I have an old D40x. I have it on a tripod and want to take some still images using aperture priority with the aperture as small as possible. Sometimes it works, and sometimes I get a flashing question mark and "ISO-A", and when I view the error message, it says a flash is recommended. But I don't want to use a flash, and I don't it won’t take the picture. How do I get the flashing to stop? What's weird is that sometime it take a picture and, without changing any settings, sometimes it doesn't. Thanks.