Hi, my name is ***** ***** it would my my pleasure to help with your issue today.
In shutter priority mode you should have full control over the shutter speed of the camera, but no control over the aperture setting. I'm just trying to make sure that I fully understand your question. Are you saying that your shutter speed is changing on it's own based on lighting conditions in shutter priority mode?
Yes this does appear to be happening in low light even if we increase iso ,strange is it not ?
I was just about to ask you if the ISO was set to auto. There should be no way for the shutter speed to change in the TV mode. The ISO if set to auto may change and affect the outcome of the shot if you were trying to take pictures in low light conditions with the shutter speed set too fast. If the shutter speed is set to a setting that can not be achieved it will blink when you press the shutter to focus.
Do you have any other issues in any other mode of operation?
If not, then try resetting your camera settings to default and see if that helps. You should be able to find that option in one of the yellow setup menus near the bottom. I don't have a 1ds here to tell you exactly where the option is, but it's usually in the last setup menu.
Hope this helps. Let me know if there's anything else that I can do to assist you. If you are pressing your shutter to autofocus in the TV mode and the shutter speed represented normally by a fraction (ie 1/200) is changing after the reset, then there's an issue with the firmware in your camera that will need to be addressed.
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Thanks having reset the camera it seems the problem occurs using back button focus,when using shutter button to achieve auto focus the problem disappears. Cheers Den