diff --git a/build-number.txt b/build-number.txt
index 1303c35..761fcd3 100644
--- a/build-number.txt
+++ b/build-number.txt
@@ -1 +1 @@
-497
+499
diff --git a/src/ov7670.c b/src/ov7670.c
index 7f9bf74..0409b64 100644
--- a/src/ov7670.c
+++ b/src/ov7670.c
@@ -154,6 +154,7 @@ uint8_t LineBuffer[CAMERA_IMAGE_WIDTH + 40];
 int LineCount = 0;
 static int FrameCount = 0;
 volatile int Camera_Captured = 0;
+unsigned int BlackPixels = 0;
 
 static uint8_t ReadRegister(uint8_t reg)
 {
@@ -310,7 +311,9 @@ void TIM1_CC_IRQHandler(void)
     CurrentLine = 0;
     FrameCount++;
 
-    if(FrameCount == 5)
+    // Check if the last frame's exposure is reasonable
+    if(BlackPixels >= (unsigned)(CAMERA_PIXELS * CAMERA_EXPOSURE_LOW)
+        && BlackPixels <= (unsigned)(CAMERA_PIXELS * CAMERA_EXPOSURE_HIGH))
     {
         Camera_Captured = 1;
         // Disable everything
@@ -318,6 +321,9 @@ void TIM1_CC_IRQHandler(void)
         TIM1->CR1 = 0;
     }
 
+    // Reset black pixel counter
+    BlackPixels = 0;
+
     // Dummy read
     TIM1->CCR2;
     TIM1->SR &= ~TIM_SR_CC2IF;
@@ -371,6 +377,7 @@ void TIM3_IRQHandler(void)
                 error = pixel;
                 ImageBuffer[(line * CAMERA_IMAGE_WIDTH + i) / 8] |=
                     0x80 >> (i % 8);
+                BlackPixels++;
             }
             else
             {
diff --git a/src/ov7670.h b/src/ov7670.h
index 95b9050..23b7fa9 100644
--- a/src/ov7670.h
+++ b/src/ov7670.h
@@ -5,6 +5,12 @@
 
 #define CAMERA_IMAGE_WIDTH      160
 #define CAMERA_IMAGE_HEIGHT     144
+#define CAMERA_PIXELS           (CAMERA_IMAGE_WIDTH * CAMERA_IMAGE_HEIGHT)
+
+// Maximum and minimum share of black pixels for an image to be accepted
+#define CAMERA_EXPOSURE_LIMIT   0.4
+#define CAMERA_EXPOSURE_LOW     (0.5 - CAMERA_EXPOSURE_LIMIT)
+#define CAMERA_EXPOSURE_HIGH    (0.5 + CAMERA_EXPOSURE_LIMIT)
 
 // Use 2D Floyd-Steinberg dithering. Uncomment to use simple line-wise
 // dithering.