I used to be able to see both aperture f-number and shutter speed, and change both values on Av mode on my Canon 550 DSLR. Now, I can only see the aperture f-number and not the shutter speed on the screen — vice versa for Tv mode. How to change this back to normal?
To see the shutter speed in Av mode, compose the shot and press the shutter button halfway. The shutter speed will show up in the viewfinder alongside the aperture you have set, in the same spot both numbers always display. Because you've pressed the shutter only halfway, the camera will not take the picture.
Keep in mind the camera is determining the appropriate shutter speed based not only on the aperture you have locked in, but also on variables such as the ISO you have set (or any auto-ISO settings you have configured), the available light in the scene, and the light-metering mode (evaluative, center-weighted average, etc.) that you have chosen.
The camera takes these settings into account and attempts to choose a shutter speed that produces an even exposure. Watch the -3...+3 bar in the viewfinder. In an even exposure, there will be a vertical line underneath the midway point of this line. A vertical line to the left of this point indicates the scene will be underexposed, while a vertical like to the right indicates the scene will be overexposed. (You may be okay with under- or over-exposures based on what it is you are shooting.)
Follow the 'press-the-shutter halfway' tip to see the aperture while in Tv mode. Everything above is still true except replace the word 'aperture' with 'shutter speed' :-)
To see both shutter speed and aperture without pressing the shutter, switch to the P or M mode. However, in these modes, the camera doesn't set either number automatically. You change them yourself based on the kind of picture you want to take.
This is normal, Av or 'aperture priority' mode is a semi automatic mode allowing selection of aperture value with shutter speed being calculated by the camera, Similarly Tv or 'shutter priority' mode allows shutter speed to be selected while automatically calculating aperture value. To enable both of these setting to be user selectable at the same time you need to be in full manual 'M' mode.
I have a Nikon so maybe this does not apply to your Canon camera. In A and S mode things work as described by others.
On my Nikon D7000 (and others before it) in P mode I can use one of the control dials to change the combination of aperture and shutter speed. This is a very easy way to change the aperture or the shutter speed to what you want without having to change to mode dial. The mode dial is on the top of the camera and the control dials are under my thumb or under my index finger.
This might apply to Canon cameras as well and it can be what you are looking for.