diff --git a/backend/genesys_gl124.cc b/backend/genesys_gl124.cc
index 169d720b2..9f6bf52f0 100644
--- a/backend/genesys_gl124.cc
+++ b/backend/genesys_gl124.cc
@@ -1092,8 +1092,7 @@ static void gl124_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
 
     debug_dump(DBG_info, session.params);
 
-    unsigned optical_res = sensor.optical_res / session.ccd_size_divisor;
-  DBG (DBG_info, "%s: optical_res=%d\n", __func__, optical_res);
+    DBG (DBG_info, "%s: optical_res=%d\n", __func__, session.optical_resolution);
 
   /* stagger */
     if (session.ccd_size_divisor == 1 && (dev->model->flags & GENESYS_FLAG_STAGGERED_LINE)) {
@@ -1105,14 +1104,14 @@ static void gl124_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
 
   /** @brief compute used resolution */
     if (session.params.flags & SCAN_FLAG_USE_OPTICAL_RES) {
-      used_res = optical_res;
+        used_res = session.optical_resolution;
     }
   else
     {
         // resolution is choosen from a fixed list and can be used directly, unless we have ydpi
         // higher than sensor's maximum one */
-        if (session.params.xres > optical_res) {
-            used_res = optical_res;
+        if (session.params.xres > session.optical_resolution) {
+            used_res = session.optical_resolution;
         } else {
             used_res = session.params.xres;
         }
@@ -1127,11 +1126,11 @@ static void gl124_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
     start |= 1;
 
   /* compute correct pixels number */
-    used_pixels = (session.params.pixels * optical_res) / session.params.xres;
+    used_pixels = (session.params.pixels * session.optical_resolution) / session.params.xres;
   DBG (DBG_info, "%s: used_pixels=%d\n", __func__, used_pixels);
 
   /* round up pixels number if needed */
-    if (used_pixels * session.params.xres < session.params.pixels * optical_res) {
+    if (used_pixels * session.params.xres < session.params.pixels * session.optical_resolution) {
         used_pixels++;
     }
 
@@ -1202,7 +1201,7 @@ static void gl124_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   /*** prepares data reordering ***/
 
   /* words_per_line */
-  bytes_per_line = (used_pixels * used_res) / optical_res;
+    bytes_per_line = (used_pixels * used_res) / session.optical_resolution;
   bytes_per_line = (bytes_per_line * session.params.channels * session.params.depth) / 8;
 
   /* since we don't have sheetfed scanners to handle,
@@ -1231,7 +1230,7 @@ static void gl124_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   dev->read_active = SANE_TRUE;
 
     dev->session = session;
-  dev->current_setup.pixels = (used_pixels * used_res) / optical_res;
+    dev->current_setup.pixels = (used_pixels * used_res) / session.optical_resolution;
   DBG(DBG_info, "%s: current_setup.pixels=%d\n", __func__, dev->current_setup.pixels);
   dev->current_setup.lines = lincnt;
   dev->current_setup.exposure_time = exposure_time;
diff --git a/backend/genesys_gl646.cc b/backend/genesys_gl646.cc
index 2dd39a502..a66a338f7 100644
--- a/backend/genesys_gl646.cc
+++ b/backend/genesys_gl646.cc
@@ -351,7 +351,7 @@ static void gl646_setup_registers(Genesys_Device* dev,
         startx |= 1;
     }
 
-    uint32_t pixels = (session.params.pixels * sensor.optical_res) / session.params.xres;
+    uint32_t pixels = (session.params.pixels * session.optical_resolution) / session.params.xres;
     // special requirement for 400 dpi on 1200 dpi sensors
     if (session.params.xres == 400) {
         pixels = (pixels / 6) * 6;
@@ -374,12 +374,9 @@ static void gl646_setup_registers(Genesys_Device* dev,
   DBG(DBG_info, "%s: startx=%d, endx=%d, linecnt=%d\n", __func__, startx, endx, linecnt);
 
