More info on CMOS sensor grid artefacts.

workshops-275 More info on CMOS sensor grid artefacts.

Cameras with bayer CMOS sensors can in certain circumstances suffer from an image artefact that appears as a grid pattern across the image. The actual artefact is normally the result of red and blue pixels that are brighter than they should be which gives a magenta type flare effect. However sometimes re-scaling an image containing this artefact can result in what looks like a grid type pattern as some pixels may be dropped or added together during the re scaling and this makes the artefact show up as a grip superimposed over the image.

grid-pattern-1024x576 More info on CMOS sensor grid artefacts.
Grid type artefact.

The cause of this artefact is most likely off-axis light somehow falling on the sensor. This off axis light could come from an internal reflection within the camera or the lens. It’s known that with the F5/F55 and FS7 cameras that a very strong light source that is just out of shot, just above or below the image frame can in some circumstances with some lenses result in this artefact. The cure is actually very simple, use a flag or lens hood to prevent off axis light from entering the lens. This is best practice anyway.

So what’s going on, on the sensor?sony-grid-artefact-explained More info on CMOS sensor grid artefacts.

When white light falls on a bayer sensor it passes through color filters before hitting the pixel that measures the light level. For white light the amount of light that passes through each color is different. don’t know the actual ratios of the different colors, it will vary from sensor to sensor, so I’ve used some made up values to illustrate what is going on.

In the illustration above when the blue pixel see’s 10%, green 70% and red 20% after processing the output would be white. If the light falling on the sensor is on axis, ie coming directly, straight through the lens then everything is fine. But if somehow the light falls on the sensor off axis at an oblique angle then it is possible that the light that passes through the blue filter may fall on the green pixel, or the light from the green filter may fall on the red pixel etc. So instead of nice white light the sensor pixels would think they are seeing light with an unusually high red and blue component. If you viewed the image pixel for pixel it would have very bright red pixels, bright blue pixels and dark green pixels. This is the kind of pattern that can result in the grid type artefact seen on many CMOS bayer sensors when there are problems with off axis light.

This is a very rare problem and only occurs in certain circumstances. But when it does occur it can spoil an otherwise good shot. I have found that adding a simple mask in front of the lens or using a matte box such as any of the Vocas matte boxes with eyebrows will eliminate the issue. I made a number of screw on masks for my lenses by taking a clear glass or UV filter and adding a couple of strips of black electrical tape to the rear of the filter to produce a mask for the top and bottom of the lens. With zoom lenses if you make this mask such that it can’t be seen in the shot at the wide end the mask is effective throughout the entire zoo range.

f5-f55-mask More info on CMOS sensor grid artefacts.


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