diff --git a/src/Image.cpp b/src/Image.cpp
index 4b1df64..838071b 100644
--- a/src/Image.cpp
+++ b/src/Image.cpp
@@ -44,16 +44,19 @@ void Image::buildImage(uint16_t * rawImage, const RawParameters & params) {
     size_t size = width*height;
     brightness = 0.0;
     max = 0;
+    min = params.black * 100; // some inital big value
     for (size_t y = 0, ry = params.topMargin; y < height; ++y, ++ry) {
         for (size_t x = 0, rx = params.leftMargin; x < width; ++x, ++rx) {
             uint16_t v = rawImage[ry*params.rawWidth + rx];
             (*this)(x, y) = v;
             brightness += v;
             if (v > max) max = v;
+            if (v < min) min = v;
         }
     }
     brightness /= size;
-    response.setLinear(params.max == 0 ? 1.0 : 65535.0 / params.max);
+    // response.setLinear(params.max == 0 ? 1.0 : 65535.0 / params.max);
+    response.setLinear(1.0);
     subtractBlack(params);
 }
 
@@ -165,9 +168,11 @@ void Image::computeResponseFunction(const Image & r) {
                 int pos = y * width + x;
                 double v = usePixels[pos];
                 double nv = rUsePixels[pos];
-                if (v >= nv && v < satThreshold) {
-                    numerator += v * r.response(nv);
-                    denom += v * v;
+                if (v <= nv && nv < satThreshold) {
+                    if ((nv - (double)r.min) / ((double)r.max - (double)r.min) > 0.1) {
+                        numerator += v * r.response(nv);
+                        denom += v * v;
+                    }
                 }
             }
         }
diff --git a/src/Image.hpp b/src/Image.hpp
index 0bc760b..f0f4ff5 100644
--- a/src/Image.hpp
+++ b/src/Image.hpp
@@ -104,7 +104,7 @@ private:
     QString filename;
 
     std::unique_ptr<Array2D<uint16_t>[]> scaled;
-    uint16_t satThreshold, max;
+    uint16_t satThreshold, max, min;
     double brightness;
     ResponseFunction response;
     double halfLightPercent;
diff --git a/src/ImageIO.cpp b/src/ImageIO.cpp
index a80ca51..3dbdfa3 100644
--- a/src/ImageIO.cpp
+++ b/src/ImageIO.cpp
@@ -183,7 +183,7 @@ void ImageIO::save(const SaveOptions & options, ProgressIndicator & progress) {
     params.width = stack.getWidth();
     params.height = stack.getHeight();
     params.adjustWhite(stack.getImage(stack.size() - 1));
-    Array2D<float> composedImage = stack.compose(params, options.featherRadius);
+    Array2D<float> composedImage = stack.compose(params, options.featherRadius, options.bps);
 
     progress.advance(33, "Rendering preview");
     QImage preview = renderPreview(composedImage, params, stack.getMaxExposure(), options.previewSize <= 1);
diff --git a/src/ImageStack.cpp b/src/ImageStack.cpp
index 6074321..5090f10 100644
--- a/src/ImageStack.cpp
+++ b/src/ImageStack.cpp
@@ -21,6 +21,7 @@
  */
 
 #include <algorithm>
+#include <cmath>
 
 #include "BoxBlur.hpp"
 #include "ImageStack.hpp"
@@ -166,8 +167,8 @@ void ImageStack::crop() {
 
 void ImageStack::computeResponseFunctions() {
     Timer t("Compute response functions");
-    for (int i = images.size() - 2; i >= 0; --i) {
-        images[i].computeResponseFunction(images[i + 1]);
+    for (int i = 1; i < images.size(); ++i) {
+        images[i].computeResponseFunction(images[i - 1]);
     }
 }
 
@@ -394,7 +395,7 @@ static Array2D<uint8_t> fattenMask(const Array2D<uint8_t> & mask, int radius) {
 }
 #endif
 
-Array2D<float> ImageStack::compose(const RawParameters & params, int featherRadius) const {
+Array2D<float> ImageStack::compose(const RawParameters & params, int featherRadius, int bit_depth) const {
     int imageMax = images.size() - 1;
     BoxBlur map(fattenMask(mask, featherRadius));
     measureTime("Blur", [&] () {
@@ -459,7 +460,10 @@ Array2D<float> ImageStack::compose(const RawParameters & params, int featherRadi
 
     dst.displace(params.leftMargin, params.topMargin);
     // Scale to params.max and recover the black levels
-    float mult = (params.max - params.maxBlack) / max;
+    int max_possible = std::pow(2.0, bit_depth); // almost always 65536
+    int max_black = params.maxBlack * ((double)max_possible / (double)params.max);
+    float mult = (max_possible - max_black) / max; // scale dst to fit the bit depth space
+
     #pragma omp parallel for
     for (size_t y = 0; y < params.rawHeight; ++y) {
         for (size_t x = 0; x < params.rawWidth; ++x) {
diff --git a/src/ImageStack.hpp b/src/ImageStack.hpp
index 87610da..a16c63a 100644
--- a/src/ImageStack.hpp
+++ b/src/ImageStack.hpp
@@ -48,7 +48,7 @@ public:
     void crop();
     void computeResponseFunctions();
     void generateMask();
-    Array2D<float> compose(const RawParameters & md, int featherRadius) const;
+    Array2D<float> compose(const RawParameters & md, int featherRadius, int bit_depth) const;
 
     size_t size() const { return images.size(); }