   /* x resolution is capped by sensor's capability */
-  if (resolution > sensor.optical_res)
-    {
-      xresolution = sensor.optical_res;
-    }
-  else
-    {
+     if (static_cast<unsigned>(resolution) > session.optical_resolution) {
+        xresolution = session.optical_resolution;
+    } else {
       xresolution = resolution;
     }
 
diff --git a/backend/genesys_gl841.cc b/backend/genesys_gl841.cc
index c901f16e3..b1ebd88a8 100644
--- a/backend/genesys_gl841.cc
+++ b/backend/genesys_gl841.cc
@@ -1764,8 +1764,6 @@ static void gl841_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   int max_shift;
   size_t requested_buffer_size, read_buffer_size;
 
-  int optical_res;
-
     debug_dump(DBG_info, session.params);
 
 /*
@@ -1796,10 +1794,6 @@ independent of our calculated values:
   dev->bytes_to_read
  */
 
-/* optical_res */
-
-    optical_res = sensor.optical_res / session.ccd_size_divisor;
-
 /* stagger */
 
     if (session.ccd_size_divisor == 1 && (dev->model->flags & GENESYS_FLAG_STAGGERED_LINE)) {
@@ -1810,39 +1804,39 @@ independent of our calculated values:
   DBG(DBG_info, "%s : stagger=%d lines\n", __func__, stagger);
 
 /* used_res */
-    i = optical_res / session.params.xres;
+    i = session.optical_resolution / session.params.xres;
 
 /* gl841 supports 1/1 1/2 1/3 1/4 1/5 1/6 1/8 1/10 1/12 1/15 averaging */
 
     if (i < 2 || (session.params.flags & SCAN_FLAG_USE_OPTICAL_RES)) {
         // optical_res >= xres > optical_res/2
-        used_res = optical_res;
+        used_res = session.optical_resolution;
     } else if (i < 3)  /* optical_res/2 >= xres > optical_res/3 */
-      used_res = optical_res/2;
+        used_res = session.optical_resolution/2;
   else if (i < 4)  /* optical_res/3 >= xres > optical_res/4 */
-      used_res = optical_res/3;
+        used_res = session.optical_resolution/3;
   else if (i < 5)  /* optical_res/4 >= xres > optical_res/5 */
-      used_res = optical_res/4;
+        used_res = session.optical_resolution/4;
   else if (i < 6)  /* optical_res/5 >= xres > optical_res/6 */
-      used_res = optical_res/5;
+        used_res = session.optical_resolution/5;
   else if (i < 8)  /* optical_res/6 >= xres > optical_res/8 */
-      used_res = optical_res/6;
+        used_res = session.optical_resolution/6;
   else if (i < 10)  /* optical_res/8 >= xres > optical_res/10 */
-      used_res = optical_res/8;
+        used_res = session.optical_resolution/8;
   else if (i < 12)  /* optical_res/10 >= xres > optical_res/12 */
-      used_res = optical_res/10;
+        used_res = session.optical_resolution/10;
   else if (i < 15)  /* optical_res/12 >= xres > optical_res/15 */
-      used_res = optical_res/12;
+        used_res = session.optical_resolution/12;
   else
-      used_res = optical_res/15;
+        used_res = session.optical_resolution/15;
 
   /* compute scan parameters values */
   /* pixels are allways given at half or full CCD optical resolution */
   /* use detected left margin  and fixed value */
     start = ((sensor.CCD_start_xoffset + session.params.startx) * used_res) / sensor.optical_res;
 
-  /* needs to be aligned for used_res */
-  start = (start * optical_res) / used_res;
+    // needs to be aligned for used_res
+    start = (start * session.optical_resolution) / used_res;
 
   start += sensor.dummy_pixel + 1;
 
@@ -1855,15 +1849,15 @@ independent of our calculated values:
    * scan, where shading data needs to be align */
   if((dev->reg.find_reg(0x01).value & REG01_SHDAREA) != 0)
     {
-      avg=optical_res/used_res;
+        avg = session.optical_resolution / used_res;
       start=(start/avg)*avg;
     }
 
