diff --git a/demo.c b/demo.c
index 262efff..dc6b731 100644
--- a/demo.c
+++ b/demo.c
@@ -8,7 +8,7 @@
 #define W 800
 #define H 600
 #define FOCAL_LEN 1.0
-#define VIEWPORT_H 2.0
+#define FOV 120
 #define SAMPLES_PER_PIXEL 100
 
 #define NELEMS(arr) (sizeof(arr) / sizeof(arr[0]))
@@ -31,8 +31,8 @@ static pix_t pixbuf[W * H];
 int main()
 {
 	img_t img = { .pix = pixbuf };
-	camera_t camera = camera_init(
-	    camera_pos, FOCAL_LEN, VIEWPORT_H, W, H, SAMPLES_PER_PIXEL);
+	camera_t camera
+	    = camera_init(camera_pos, FOCAL_LEN, FOV, W, H, SAMPLES_PER_PIXEL);
 	camera_render(&camera, scene, NELEMS(scene), &img);
 
 	ff_write(STDOUT_FILENO, img);
diff --git a/include/camera.h b/include/camera.h
index 354f14c..a9fdbf5 100644
--- a/include/camera.h
+++ b/include/camera.h
@@ -15,7 +15,7 @@ typedef struct {
 } camera_t;
 
 camera_t camera_init(
-    vec3_t pos, double focal_len, double viewport_height, uint32_t img_width,
+    vec3_t pos, double focal_len, double fov, uint32_t img_width,
     uint32_t img_height, unsigned samples_per_pixel);
 
 void camera_render(
diff --git a/src/camera.c b/src/camera.c
index 2a41393..efa4f2b 100644
--- a/src/camera.c
+++ b/src/camera.c
@@ -9,6 +9,10 @@
 #include <stdio.h>
 #include <threads.h>
 
+#ifndef M_PI
+#define M_PI 3.14159265258979323846264
+#endif
+
 #define MAX_ITER 10
 #define MIN_T 1e-6
 #define SAMPLE_STDDEV 0.333
@@ -113,12 +117,16 @@ static int render_thread(void *arg)
 }
 
 camera_t camera_init(
-    vec3_t pos, double focal_len, double viewport_height, uint32_t img_width,
+    vec3_t pos, double focal_len, double fov, uint32_t img_width,
     uint32_t img_height, unsigned samples_per_pixel)
 {
 	const double aspect = (double)img_width / (double)img_height;
+
+	const double fov_rad = M_PI * fov / 180.0;
+	const double viewport_height = focal_len * tan(fov_rad / 2);
 	const double viewport_width = viewport_height * aspect;
 	const vec3_t viewport_disp = { 0, 0, focal_len };
+
 	const vec3_t u = { viewport_width, 0, 0 };
 	const vec3_t v = { 0, -viewport_height, 0 };
 	const vec3_t topleft = vec3_sub(