Why do you insist on setting the aperture dial to A? Is it a matter of choice or you just think it is difficult to guess the right aperture on your own?
There is a good reason. The camera was designed to be used this way. The "A" setting should generally be used for all shots without a flash. The reason is that the coupled metering system sets both the aperture and the shutter speed when in the "A" position. There are two shutter speeds: 1/40th second and 1/200th second. Setting an aperture other than "A" will restrict you to 1/40th second.
You can of course experiment with taking pictures at different apertures and use a separate light meter to establish the correct aperture. However, the nearest shutter speed you will find on most light meters is 1/30th second to the the recommendations given by the meter won't be exact.
One time when I use the manual settings without a flash is when the camera is prevented from taking pictures on the "A" setting because there is insufficient light. If you set the aperture to f2.8 (wide open) it allows you to take a shot and in some cases the results will be fine, particularly where the subject is lit well but the background and surroundings are dark. It's a bit hit and miss but at least you get a shot.
When using a flash, the aperture should be set to correspond to the setting given on the flashgun for the camera to subject distance you require.
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