   /* compute correct pixels number */
-    used_pixels = (session.params.pixels * optical_res) / session.params.xres;
+    used_pixels = (session.params.pixels * session.optical_resolution) / session.params.xres;
 
   /* round up pixels number if needed */
-    if (used_pixels * session.params.xres < session.params.pixels * optical_res) {
+    if (used_pixels * session.params.xres < session.params.pixels * session.optical_resolution) {
         used_pixels++;
     }
 
@@ -1967,7 +1961,7 @@ dummy \ scanned lines
   /*** prepares data reordering ***/
 
 /* words_per_line */
-  bytes_per_line = (used_pixels * used_res) / optical_res;
+    bytes_per_line = (used_pixels * used_res) / session.optical_resolution;
     bytes_per_line = (bytes_per_line *  session.params.channels * session.params.depth) / 8;
 
   requested_buffer_size = 8 * bytes_per_line;
@@ -1997,7 +1991,7 @@ dummy \ scanned lines
   dev->read_active = SANE_TRUE;
 
     dev->session = session;
-  dev->current_setup.pixels = (used_pixels * used_res)/optical_res;
+    dev->current_setup.pixels = (used_pixels * used_res) / session.optical_resolution;
   dev->current_setup.lines = lincnt;
   dev->current_setup.exposure_time = exposure_time;
   dev->current_setup.xres = used_res;
@@ -2049,8 +2043,6 @@ static void gl841_calculate_current_setup(Genesys_Device * dev, const Genesys_Se
   int scan_step_type = 1;
   int max_shift;
 
-  int optical_res;
-
     DBG(DBG_info, "%s ", __func__);
     debug_dump(DBG_info, dev->settings);
 
@@ -2080,10 +2072,6 @@ static void gl841_calculate_current_setup(Genesys_Device * dev, const Genesys_Se
     DBG(DBG_info, "%s ", __func__);
     debug_dump(DBG_info, session.params);
 
-/* optical_res */
-
-    optical_res = sensor.optical_res / session.ccd_size_divisor;
-
 /* stagger */
 
     if (session.ccd_size_divisor == 1 && (dev->model->flags & GENESYS_FLAG_STAGGERED_LINE)) {
@@ -2094,38 +2082,38 @@ static void gl841_calculate_current_setup(Genesys_Device * dev, const Genesys_Se
   DBG(DBG_info, "%s: stagger=%d lines\n", __func__, stagger);
 
 /* used_res */
-  i = optical_res / session.params.xres;
+  i = session.optical_resolution / session.params.xres;
 
 /* gl841 supports 1/1 1/2 1/3 1/4 1/5 1/6 1/8 1/10 1/12 1/15 averaging */
 
   if (i < 2) /* optical_res >= xres > optical_res/2 */
-      used_res = optical_res;
+        used_res = session.optical_resolution;
   else if (i < 3)  /* optical_res/2 >= xres > optical_res/3 */
-      used_res = optical_res/2;
+        used_res = session.optical_resolution / 2;
   else if (i < 4)  /* optical_res/3 >= xres > optical_res/4 */
-      used_res = optical_res/3;
+        used_res = session.optical_resolution / 3;
   else if (i < 5)  /* optical_res/4 >= xres > optical_res/5 */
-      used_res = optical_res/4;
+        used_res = session.optical_resolution / 4;
   else if (i < 6)  /* optical_res/5 >= xres > optical_res/6 */
-      used_res = optical_res/5;
+        used_res = session.optical_resolution / 5;
   else if (i < 8)  /* optical_res/6 >= xres > optical_res/8 */
-      used_res = optical_res/6;
+        used_res = session.optical_resolution / 6;
   else if (i < 10)  /* optical_res/8 >= xres > optical_res/10 */
-      used_res = optical_res/8;
+        used_res = session.optical_resolution / 8;
   else if (i < 12)  /* optical_res/10 >= xres > optical_res/12 */
-      used_res = optical_res/10;
+        used_res = session.optical_resolution / 10;
   else if (i < 15)  /* optical_res/12 >= xres > optical_res/15 */
-      used_res = optical_res/12;
+        used_res = session.optical_resolution / 12;
   else
-      used_res = optical_res/15;
+        used_res = session.optical_resolution / 15;
 
