在magento admin中的产品编辑中不保存自定义选项卡数据

I have created a custom tab in Magento but while saving the product data is not reflected in database.

I have created below from with input field.

<div class="input-field">
    <label for="custom_field">Custom Field</label>
    <input class="input-text" name="custom_field" id="custom_field" />
</div>

I have wrote below code in Observer.

public function saveProductTabData(Varien_Event_Observer $observer)
{
    if (!self::$_singletonFlag) {
        self::$_singletonFlag = true;   
        $product = $observer->getEvent()->getProduct();
        try {
            $customFieldValue =  $this->_getRequest()->getPost('custom_field');
            $product->setNewAttribute($customFieldValue); 
            #$product->custom_field = $customFieldValue;
            $product->save();
        }
        catch (Exception $e) {
            Mage::getSingleton('adminhtml/session')->addError($e->getMessage());
        }
    }
}

I have followed all steps of this site https://fishpig.co.uk/magento/tutorials/custom-tabs-magento-product-admin/

$setup->addAttribute('catalog_product', 'custom_field', array(
'entity_model' => 'catalog/product',
'label' => 'custom',
'group' => 'General',
'input' => 'text',
'type' => 'text',
'is_html_allowed_on_front' => false,
'backend' => 'catalog/product_attribute_backend_price',
'global' => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
'visible' => false,
'apply_to' => Mage_Catalog_Model_Product_Type::TYPE_SIMPLE,
'required' => false,
'user_defined' => true,
'unique' => false,
'visible_on_front' => false,
'note' => 'custom'
));

use this function in tab.php block
public function getProduct()
{
return Mage::registry('product');
}