Somebody told me that there is a difference in sharpness between a zoom lens vs. prime lens. A  58-200mm will give sharp focus in the center of the frame but blurry performance at the corners,  while in the meantime a prime 200mm will give a sharp definition to the whole frame of the photograph. Is this true? 


Question: How do I fill a textbox with text if it is empty?
Answer: the sender of your object should be the textbox. There you can get the text you want:

void c_TextChanged(object sender, EventArgs e)
{
    TextBox box = sender as TextBox;
    if (box != null)
    {
        textBox1.Text = box.Text;
    }
}