How do I keep smartphone camera exposure constant when a bright object enters the frame?
Asked 10/3/2012
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2 answers
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I’m building an iOS/Android app that uses the phone camera for image processing. During recording, if a very bright or mostly white object enters the frame, the image brightness changes and the whole scene gets darker. I’ve tried adjusting focus, white balance, and exposure settings, but I want the lighting/exposure to stay constant instead of the camera reacting to the scene. How can I do that through the camera API?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
2
If I understand you right you have your camera on your phone and you're composing the picture and when something bright enters the screen the lighting 'appears' to change (probably the whole screen gets darker).
The camera will adjust what it thinks the shutter/aperture/iso should be to get the "proper" lighting. How does it know what the proper lighting is? It thinks the frame should average out to 18% grey (tones, color doesn't matter). This 18% works for "most" pictures, but if you have a predominately white/light image it will appear dark and vice versa for dark images appearing bright (blacks appear grey).
The fix is that once you figure out what the "proper" exposure is (a grey card can help) then you lock in your ISO/shutter speed/aperture and then as you move about and zoom in/out and change the proportions of the screen that are light or dark then it won't change. This is why people shoot in manual (one of the reasons anyway).
Edit: you can see a similar question here Can the camera select wrong exposure just because of color?
Originally by user9510. Source · Licensed CC BY-SA 4.0
user9510
14y ago
0
Generated from our catalog & community — verify before relying on it.
What you’re seeing is normal auto-exposure behavior. The camera is trying to make the overall frame average to a midtone (often described as about 18% gray), so when a large bright/white object enters the shot, the camera reduces exposure and the whole image looks darker.
To keep brightness constant, you need to stop the camera from continuously auto-adjusting exposure. In practice, that means:
- determine the correct exposure for the scene first
- then lock exposure / switch to manual exposure controls if the API allows it
- keep shutter/ISO (and aperture if the device supports it) fixed while recording
Focus and white balance won’t solve this exposure shift. White balance affects color, not brightness.
If your app can meter first, using a neutral target such as a gray card can help you choose a good fixed exposure before locking it. Once exposure is locked, bright objects entering the frame won’t cause the camera to darken the image—though those bright areas may clip if they exceed the sensor’s range.
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UniqueBot
AI14y ago
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