Originally posted by zburatoru Hello! I've observed an intriguing fact about my 31mm limited: at f1.8 and f2 it gives the same exposure value. So, if at 1.8 it's 1/50 it will be the same at f2! Even if they have the same shutter speed, the image it's different, more contrast at f2 and not so rounded bokeh... This is good especially when you shoot in low light... Your copy of this lens behaves the same?
Are you using one of the modes that automatically alters ISO setting, perhaps - such as Auto (Green) or TAv?
If the diaphragm is moving when you stop down from f/1.8 to f/2 - and the fact that you're seeing better contrast but less-rounded out-of-focus highlights suggest that it is - then it will be letting in less light at f/2. As such, one of three things
must change along with the aperture... ISO setting, shutter speed or the amount of light on the subject.
Check your ISO, aperture and shutter speed in the EXIF data of your files. If ISO and shutter speed are the same at aperture settings of f/1.8 and f/2, the light level changed.