Tuesday, 12 April 2011

GH1 and dead pixels

Back in the day when people started replacing CRT computer monitors with LCD panels, dead pixels was a big deal. When you received the LCD panel you had ordered, you would have to review it to find the number of dead pixels, and consider whether or not to return it. Some claimed that dead pixels could be massaged with your fingertip and revived. After the production process improved, though, dead pixels has ceased to be a problem with computer monitors.

A computer monitor with 1280x1024 pixels has a total of 1280x1024x3 individual dots, one for each primary colour. This makes almost four million individual dots that make up the image you see. Previously, I took a macro closeup picture of a computer monitor to reveal the pattern of red, green and blue dots.

With camera sensors, the number of pixels is counted as the number of individual dots, each capable of seeing only one of the primary colour. Generally, these pixels are arranged in a Bayer-pattern, with two green pixels for each red and blue:


So a camera with 12 megapixels has 12 million individual light measuring cells, three times as many as the dots on a typical computer monitor. So is dead pixels a problem with cameras?

I tested my Panasonic GH1 camera. I took one very underexposed picture (which turned out black) and one very overexposed picture (which became white). The JPEG images are here, straight from the camera:



You don't need to look at them, though. Trust me, they are completely even, with no signs of dead pixels whatsoever.

However, what if the camera corrects the dead pixels in the JPEG files it generates? Perhaps there are "holes" in the image data, which is filled in by the JPEG engine in the camera's algorithm.

To check this, I opened the RAW files in a third party RAW converter program, the UFRAW. This did in fact reveal some dead pixels. I found eight pink dots. The colour pink is due to the green sensor element being dead, I suppose.

In the image below, a pink ring has been put around the dead pink pixels. After scaling the image down to 1000 pixels wide, the dead pixels themselves are of course not easy to spot anymore.


When investigating the black image in the RAW converter, I found no evidence of stuck pixels. So no pixels were generating a "phantom" light even though the exposure was non-existent.

I looked at other exposures, and found the dead, pink pixels in the same spots. So these pixels are definitively permanently dead on my GH1 camera.

Is this a problem? Hardly. As we saw, the JPEG engine is clever enough to mask these dead pixels, so when using JPEG images out of the camera, don't worry.

When using the RAW images, though, the dead pixels might disturb the image when using high resolution prints, for example. I would guess that the supplied RAW conversion program fixes these problems automatically, so this is likely only to be a potential problem with third party converters. And even then, eight dead pixels out of a total of 12 million is not exactly a huge percentage. It is very unlikely that this will cause any unwanted side-effects.

Appendix

Panasonic G series cameras do have a function to map these dead pixels. To do so, use the function "Pixel Refresh", which can be found in the custom menu (on the last page). The custom menu is the one with the "C" and wrench icon.

1 comment:

  1. The Olympus E-P1 has a function to map out dead/stuck/hot pixels in the camera. I had to use it once, and I think it works directly on the RAW data. Very helpful!

    With my Leica M8, it's a different matter. It has several dyfunctional pixels, three of which cause vertical lines to appear in JPEGs. As it is with Leica, you can choose between an expensive visit to the secret Leica factory, or you can try and help yourself. I did so with Pixel Fixer, a software which corrects dead pixels and vertical lines in RAW data.

    It also seems that Lightroom is capable of detecting and correcting dead pixels in RAW files.

    ReplyDelete