diff --git a/core/modules/field/field.api.php b/core/modules/field/field.api.php
index 4b71248..63c388d 100644
--- a/core/modules/field/field.api.php
+++ b/core/modules/field/field.api.php
@@ -274,6 +274,32 @@ function hook_field_schema($field) {
 }
 
 /**
+ * Allow modules to alter the schema for a field.
+ *
+ * @param $schema
+ *   The schema definition as returned by hook_field_schema().
+ * @param $field
+ *   The field definition.
+ *
+ * @see field_retrieve_schema()
+ */
+function hook_field_schema_alter(&$schema, $field) {
+  if ($field['type'] == 'image') {
+    // Alter the length of a field.
+    $schema['columns']['alt']['length'] = 2048;
+    // Add an addtional column of data.
+    $schema['columns']['additional_column'] = array(
+        'description' => "Addtional column added to image field table.",
+        'type' => 'varchar',
+        'length' => 128,
+        'not null' => FALSE,
+    );
+    // Add an additional index.
+    $schema['indexes']['fid_addtional_column'] = array('fid', 'additional_column');
+  }
+}
+
+/**
  * Define custom load behavior for this module's field types.
  *
  * Unlike most other field hooks, this hook operates on multiple entities. The
diff --git a/core/modules/field/field.crud.inc b/core/modules/field/field.crud.inc
index ff39f22..bc2d6fb 100644
--- a/core/modules/field/field.crud.inc
+++ b/core/modules/field/field.crud.inc
@@ -20,6 +20,24 @@
  */
 
 /**
+ * Retrieves the schema for a field.
+ *
+ * @param $field
+ *   The field array to get the schema definition against.
+ * @return
+ *   The field schema definition array.
+ */
+function field_retrieve_schema($field) {
+  module_load_install($field['module']);
+  $schema = (array) module_invoke($field['module'], 'field_schema', $field);
+  $schema += array('columns' => array(), 'indexes' => array(), 'foreign keys' => array());
+  // Give other modules a chance to alter this definition.
+  // @see hook_field_schema_alter()
+  drupal_alter('field_schema', $schema, $field);
+  return $schema;
+}
+
+/**
  * Creates a field.
  *
  * This function does not bind the field to any bundle; use
@@ -129,9 +147,7 @@ function field_create_field($field) {
   $field['storage']['module'] = $storage_type['module'];
   $field['storage']['active'] = 1;
   // Collect storage information.
-  module_load_install($field['module']);
-  $schema = (array) module_invoke($field['module'], 'field_schema', $field);
-  $schema += array('columns' => array(), 'indexes' => array(), 'foreign keys' => array());
+  $schema = field_retrieve_schema($field);
   // 'columns' are hardcoded in the field type.
   $field['columns'] = $schema['columns'];
   // 'foreign keys' are hardcoded in the field type.
@@ -241,9 +257,7 @@ function field_update_field($field) {
 
   // Collect the new storage information, since what is in
   // $prior_field may no longer be right.
-  module_load_install($field['module']);
-  $schema = (array) module_invoke($field['module'], 'field_schema', $field);
-  $schema += array('columns' => array(), 'indexes' => array());
+  $schema = field_retrieve_schema($field);
   // 'columns' are hardcoded in the field type.
   $field['columns'] = $schema['columns'];
   // 'indexes' can be both hardcoded in the field type, and specified in the
@@ -368,9 +382,7 @@ function field_read_fields($params = array(), $include_additional = array()) {
     module_invoke_all('field_read_field', $field);
 
     // Populate storage information.
-    module_load_install($field['module']);
-    $schema = (array) module_invoke($field['module'], 'field_schema', $field);
-    $schema += array('columns' => array(), 'indexes' => array());
+    $schema = field_retrieve_schema($field);
     $field['columns'] = $schema['columns'];
 
     $field_name = $field['field_name'];
diff --git a/core/modules/field/tests/field.test b/core/modules/field/tests/field.test
index 214eab4..9df3fb8 100644
--- a/core/modules/field/tests/field.test
+++ b/core/modules/field/tests/field.test
@@ -2379,6 +2379,52 @@ class FieldCrudTestCase extends FieldTestCase {
   }
 }
 
+class FieldSchemaAlterTestCase extends FieldTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'Field schema alteration tests',
+      'description' => 'Alter the schema for a given type of field.',
+      'group' => 'Field API',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('image', 'field_test');
+  }
+
+  /**
+   * Test that new image field gets an altered alt column and a new column.
+   */
+  function testImageFieldSchemaAlter() {
+    $image_field = array(
+      'field_name' => drupal_strtolower($this->randomName()),
+      'type' => 'image',
+      'settings' => array(),
+      'cardinality' => 1,
+    );
+    field_create_field($image_field);
+    $image_field_name = $image_field['field_name'];
+    $image_field_instance_settings = array(
+      'field_name' => $image_field_name,
+      'entity_type' => 'node',
+      'bundle' => 'article',
+      'required' => TRUE,
+      'settings' => array(),
+      'widget' => array(
+        'type' => 'image_image',
+        'settings' => array(),
+      ),
+      'deleted' => 0,
+    );
+    $image_field_instance = field_create_instance($image_field_instance_settings);
+
+    $table_name = _field_sql_storage_tablename($image_field_instance);
+    $schema = drupal_get_schema($table_name, TRUE);
+    $this->assertTrue(db_field_exists($table_name, $image_field_name .'_additional_column'));
+    $this->assertEqual(2048, $schema['fields'][$image_field_name .'_alt']['length']);
+  }
+}
+
 class FieldInstanceCrudTestCase extends FieldTestCase {
   protected $field;
 
diff --git a/core/modules/field/tests/field_test.module b/core/modules/field/tests/field_test.module
index a43a03a..17269d6 100644
--- a/core/modules/field/tests/field_test.module
+++ b/core/modules/field/tests/field_test.module
@@ -285,3 +285,20 @@ function field_test_field_widget_form_alter(&$element, &$form_state, $context) {
       break;
   }
 }
+
+/**
+ * Implements hook_field_schema_alter().
+ */
+function field_test_field_schema_alter(&$schema, $field) {
+  if ($field['type'] == 'image') {
+    // Alter the length of a field.
+    $schema['columns']['alt']['length'] = 2048;
+    // Add an addtional column of data.
+    $schema['columns']['additional_column'] = array(
+        'description' => "Addtional column added to image field table.",
+        'type' => 'varchar',
+        'length' => 128,
+        'not null' => FALSE,
+    );
+  }
+}