Do point-and-shoot cameras use a separate metering sensor, and how does IR conversion affect focus?
Asked 8/1/2012
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I converted a Pentax point-and-shoot camera for infrared by removing the IR-cut filter from the CCD and adding a 760nm IR filter. After the conversion, autofocus is no longer accurate. I have heard that I may need to replace the removed filter with clear glass of similar thickness to maintain the correct optical spacing. I also heard that some cameras use a separate metering sensor that could affect autofocus after an IR conversion. Do point-and-shoot cameras have a separate metering sensor, or do they use the main imaging sensor for metering and autofocus? If they use the main sensor, what issues should I expect when converting a point-and-shoot to infrared?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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Most if not all fixed lens camera do not have any such sensor. They only have the imaging sensor which is read continuously to give the preview image. At the same time, part of that data is used to meter, compute white-balance, compute a live-histogram (optionally) and focus (by measuring contrast between adjacent pixels).
In other words, the imaging sensor does everything. Unfortunately for you, the removal of the IR sensor changes everything the imaging sensor sees which means that all functions are operating outside of expected conditions and it is unpredictable what the algorithms in a particular camera will behave. For example, some simple metering sensors just read the intensity of a few green pixels scattered around the image, with you IR filter in place they may get very little to work with.
So, to answer your title question, NO. How to fix it is a wild guess and completely depends on the particular camera.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
14y ago
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Generated from our catalog & community — verify before relying on it.
Most point-and-shoot cameras do not have a separate metering sensor. Like many mirrorless designs, they typically use the main imaging sensor continuously for preview, metering, white balance, and autofocus, usually by contrast detection.
That means IR conversion changes what the camera’s only sensor sees, so exposure, white balance, and autofocus can all behave unpredictably. The issue is usually not a separate metering sensor needing adjustment.
Your focus problem is more likely caused by two things:
- Optical thickness change: removing the original IR-cut filter changes the sensor path length, so replacing it with clear glass of similar thickness is often needed to restore correct focus calibration.
- Infrared focus shift / AF limitations: lenses focus IR differently from visible light, and contrast-detect AF may struggle because the camera’s algorithms were designed for normal visible-light imaging.
So for a point-and-shoot, there usually isn’t a separate metering sensor to fix. The practical fixes are restoring the original optical spacing with clear glass and accepting that metering/AF performance in IR may remain inconsistent depending on the camera.
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