1. An imaging apparatus which comprises an image pickup device, the image pickup device comprising imaging pixels including light receiving portions and focus detection pixels in which opening positions of light receiving portions are different from opening positions of the imaging pixels, the imaging apparatus comprising: a reading unit which reads pixel signals from the imaging pixels and the focus detection pixels; a display unit which displays an image based on the pixel signals read from the imaging pixels; and a control unit which controls to perform the display of the image on the display unit and a read operation of the pixel signals of the focus detection pixels within a time corresponding to a display frame rate of the display unit, wherein when both exposure of the imaging pixels and reading of the pixel signals of the imaging pixels and exposure of the focus detection pixels and reading of the pixel signals of the focus detection pixels are achievable within the time corresponding to the display frame rate of the display unit, the control unit controls to alternate the exposure of the imaging pixels and the reading of the pixel signals of the imaging pixels and the exposure of the focus detection pixels and the reading of the pixel signals of the focus detection pixels, and when both the exposure of the imaging pixels and the reading of the pixel signals of the imaging pixels and the exposure of the focus detection pixels and the reading of the pixel signals of the focus detection pixels are not achievable within the time corresponding to the display frame rate of the display unit, the control unit controls to perform the exposure of the imaging pixels and the focus detection pixels at the same time and read the pixel signals of the imaging pixels and the pixel signals of the focus detection pixels after the end of the exposure of the imaging pixels and the focus detection pixels.