   /* compute scan parameters values */
   /* pixels are allways given at half or full CCD optical resolution */
   /* use detected left margin  and fixed value */
     start = ((sensor.CCD_start_xoffset + session.params.startx) * used_res) / sensor.optical_res;
 
-/* needs to be aligned for used_res */
-  start = (start * optical_res) / used_res;
+    // needs to be aligned for used_res
+    start = (start * session.optical_resolution) / used_res;
 
   start += sensor.dummy_pixel + 1;
 
@@ -2133,10 +2121,10 @@ static void gl841_calculate_current_setup(Genesys_Device * dev, const Genesys_Se
     start |= 1;
   }
 
-    used_pixels = (session.params.pixels * optical_res) / session.params.xres;
+    used_pixels = (session.params.pixels * session.optical_resolution) / session.params.xres;
 
     // round up pixels number if needed
-    if (used_pixels * session.params.xres < session.params.pixels * optical_res) {
+    if (used_pixels * session.params.xres < session.params.pixels * session.optical_resolution) {
         used_pixels++;
     }
 
@@ -2191,7 +2179,7 @@ dummy \ scanned lines
     lincnt = session.params.lines + max_shift + stagger;
 
     dev->session = session;
-  dev->current_setup.pixels = (used_pixels * used_res)/optical_res;
+    dev->current_setup.pixels = (used_pixels * used_res) / session.optical_resolution;
   dev->current_setup.lines = lincnt;
   dev->current_setup.exposure_time = exposure_time;
   dev->current_setup.xres = used_res;
diff --git a/backend/genesys_gl843.cc b/backend/genesys_gl843.cc
index b2b12a1d4..a78aa5c0b 100644
--- a/backend/genesys_gl843.cc
+++ b/backend/genesys_gl843.cc
@@ -1201,9 +1201,6 @@ static void gl843_compute_session(Genesys_Device* dev, ScanSession& s,
     // compute optical and output resolutions
     s.hwdpi_divisor = sensor.get_hwdpi_divisor_for_dpi(s.params.xres);
 
-    s.optical_resolution = sensor.optical_res / s.ccd_size_divisor;
-    s.output_resolution = s.params.xres;
-
     if (s.output_resolution > s.optical_resolution) {
         throw std::runtime_error("output resolution higher than optical resolution");
     }
@@ -1413,8 +1410,6 @@ gl843_calculate_current_setup(Genesys_Device * dev, const Genesys_Sensor& sensor
 
   int max_shift;
 
-  int optical_res;
-
     DBG(DBG_info, "%s ", __func__);
     debug_dump(DBG_info, dev->settings);
 
@@ -1458,9 +1453,6 @@ gl843_calculate_current_setup(Genesys_Device * dev, const Genesys_Sensor& sensor
     DBG(DBG_info, "%s ", __func__);
     debug_dump(DBG_info, session.params);
 
-    // optical_res
-    optical_res = sensor.optical_res / session.ccd_size_divisor;
-
   /* stagger */
     if (session.ccd_size_divisor == 1 && (dev->model->flags & GENESYS_FLAG_STAGGERED_LINE)) {
         stagger = (4 * session.params.yres) / dev->motor.base_ydpi;
@@ -1469,10 +1461,10 @@ gl843_calculate_current_setup(Genesys_Device * dev, const Genesys_Sensor& sensor
     }
   DBG(DBG_info, "%s: stagger=%d lines\n", __func__, stagger);
 
