Getting the std deviation of multiple pictures using Opencv

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I'm trying to get the standard deviation of multiple pictures using, OpenCV, here what I've done :

#include <opencv2\core\core.hpp>
#include <opencv2\highgui\highgui.hpp>
#include <opencv2\opencv.hpp>

using namespace std;
using namespace cv;

int main() {
    cv::Mat frame,frame32f;
    char filename[40];
    cv::Mat mean;
    const int count = 273;
    const int width  = 1920;
    const int height = 1080;
    cv::Mat resultframe = cv::Mat::zeros(height,width,CV_32FC3);
    cv::Mat deviationframe = cv::Mat ::zeros(height,width,CV_32FC3);
    for(int i = 1 ; i<= count; i++){
    //int i = 3;
    sprintf(filename,"d:\\BMdvideos\\images\\image%d.tiff",i);
    frame = imread(filename,CV_LOAD_IMAGE_COLOR);
    frame.convertTo(frame32f,CV_32FC3 );
    //cout << " frame type " << frame.type() << endl;
    //imshow(" frame32f" , frame32f);
    resultframe +=frame32f;
    //cout<< "rows number " << frame.rows<< std::endl;
    //cout<<"column number " << frame.cols<<std::endl;
    cout << " i = " << i<<endl;
    frame.release();
    }
    resultframe *= (1.0/count/255);
    for(int j =1; j< count; j++){
        sprintf(filename,"d:\\BMdvideos\\images\\image%d.tiff",j);
        frame = imread(filename,CV_LOAD_IMAGE_COLOR);
        frame.convertTo(frame32f,CV_32FC3);
        deviationframe =(frame32f - resultframe);
        deviationframe= deviationframe.mul(deviationframe);
    }
    deviationframe = deviationframe /(count -1 );

    cv::sqrt(deviationframe,deviationframe);
    //deviationframe *= (1.0/255); doesn't change anything ? ? 
    imshow("mean ",resultframe);
    imshow("deviation frame ",deviationframe);
    waitKey(0);
    return 0;
}

when I see what I get the result frame "mean value" is correct but the std deviation is just wrong. any idea what I'm doing wrong, thanks in advance for help.