[ios] calculate EFL

ios/Runner/AppDelegate.swift

@@ -9,6 +9,7 @@ import Flutter
     ) -> Bool {
         let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
         let caffeinePlatformChannel = CaffeinePlatformChannel(binaryMessenger: controller.binaryMessenger)
+        let cameraInfoPlatformChannel = CameraInfoPlatformChannel(binaryMessenger: controller.binaryMessenger)
         GeneratedPluginRegistrant.register(with: self)
         return super.application(application, didFinishLaunchingWithOptions: launchOptions)
     }

ios/Runner/PlatformChannels/CaffeinePlatformChannel.swift

@@ -31,6 +31,5 @@ public class CaffeinePlatformChannel: NSObject {
                     result(FlutterMethodNotImplemented)
             }
         })
-        
     }
 }

ios/Runner/PlatformChannels/CameraInfoPlatformChannel.swift

@@ -0,0 +1,45 @@
+//
+//  CameraInfoPlatformChannel.swift
+//  Runner
+//
+//  Created by Vadim Turko on 2025-05-11.
+//
+import AVFoundation
+import Flutter
+
+public class CameraInfoPlatformChannel: NSObject {
+    let methodChannel: FlutterMethodChannel
+    
+    init(binaryMessenger: FlutterBinaryMessenger) {
+        self.methodChannel = FlutterMethodChannel(
+            name: "com.vodemn.lightmeter.CameraInfoPlatformChannel.MethodChannel",
+            binaryMessenger: binaryMessenger
+        )
+        super.init()
+        methodChannel.setMethodCallHandler({
+          (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
+            switch call.method {
+                case "mainCameraEfl":
+                    if let device = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back) {
+                        result(self.get35mmEquivalentFocalLength(format: device.activeFormat))
+                    } else {
+                        result(nil)
+                    }
+                default:
+                    result(FlutterMethodNotImplemented)
+            }
+        })
+    }
+    
+    func get35mmEquivalentFocalLength(format : AVCaptureDevice.Format) -> Float {
+        // get reported field of view. Documentation says this is the horizontal field of view
+        var fov = format.videoFieldOfView
+        // convert to radians
+        fov *= Float.pi/180.0
+        // angle and opposite of right angle triangle are half the fov and half the width of
+        // 35mm film (ie 18mm). The adjacent value of the right angle triangle is the equivalent
+        // focal length. Using some right angle triangle math you can work out focal length
+        let focalLen = 18 / tan(fov/2)
+        return focalLen
+    }
+}

lib/application_wrapper.dart

@@ -100,11 +100,13 @@ class _ApplicationWrapperState extends State<ApplicationWrapper> {
     await Future.wait([
       SharedPreferences.getInstance(),
       const LightSensorService(LocalPlatform()).hasSensor(),
+      const CameraInfoService().mainCameraEfl(),
       remoteConfigService.activeAndFetchFeatures(),
       equipmentProfilesStorageService.init(),
       filmsStorageService.init(),
     ]).then((value) {
-      userPreferencesService = UserPreferencesService((value[0] as SharedPreferences?)!);
+      userPreferencesService = UserPreferencesService((value[0] as SharedPreferences?)!)
+        ..cameraFocalLength = value[2] as int?;
       hasLightSensor = value[1] as bool? ?? false;
     });
   }

lib/data/camera_info_service.dart

@@ -1,5 +1,3 @@
-import 'dart:io';
-
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
@@ -12,9 +10,7 @@ class CameraInfoService {
   const CameraInfoService();
 
   Future<int?> mainCameraEfl() async {
-    if (Platform.isIOS) {
+    final focalLength = await cameraInfoPlatformChannel.invokeMethod<double?>('mainCameraEfl');
-      return null;
+    return focalLength?.round();
-    }
-    return (await cameraInfoPlatformChannel.invokeMethod<double?>('mainCameraEfl'))?.round();
   }
 }