-    if (session.params.xres <= (unsigned) optical_res) {
+    if (session.params.xres <= (unsigned) session.optical_resolution) {
         used_res = session.params.xres;
     } else {
-        used_res = optical_res;
+        used_res = session.optical_resolution;
     }
 
   /* compute scan parameters values */
@@ -1480,7 +1472,7 @@ gl843_calculate_current_setup(Genesys_Device * dev, const Genesys_Sensor& sensor
   /* use detected left margin  and fixed value */
 
   /* compute correct pixels number */
-    used_pixels = (session.params.pixels * optical_res) / session.params.xres;
+    used_pixels = (session.params.pixels * session.optical_resolution) / session.params.xres;
   DBG(DBG_info, "%s: used_pixels=%d\n", __func__, used_pixels);
 
   /* exposure */
@@ -1509,7 +1501,7 @@ gl843_calculate_current_setup(Genesys_Device * dev, const Genesys_Sensor& sensor
     lincnt = session.params.lines + max_shift + stagger;
 
     dev->session = session;
-  dev->current_setup.pixels = (used_pixels * used_res) / optical_res;
+    dev->current_setup.pixels = (used_pixels * used_res) / session.optical_resolution;
   DBG(DBG_info, "%s: current_setup.pixels=%d\n", __func__, dev->current_setup.pixels);
   dev->current_setup.lines = lincnt;
   dev->current_setup.exposure_time = exposure;
diff --git a/backend/genesys_gl846.cc b/backend/genesys_gl846.cc
index 84de797ce..c395659ee 100644
--- a/backend/genesys_gl846.cc
+++ b/backend/genesys_gl846.cc
@@ -971,12 +971,8 @@ static void gl846_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   int max_shift;
   size_t requested_buffer_size, read_buffer_size;
 
-  int optical_res;
-
     debug_dump(DBG_info, session.params);
 
-    optical_res = sensor.optical_res / session.ccd_size_divisor;
-
   /* stagger */
     if (session.ccd_size_divisor == 1 && (dev->model->flags & GENESYS_FLAG_STAGGERED_LINE)) {
         stagger = (4 * session.params.yres) / dev->motor.base_ydpi;
@@ -986,7 +982,7 @@ static void gl846_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   DBG(DBG_info, "%s : stagger=%d lines\n", __func__, stagger);
 
     if (session.params.flags & SCAN_FLAG_USE_OPTICAL_RES) {
-      used_res = optical_res;
+        used_res = session.optical_resolution;
     }
   else
     {
@@ -1006,10 +1002,10 @@ static void gl846_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
     }
 
   /* compute correct pixels number */
-    used_pixels = (session.params.pixels * optical_res) / session.params.xres;
+    used_pixels = (session.params.pixels * session.optical_resolution) / session.params.xres;
 
   /* round up pixels number if needed */
-    if (used_pixels * session.params.xres < session.params.pixels * optical_res) {
+    if (used_pixels * session.params.xres < session.params.pixels * session.optical_resolution) {
         used_pixels++;
     }
 
@@ -1074,7 +1070,7 @@ static void gl846_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   /*** prepares data reordering ***/
 
 /* words_per_line */
-  bytes_per_line = (used_pixels * used_res) / optical_res;
+    bytes_per_line = (used_pixels * used_res) / session.optical_resolution;
     bytes_per_line = (bytes_per_line * session.params.channels * session.params.depth) / 8;
 
   requested_buffer_size = 8 * bytes_per_line;
@@ -1100,7 +1096,7 @@ static void gl846_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   dev->read_active = SANE_TRUE;
 
     dev->session = session;
-  dev->current_setup.pixels = (used_pixels * used_res) / optical_res;
+    dev->current_setup.pixels = (used_pixels * used_res) / session.optical_resolution;
   dev->current_setup.lines = lincnt;
   dev->current_setup.exposure_time = exposure_time;
   dev->current_setup.xres = used_res;
diff --git a/backend/genesys_gl847.cc b/backend/genesys_gl847.cc
index e67f73fd9..35e313fca 100644
--- a/backend/genesys_gl847.cc
+++ b/backend/genesys_gl847.cc
@@ -990,12 +990,8 @@ static void gl847_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   int max_shift;
   size_t requested_buffer_size, read_buffer_size;
 
