I was shooting photographs of birds at my home using my Canon 7D and a Sigma 170-500 lens (non-OS- approx. 4 years old) lens and a Yongnuo remote shutter operating system. I used the Aperture Priority setting. I shot the first first four photos at f6.3, ISO 640 with the lens at 500 mm. The Canon 7D selected a shutter speed of 1/1000 of a second and those photos were properly exposed. However, I then closed the f-stop to f8 (to increase sharpness) and left everything else the same. The outdoor lighting remained unchanged. After that change, the camera now selected 1/2000 of a second as a shutter speed rather than a slower shutter speed grossly under exposing the 50 or so photographs that I shot thereafter by at least two f-stops. At all times, exposure compensation was set to 0. I thought there was a problem with the Sigma lens and sent it to Sigma who informed me that the lens is functioning correctly. I then used a Canon 100-400L on the Canon 7D and it worked properly with the same settings. I have not had this problem using my other Sigma Lenses (70-200 f2.8 and 18-50 f2.8) Also, last year, I used the Sigma 170-500 and did not have that problem. The tech.s at Sigma believe that it is a problem with the Canon 7D. Has anyone had this type of problem or know why the 7D is selecting the wrong shutter speed?