(defvar sacha/text-zoom-factor 40
  "Points to add or subtract during zooms.")

(defun sacha/zoom-in ()
  (interactive)
  (set-face-attribute 'default nil :height
                      (+ (face-attribute 'default :height) sacha/text-zoom-factor)))

(defun sacha/zoom-out ()
  (interactive)
  (set-face-attribute 'default nil :height
                      (- (face-attribute 'default :height) sacha/text-zoom-factor)))

(global-set-key (kbd "<f9> z +") 'sacha/zoom-in)
(global-set-key (kbd "<f9> z -") 'sacha/zoom-out)