-  int optical_res;
-
     debug_dump(DBG_info, session.params);
 
-    optical_res = sensor.optical_res / session.ccd_size_divisor;
-
   /* stagger */
     if (session.ccd_size_divisor == 1 && (dev->model->flags & GENESYS_FLAG_STAGGERED_LINE)) {
         stagger = (4 * session.params.yres) / dev->motor.base_ydpi;
@@ -1006,10 +1002,8 @@ static void gl847_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
 
   /* used_res */
     if (session.params.flags & SCAN_FLAG_USE_OPTICAL_RES) {
-      used_res = optical_res;
-    }
-  else
-    {
+        used_res = session.optical_resolution;
+    } else {
       /* resolution is choosen from a list */
         used_res = session.params.xres;
     }
@@ -1026,10 +1020,10 @@ static void gl847_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
 
   /* compute correct pixels number */
   /* pixels */
-    used_pixels = (session.params.pixels * optical_res) / session.params.xres;
+    used_pixels = (session.params.pixels * session.optical_resolution) / session.params.xres;
 
   /* round up pixels number if needed */
-    if (used_pixels * session.params.xres < session.params.pixels * optical_res) {
+    if (used_pixels * session.params.xres < session.params.pixels * session.optical_resolution) {
         used_pixels++;
     }
     dummy = 3 - session.params.channels;
@@ -1091,7 +1085,7 @@ static void gl847_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   /*** prepares data reordering ***/
 
 /* words_per_line */
-  bytes_per_line = (used_pixels * used_res) / optical_res;
+    bytes_per_line = (used_pixels * used_res) / session.optical_resolution;
     bytes_per_line = (bytes_per_line * session.params.channels * session.params.depth) / 8;
 
   requested_buffer_size = 8 * bytes_per_line;
@@ -1117,7 +1111,7 @@ static void gl847_init_scan_regs(Genesys_Device* dev, const Genesys_Sensor& sens
   dev->read_active = SANE_TRUE;
 
     dev->session = session;
-  dev->current_setup.pixels = (used_pixels * used_res) / optical_res;
+    dev->current_setup.pixels = (used_pixels * used_res) / session.optical_resolution;
   dev->current_setup.lines = lincnt;
   dev->current_setup.exposure_time = exposure_time;
   dev->current_setup.xres = used_res;
diff --git a/backend/genesys_low.cc b/backend/genesys_low.cc
index b761ffc0c..cb43160b8 100644
--- a/backend/genesys_low.cc
+++ b/backend/genesys_low.cc
@@ -1139,6 +1139,13 @@ void compute_session(Genesys_Device* dev, ScanSession& s, const Genesys_Sensor&
 
     // compute optical and output resolutions
     s.ccd_size_divisor = sensor.get_ccd_size_divisor_for_dpi(s.params.xres);
+
+    if (dev->model->asic_type == AsicType::GL646) {
+        s.optical_resolution = sensor.optical_res;
+    } else {
+        s.optical_resolution = sensor.optical_res / s.ccd_size_divisor;
+    }
+    s.output_resolution = s.params.xres;
 }
 
 /** @brief initialize device
diff --git a/backend/genesys_settings.h b/backend/genesys_settings.h
index 6551db1c2..6a391ecf2 100644
--- a/backend/genesys_settings.h
+++ b/backend/genesys_settings.h
@@ -205,7 +205,6 @@ struct ScanSession {
     unsigned ccd_size_divisor = 1;
 
     // the optical resolution of the scanner.
-    // gl843-only
     unsigned optical_resolution = 0;
 
     // the number of pixels at the optical resolution.