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by Aditya

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I have a Pentax point and shot camera and I tried to convert it to Infrared. I was successful to remove the front IR filter from CCD and then I had a 760 nm IR filter to kick start with the infrared photography.

But the fun was spoilt as the camera won't focus correctly. I heard that I must replace a clear glass to compensate the thickness of removed IR filter (which is to be done, waiting for a glass sheet to come). But then, I heard someone saying in some of the question regarding IR conversion that there is a metering sensor which will ultimately decide the fate of auto focusing system in IR camera conversion.

So, my question is, do point and shoot cameras are equipped with metering sensor? If yes, how to fix it for infrared camera conversion?

Thanks in advance!

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Awesome question, wish we had more like this! –  dpollitt Aug 2 '12 at 1:49
    
Following the logic in the answers so far, would you be able to put the original IR filter in front of the lens just so the camera achieves focus, then remove it prior to the actual shot? Would that be proof of the green pixel contrast focus algorithm? –  Jahaziel Aug 2 '12 at 15:12

2 Answers 2

up vote 7 down vote accepted

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.

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It seems the information you've received applies to DSLR cameras.

DSLRs have separate autofocus and metering sensors, this is because the main sensor is unavailable prior to the shutter opening as there's a mirror in the way! However the metering sensor doesn't normally affect autofocus performance in any way*

With a DSLR it is very important for the AF sensor and main sensor to be in alignment as there's no communication between the two, hence the need to add a clear glass to ensure the sensor is the correct distance otherwise focus will be consistently off by a small amount.

Mirrorless cameras such as point and shoots don't have a separate metering sensor as metering can take place using the main sensor (and there's nowhere to put it anyway). Likewise autofocus takes place using the main sensor via a contrast detect feedback loop (where the lens is moved forward or backward looking for the sharpest image). Because of this feedback loop the tolerance on sensor position is not as tight, so there ought to be no need for a clear glass.

Likewise there should be no need to adjust for the difference in focusing distance between visible and IR light (like you do with a DSLR). However as the AF algorithm was never designed to work with the infrared spectrum you may have uncovered a flaw in the method which is impossible to fix.

*some high end DSLRs do use the metering sensor to aid AF tracking

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I do not agree that there is no need for the clear glass for point and shot camera. I have searched many article on infrared conversion and auto-focus has fixed in many cases when they replaced a piece of clear glass. In my camera without glass, I can see in live view that it moves forward and backward to focus, there is even a sharp view before the camera set the final position at which the view is blurry. I will check it myself as soon as I will have a piece of glass over CCD. –  Nitin Kumar Aug 2 '12 at 14:58
    
@Nitin Can you provide links to these articles? Other than to correct infinity focus I don't see why the glass is required for contrast detect autofocus as it's a closed loop system (i.e. the process stops when the sharpest image is observed on the main imaging sensor). –  Matt Grum Aug 2 '12 at 16:00

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