<p>I noticed this report from the first day it was posted, but hesitated to reply. Being an amateur astronomer for many years and also an electronics engineer by profession I may be useful on this issue. It is not something new in cameras and is found on most cameras used on demanding applications with ultra high contrast as star fields on an almost dark background. Oversaturated pixels might lead to neighbour pixels with abnormal values in the direction that pixels are sampled. The first time I faced this issue was on a special monochrome camera made for astronomical photos back in 1998. Trying to use DSLRs for astro imaging I have noticed this behaviour even at the time of D10. <br>
It is a normal behaviour for a ccd or cmos camera that has to overcome saturated pixels while sampling sequential pixels. Above all emotional thoughts and expectations, these marvels of technology cannot escape basic troubles of electronics. I will try to be as simple as I can but not too much as not to miss the meaning. The photons captured by a single pixel (any kind of photo sensitive device used) result to a charge stored on this insulated device. This charge is proportional to the amount of photons that have struck the sensitive area until the limit of storage (known as ‘well capacity’ normally measured in electrons), where voltage has the max value and result in a saturated pixel reading. Today's pixels are insulated to preserve the charge until the processing circuitry is able to read the value. Older versions of image sensors had leakage of charge to their neighbour pixels when oversaturated, usually in the row that they were wired and created streaks on the image with saturated pixels, an effect known as blooming.<br>
The stored signal (el. charge) has to be read by an analogue to digital converter but at first has to be buffered and amplified. These circuits take care of the conversion of the stored charge to a signal at levels usable as the input of A/D converters, but also take care of the required amplification as advised by user's ISO setting. Canon sensors are advertised to have amplifiers on each pixel (perhaps buffering for low noise operation) but also final row amplifiers acting as conditioners of the sampled signal. Sampling subsequent pixels connects these amplifiers to each pixel. Saturated pixels will lead these amplifiers to their physical limits and moving from a saturated (overcharged) pixel to a contrasty dark pixel (with very low charge) does not mean that the amplifier will be able to respond linearly as a perfect follower. It will overshoot, meaning it might read smaller values than the real ones, depending on the speed that we try to sample the pixels. There is always a trade-off between speed and accuracy of an amplifier, something decided at design time of the circuit. When seeking high accuracy and linear operation this amplifier must have the time to return to normal operating range and respond, and also feedback is chosen accordingly. Perhaps the "need for speed" is today’s must but it does not come without sacrifice.<br>
This is the problem with these black pixels, and if sampling and timing values are incorporated in the firmware files, something can be done through a new firmware that distinguishes movie sampling mode and high quality photo sampling settings. <strong>Until then, a user in demanding photos should use as low amplification as can be used (low ISO setting). This will also lead to high dynamic range and smoother tones. Higher amplification (higher ISO) will create more pronounced black pixels.</strong><br>
Noise reduction is the technique of subtracting a dark (unexposed) image of the same duration from the light (exposed) frame. In this case the black pixel effect could be even worse because of errors in mathematics of an exposed pixel being darker than the corresponding dark frame's pixel. <strong>Avoid noise reduction when shooting ultra high contrast shots and really want to avoid black pixels.</strong><br>
I did try to explain it as simple as i could. Hope this helped.<br>
5d mark II strange black dots?
in Canon EOS Mount
Posted
<p>I noticed this report from the first day it was posted, but hesitated to reply. Being an amateur astronomer for many years and also an electronics engineer by profession I may be useful on this issue. It is not something new in cameras and is found on most cameras used on demanding applications with ultra high contrast as star fields on an almost dark background. Oversaturated pixels might lead to neighbour pixels with abnormal values in the direction that pixels are sampled. The first time I faced this issue was on a special monochrome camera made for astronomical photos back in 1998. Trying to use DSLRs for astro imaging I have noticed this behaviour even at the time of D10. <br>
It is a normal behaviour for a ccd or cmos camera that has to overcome saturated pixels while sampling sequential pixels. Above all emotional thoughts and expectations, these marvels of technology cannot escape basic troubles of electronics. I will try to be as simple as I can but not too much as not to miss the meaning. The photons captured by a single pixel (any kind of photo sensitive device used) result to a charge stored on this insulated device. This charge is proportional to the amount of photons that have struck the sensitive area until the limit of storage (known as ‘well capacity’ normally measured in electrons), where voltage has the max value and result in a saturated pixel reading. Today's pixels are insulated to preserve the charge until the processing circuitry is able to read the value. Older versions of image sensors had leakage of charge to their neighbour pixels when oversaturated, usually in the row that they were wired and created streaks on the image with saturated pixels, an effect known as blooming.<br>
The stored signal (el. charge) has to be read by an analogue to digital converter but at first has to be buffered and amplified. These circuits take care of the conversion of the stored charge to a signal at levels usable as the input of A/D converters, but also take care of the required amplification as advised by user's ISO setting. Canon sensors are advertised to have amplifiers on each pixel (perhaps buffering for low noise operation) but also final row amplifiers acting as conditioners of the sampled signal. Sampling subsequent pixels connects these amplifiers to each pixel. Saturated pixels will lead these amplifiers to their physical limits and moving from a saturated (overcharged) pixel to a contrasty dark pixel (with very low charge) does not mean that the amplifier will be able to respond linearly as a perfect follower. It will overshoot, meaning it might read smaller values than the real ones, depending on the speed that we try to sample the pixels. There is always a trade-off between speed and accuracy of an amplifier, something decided at design time of the circuit. When seeking high accuracy and linear operation this amplifier must have the time to return to normal operating range and respond, and also feedback is chosen accordingly. Perhaps the "need for speed" is today’s must but it does not come without sacrifice.<br>
This is the problem with these black pixels, and if sampling and timing values are incorporated in the firmware files, something can be done through a new firmware that distinguishes movie sampling mode and high quality photo sampling settings. <strong>Until then, a user in demanding photos should use as low amplification as can be used (low ISO setting). This will also lead to high dynamic range and smoother tones. Higher amplification (higher ISO) will create more pronounced black pixels.</strong><br>
Noise reduction is the technique of subtracting a dark (unexposed) image of the same duration from the light (exposed) frame. In this case the black pixel effect could be even worse because of errors in mathematics of an exposed pixel being darker than the corresponding dark frame's pixel. <strong>Avoid noise reduction when shooting ultra high contrast shots and really want to avoid black pixels.</strong><br>
I did try to explain it as simple as i could. Hope this helped.<br>
Yiannis</p>