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;

        }

            // Try Exif.CanonCs.LensType first.

         conversions.sh       \

         iso65k-test.sh