Notes:
Use the threshold adjuster to create a mask seperating background and objects.
You choose the min and max values.
All values below min and above max will be set to 0. All values between min and max will be set to 255. Leaving you with the mask, that is an image that contains only two intensities.
You can use a magic wand tool to create a selection from a mask. You can then transfer the selection to the original image.