is able to focus. AF-S Mode: For stationary subjects. Focus locks when shutter-release button AF-S is pressed halfway. Shutter can only be released when in-focus indicator is displayed.
AF-C Mode: For moving subjects. Camera focuses continuously while AF-C shutter-release button is pressed halfway. Photographs can be taken even when in-focus indicator is not displayed.

The description of the manual focus is available in part 2 of the manual, page 40. The manual is available as a PDF here: http://ca.konicaminolta.com/support/manuals/film-cameras/film_af_slr/index.html#si_series . It is the seventh camera in the SI series in the list.

To manually focus the lens, press the focus-mode button on the front of the camera (bottom left when you are behind the camera) until M. focus appears in the data panel. If you have an AF lens, turn the focusing ring until the subject appears sharp. If you have an xi-Series or AF Power Zoom lens, pull and turn the lens control ring.

The focus indicator will light when the subject is in focus. To return to autofocus mode, press the focus-mode button again. The camera will also return to the autofocus mode if you press the program reset button.

I hope this helps.

Cindy Wells
(in manual focus, you can take a picture even if the subject in the focus frame is not in focus. Often it means you need to take multiple pictures to get a good shot of the subject. The advantage is that you can get some interesting shots.)

There may be a few things preventing the camera from taking a picture. I don't know the lens you are using but by chance does it have an auto/manual switch that has been switched to manual? Also make sure the lens is locked in position. The camera won't take a picture unless it has focus in the auto focus mode, this is normal. What are you trying to focus on? The camera auto focus system will search and fail to auto focus on surfaces that have little or nor contrast. For example glass, chrome, a blank wall, water, blue sky. Other conditions that auto focus fails is being to close to your subject, heavy fog, a dim lit subject, available light and night photography. Once the little green confirmation light comes on the camera has focus and will operate as it should.

Minolta had two lens mounts. One for manual focus SLR film cameras and one for Auto Focus SLR Maxxum film and digital SLR cameras. Sony uses Minolta Maxxum lens mount.
If the Sigma 400 f5.6 lens was made for Minolta X-700 series, XG, series, XD series, SRT series, then it will not fit on your camera.

They will fit but will work only in manual mode

By manual zoom lens I am assuming you mean a film camera lens. If it is one of our OM 1 or 2 lenses they can be used on our Olympus digital SLR camera but you will need to purchase an MF-OM1 converter to be able to adapt those lenses to fit on our digital SLR camera. Then you will only be able to shoot using those lens in manual focus mode. Auto focus mode will not be available.

Is there a micro focus switch on the lens
On my lens if switch to Micro Mode the Auto focus is disabled, Also it is a stupid question but you are using a AF lens and not a xi series or manual focus lens.