I have a couple manual lenses that I use occasionally. One is a 25mm f/1.8 Meike EF-M and the other is an old 35mm f/2.8 Hanimex M42 mount that I use with an adapter. With both lenses I've had issues with overexposure when taking outdoor photos in daylight with a low (open) aperture setting in P (Program AE) mode. While the camera will automatically drop the ISO to 100 in these cases, it doesn't seem to want to increase the shutter speed to the level necessary to get a proper exposure. These cases are of course challenging situations for such a bright lens when such an open aperture is used. However, I can usually get the results I want by going to full manual mode and selecting a high shutter speed. What I don't understand is why the camera won't do that itself in Program AE mode. Is there a high limit for automatically-selected shutter speeds, even when the camera can't control the aperture (which I'm sure it wants to do in these situations)?
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I get better results with fully manual lenses using M mode and ISO Auto. For astrophotography you would want to dial in your own ISO value.
I do not try to capture every shot at ISO 100, either. Between 200 and 400 is where I like to be on bright sunny days.
I tried both lenses in Av mode and it solved the problem. It ran the shutter speed to 1/1600 with ISO set to auto (where it chose 100) and 1/2500 with ISO set to 200. I guess the rule of thumb is that when using a lens where the camera cannot control the aperture, don't use a mode where the camera normally controls the aperture. I know about using M mode for astrophotography.