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Thread: Is this a sensor problem?

  1. #1
    Senior Member Jason Muelver's Avatar
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    Is this a sensor problem?

    I found some spots on some images that looked a lot like sensor dust. So I opened my M8 up and did the rocket blow thing like I'd do with any other sensor.

    Then for a test, I shoot into a white drape massively over exposing shooting at 1.4 1/60th ISO 160. I expect a white image except where sensor dust would be.

    Instead, I got this:



    Now I do not have a UV cut filter on my lens, so I don't know if that's causing this? Or do I begin to panic?

    I've seen nothing like these artifacts in any images, but needless to say, I'm a bit concerned.

    Thanks!
    http://jasonedwardphoto.com http://jasonmuelver.tumblr.com
    Nikon FX, Leica M8, Mamiya 645, Canon F-1

  2. #2
    Senior Member Jason Muelver's Avatar
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    Re: Is this a sensor problem?

    I'm so confused.

    I put on a friends 50 'cron (ancient... like 174xxxx serial) and same exposure and the image is clean.

    Is that what CA looks like on an 8 stop overexposed image with my Nokton?
    http://jasonedwardphoto.com http://jasonmuelver.tumblr.com
    Nikon FX, Leica M8, Mamiya 645, Canon F-1

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    Senior Member Per Ofverbeck's Avatar
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    Re: Is this a sensor problem?

    Why the massive overexposure if the purpose is to see dust on the sensor? Around one step over measured exposure is about right for that: unfocussed surface (the sky works), fully stopped down lens, then contrast boost in Lightroom (or whatever software you use).

    I wouldn´t worry about the artefacts you see when the sensor is so severely oversaturated. If you see it within the normal dynamic range would be another matter.

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    Administrator Bob's Avatar
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    Re: Is this a sensor problem?

    It is hard to tell.
    If the image is almost completely blown, then the nokton might show something at the edges due to fall-off.

    It would be better to shoot a mid-gray exposure so you can see what is actually going on.
    Meanwhile, all you have to worry about is over-exposure.
    -bob

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    Super Duper
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    Re: Is this a sensor problem?

    Hi Jason,

    As others have suggested, this appears to be artifacts created either due to severly overexposing and overwhelming the sensor or possibly the usual crud one often gets in post processing a uniformly confluent light color image and pushing exposure/contrast in trying to bring out sensor dust. I've seen this often with almost every digital camera. It happens more often when shooting say a white wall than say a uniformly clear blue sky. Here's what I would try. If the sky is clear blue with absolutely no clouds in the frame, open your lens wide open and set to min focus distance. Make sure nothing but clear blue sky in the frame? Meter normally and take a shot wide open and then successive shots by stopping down one whole f-stop with each successive shot. Open images in Photoshop etc. and examine each one. Those shot at around f5.6 and smaller will show the residual dust specs. You probably won't see any of those corner artifacts. Now with each image you can do a "auto contrast" command or uniformly boost careful contrast in each image to highlight the dust. If you push this contrast adjustment too far, you might start seeing all sorts of artifacts, especially in the corners. Push further and see what happens. Adjustment of exposure instead will also yield some strange stuff. When the above is performed, it should help in determining if there is something else at play, which I would say at is point there isn't. Again overexposing for 8 f-stops is bound to overwhelm the sensor and depending on the camera's kind of sensor, often yields things like this.

    Why a different lens resulted in differences, I can't at the moment come up with a reason why. Let us know any other addition observations you might come across.

    Dave (D&A)

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    Super Duper
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    Re: Is this a sensor problem?

    Sorry my post above is one long paragraph. I'm having trouble typing with my cell phone and now it won't let me insert paragraph breaks.

    Dave

  7. #7
    Senior Member Jason Muelver's Avatar
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    Re: Is this a sensor problem?

    Thanks for the replies.

    I guess I thought that was the best way to identify junk on a sensor, to completely blow it out so that only the crud shows up.

    I will play around more the next couple of days.
    http://jasonedwardphoto.com http://jasonmuelver.tumblr.com
    Nikon FX, Leica M8, Mamiya 645, Canon F-1

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    Re: Is this a sensor problem?

    on the topic of junk on the sensor, my new M9 came with the sensor covered with some round shaped semi-translucent junk, as if drops of some dried liquid. could not blow it away with the rocket blower, had to use eclipse and pecpad spatulas repeatedly but the junk keeps re-appearing in different places. I have now resigned, having spent about two hours cleaning the sensor repeatedly and accepting that it will never be perfectly clean. Irritating....

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    Administrator Bob's Avatar
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    Re: Is this a sensor problem?

    Quote Originally Posted by baudolino View Post
    on the topic of junk on the sensor, my new M9 came with the sensor covered with some round shaped semi-translucent junk, as if drops of some dried liquid. could not blow it away with the rocket blower, had to use eclipse and pecpad spatulas repeatedly but the junk keeps re-appearing in different places. I have now resigned, having spent about two hours cleaning the sensor repeatedly and accepting that it will never be perfectly clean. Irritating....
    Reappearing droplets are often caused by excess lubricant in the camera and will disappear after awhile once the excess has all been shaken loose.
    I don't often hear about this on M9s though I suppose it can happen.
    -bob

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