dclark
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 Re: Does mechanical shutter on A7CR or A7CII eliminate LED banding? | |
When using pulsed lighting, the amount of light illuminating the subject during the camera’s exposure interval depends on shutter speed and the synchronization of the shutter with the light pulses. The reason banding is seen is that the synchronization changes from the top of the image to the bottom of the image. The reason the synchronization changes is that the shutter curtains, mechanical or electronic, are moving slowly and the top of the image is exposed earlier than the bottom. This has been discussed quite a lot in the context of the image distortion caused by time aliasing. See for example https://www.fredmiranda.com/forum/topic/1663827/3/.
There are two basic approaches to mitigating banding:
(1) adjust the exposure interval so that a constant amount of light illuminates the subject regardless of the synchronization of the exposure interval relative to the light pulses,
(2) synchronize the camera exposure interval with light pulsing interval.
For example, if the illumination comes from LED lights that are pulsed 120 times per second, if the camera shutter speed is set to 1/120 sec, all the pixels will see one pulse of light regardless of the synchronization of the LED pulses with the shutter opening. That means as the shutter opening traverses from top to bottom of image (from bottom to top of the sensor) the pixels all get one LED pulse of light illuminating the subject. Some pixels get the pulse at the beginning the exposure, then as the shutter slit moves down the image the light from the first pulse goes away and the next pulse provides illumination at the end of the exposure, and as the shutter continues to move down the image the second pulse is earlier in the exposure interval. If the shutter speed is 1/60 sec, 2 pulses of illumination fit in the exposure interval. In between 1/120 and 1/60, the amount of light will vary between 1 pulse and 2 pulses and will depend on the synchronization of the exposure interval with the LED pulses and the width of the LED pulses.
This method is effective even if the shutter curtains are moving slowly.
Sometimes the pulsing of the lights can be fast and the available shutter speeds will not align with an integer number of light pulses. In this case some cameras allow the shutter speed to be adjusted so that an integer number of pulses are included in the exposure interval, and banding is eliminated.
(2) If the camera shutter is synchronized with the LED pulsing or fluorescent light pulsing the banding can be greatly reduced or eliminated. Some cameras can detect the pulsing light and center the light pulse in the exposure interval of the camera when the shutter slit is at the center of the sensor. As the exposure interval moves from the top to the bottom of the image, the change in exposure is minimized. This is useful and effective for shorter shutter speeds that cannot include an integer number of light pulses. This method needs for the shutter transit time to be short enough that pulse that is centered does not move outside the exposure interval as the synchronization changes at the top and bottom of the image. The means a fast mechanical or electronic shutter.
Different camera have different capabilities, settings and terminology, but I believe they all are implementations of the same basic methods.
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