Update Canon decoder for lens detection (#2057)

The `Canon RF 24-105 F4-7.1 IS STM` lens is reported as using an
`Exif.CanonCs.MaxAperture` of `F7.3` instead of `F7.1`. As a result,
the translation of `Exif.CanonCs.LensType` is missing.

Fix updates the `CanonEV()` decode function to provide the correct
value.

Author: postscript-dev

src/canonmn_int.cpp

@@ -2527,6 +2527,13 @@ std::ostream& printCsLensTypeByMetadata(std::ostream& os, const Value& value, co
     return printCsLensFFFF(os, value, metadata);
   }
 
+  // TODO: The lens identification could be improved further:
+  // 1. RF lenses also set Exif.CanonFi.RFLensType. If a lens cannot be found here then
+  //    the RF mechanism could be used instead.
+  // 2. Exif.Photo.LensModel and Exif.Canon.LensModel provide a text description of the lens
+  //    (e.g., "RF24-105mm F4-7.1 IS STM" - Note: no manufacturer or space after "RF").
+  //    After parsing, the values could be used to search in the lens array.
+
   // get the values we need from the metadata container
   ExifKey lensKey("Exif.CanonCs.Lens");
   auto pos = metadata->findKey(lensKey);
@@ -2829,6 +2836,8 @@ float canonEv(int64_t val) {
     frac = 64.0F / 3;
   } else if ((val == 160) && (frac == 0x08)) {  // for Sigma f/6.3 lenses that report f/6.2 to camera
     frac = 30.0F / 3;
+  } else if ((val == 160) && (frac == 0x18)) {  // for Canon F4-7.1 lens that reports max aperture as f/7.3
+    frac = 21.0F;
   }
   return sign * (val + frac) / 32.0F;
 }

test/data/issue_2057_poc1.exv

@@ -87,7 +87,7 @@ Exif.CanonSi.AEBBracketValue                 Short       1  0  0
 Exif.CanonSi.ControlMode                     Short       1  1  1
 Exif.CanonSi.SubjectDistance                 Short       1  0  0 m
 Exif.CanonSi.ApertureValue                   Short       1  148  F5
-Exif.CanonSi.ShutterSpeedValue               Short       1  184  1/54 s
+Exif.CanonSi.ShutterSpeedValue               Short       1  184  1/50 s
 Exif.CanonSi.MeasuredEV2                     Short       1  123  9.38
 Exif.CanonSi.BulbDuration                    Short       1  0  0
 Exif.CanonSi.CameraType                      Short       1  248  EOS High-end

test/data/test_reference_files/Sigma_50mm_F1.4_DG_HSM_A_for_EOS.exv.out

@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+
+from system_tests import CaseMeta, path
+
+class AddLens_Canon_RF_24_105mm_F4_7_1_IS_STM(metaclass=CaseMeta):
+    """
+    Regression test for the bug described in:
+    https://github.com/Exiv2/exiv2/issues/2057
+    """
+    url = "https://github.com/Exiv2/exiv2/issues/2057"
+
+    filename = path("$data_path/issue_2057_poc1.exv")
+    commands = ["$exiv2 --Print kyyvt --key Exif.CanonCs.LensType $filename"]
+    stderr = [""]
+    stdout = ["""Exif.CanonCs.LensType                        Short      61182  Canon RF 24-105mm F4-7.1 IS STM
+"""]
+    retval = [0]