Exiv2

	struct SensKeyNameList {

	    int count;

	    const char* keys[3];

	};

	// covers Exif.Phot.SensitivityType values 1-7. Note that SOS, REI and

	// ISO do differ in their meaning. Values coming first in a list (and

	// existing as a tag) are picked up first and used as the "ISO" value.

	static const SensKeyNameList sensitivityKey[] = {

	    { 1, { "Exif.Photo.StandardOutputSensitivity" }},

	    { 1, { "Exif.Photo.RecommendedExposureIndex" }},

	    { 1, { "Exif.Photo.ISOSpeed" }},

	    { 2, { "Exif.Photo.RecommendedExposureIndex", "Exif.Photo.StandardOutputSensitivity" }},

	    { 2, { "Exif.Photo.ISOSpeed", "Exif.Photo.StandardOutputSensitivity" }},

	    { 2, { "Exif.Photo.ISOSpeed", "Exif.Photo.RecommendedExposureIndex" }},

	    { 3, { "Exif.Photo.ISOSpeed", "Exif.Photo.RecommendedExposureIndex", "Exif.Photo.StandardOutputSensitivity" }}

	};

	static const char* sensitivityType[] = {

	    "Exif.Photo.SensitivityType"

	};

	long iso_val = -1;

            iso_val = parseLong(os.str(), ok);

            if (ok && iso_val > 0) break;

	// there is either a possible ISO "overflow" or no legacy

	// ISO tag at all. Check for SensitivityType tag and the referenced

	// ISO value (see EXIF 2.3 Annex G)

	long iso_tmp_val = -1;

	while (iso_tmp_val == -1 && (iso_val == 65535 || md == ed.end())) {

	    ExifData::const_iterator md_st = ed.end();

	    md_st = findMetadatum(ed, sensitivityType, 1);

	    // no SensitivityType? exit with existing data

	    if (md_st == ed.end()) break;

	    // otherwise pick up actual value and grab value accordingly

	    const char *st_key = 0; // nullptr

	    std::ostringstream os;

	    md_st->write(os, &ed);

	    bool ok = false;

	    long st_val = parseLong(os.str(), ok);

	    // SensivityType out of range or cannot be parsed properly

	    if (!ok || st_val < 1 || st_val > 7) break;

	    // pick up list of ISO tags, and check for at least one of

	    // them available.

	    const SensKeyNameList *sensKeys = &sensitivityKey[st_val - 1];

	    md_st = ed.end();

	    for (int idx = 0; idx < sensKeys->count; md_st = ed.end()) {

		md_st = findMetadatum(ed, const_cast<const char**>(sensKeys->keys), sensKeys->count);

		if (md_st == ed.end()) break;

		std::ostringstream os_iso;

		md_st->write(os_iso, &ed);

		ok = false;

		iso_tmp_val = parseLong(os_iso.str(), ok);

		// something wrong with the value

		if (ok || iso_tmp_val > 0) {

		    md = md_st;

		    break;

		}

		while (strcmp(sensKeys->keys[idx++], md_st->key().c_str()) != 0 && idx < cnt) {}

	    }

	    break;

	}