diff --git a/src/maths/geometry.lisp b/src/maths/geometry.lisp
index 4681940..b9ed6cb 100644
--- a/src/maths/geometry.lisp
+++ b/src/maths/geometry.lisp
@@ -54,14 +54,18 @@
          mat)))))
 
 (defun calc-frustum-scale (fovy)
-  (/ 1.0 (tan (/ (maths:deg-to-rad fovy) 2.0))))
+  (tan (/ (maths:deg-to-rad fovy) 2.0)))
 
 (defun make-persp-matrix (fovy aspect near far)
-  (let ((frustum-scale (calc-frustum-scale fovy)))
-    (f44:mat frustum-scale                      0.0                              0.0  0.0
-                       0.0 (/ frustum-scale aspect)                              0.0  0.0
-                       0.0                      0.0    (/ (+ far near) (- near far)) -1.0
-                       0.0                      0.0 (/ (* 2.0 far near) (- near far)) 0.0)))
+  (let ((range (calc-frustum-scale fovy)))
+    (let ((left (* (- range) aspect))
+          (right (* range aspect))
+          (bottom (- range))
+          (top range))
+      (f44:mat (/ (* near 2) (- right left)) 0.0 0.0 0.0
+               0.0 (/ (* near 2) (- top bottom)) 0.0 0.0
+               0.0 0.0 (/ (+ far near) (- near far)) -1.0
+               0.0 0.0 (/ (* 2.0 far near) (- near far)) 0.0))))
 
 (defun make-ortho-matrix (width height)
   (f44:mat (/ 2.0 width)             0.0 0.0 -1.0