Diffraction

function diffcalc(form) {
    coc1 = (document.form.format.options[document.form.format.selectedIndex].value);
    aperture = (document.form.aperture.options[document.form.aperture.selectedIndex].value);
    unit = form.unit.value;
    view_dist = (form.view_dist.value) / 100;
    print_size = form.print_size.value;
    eyesight = (document.form.eyesight.options[document.form.eyesight.selectedIndex].value);
    megapixels = form.megapixels.value;
    if (isNaN(print_size)) {
        alert('Please enter a numerical value for the maximum print dimension.');
        document.form.print_size.focus();
        document.form.print_size.select();
    }
    if (isNaN(megapixels)) {
        if (megapixels != "?") {
            alert('Please enter a numerical value for the megapixel resolution.');
            document.form.megapixels.focus();
            document.form.megapixels.select();
        }
    }
    if (megapixels == "?") {
        form.pix_out.value = "?";
    } else {
        if (coc1 < 0.0165) {
            aspectratio = 4 / 3;
        } else {
            aspectratio = 3 / 2;
        }
        pixelsize = (coc1 / 0.032) * 43266.6 / (Math.sqrt(aspectratio * megapixels * 1e6) * Math.sqrt(1 + 1 / (aspectratio * aspectratio)));
        form.pix_out.value = Math.round(10 * pixelsize) / 10 + " \u00B5" + "m";
    }
    if (document.form.usepixels.checked == true) {
        if (megapixels == "?") {
            alert('Please enter a numerical value for the megapixel resolution.');
            document.form.megapixels.focus();
            document.form.megapixels.select();
        } else {
            CoC = 2.5 * pixelsize;
            form.coc_out.value = Math.round(100 * CoC) / 100 + " \u00B5" + "m";
        }
    } else {
        CoC = 1000 * coc1 * eyesight * (view_dist / 0.25) * (10 * unit / print_size);
        form.coc_out.value = Math.round(100 * CoC) / 100 + " \u00B5" + "m";
    }
    airy = 2.43932 * 546e-9 * aperture * 1e6;
    form.airy_out.value = Math.round(10 * airy) / 10 + " \u00B5" + "m";
    if (CoC < airy) {
        form.diff_out.value = "YES";
    } else {
        form.diff_out.value = "NO";
    }
}