1. A camera system, comprising: an imaging optical system having an aperture, and configured to focus the light from a subject; an imaging unit configured to convert the light from said imaging optical system into an electrical signal to obtain an image of the subject; an image display unit including a display region having a first display region and a second display region, the image display unit displaying on said display region the image of the subject obtained by said imaging unit; an aperture setting unit configured to adjust said aperture and set an aperture value of said imaging optical system; a depth-of-field preview mode setting unit in which information for setting the mode to a depth-of-field preview mode is inputted; and a memory configured to store an image, wherein said image display unit displays a first image on said first display region and a second image on said second display region at a time, the first image being the image of the subject obtained by said imaging unit at a first aperture value of the aperture value set by said aperture setting unit, the second image being the image of the subject obtained by said imaging unit at a second aperture value of the aperture set by said aperture setting unit, wherein while the depth-of-field preview mode is being set by the depth-of-field preview mode setting unit, the image display unit keeps displaying on the first display region the first image stored in the memory, and at the same time, the imaging unit obtains the second image and the image display unit displays, as a live view image, the second image on the said second display region, and wherein said image display unit includes said display region that further includes a third display region that is associated with said first display region, and a fourth display region that is associated with said second display region, said image display unit displays on said third display region said aperture value set for said first image while the first image is displayed on said first display region, and said image display unit displays on said fourth display region said aperture value set for said second image while the second image is displayed on said second display region.