When a camera is set to a lower resolution, at what stage is the image downscaled, and does it reduce noise?
Asked 8/23/2017
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On a Nikon D5600, the sensor is 24MP, but the camera can be set to save smaller files such as 6MP JPEGs. At what point in the imaging pipeline is that lower resolution created?
Is the image reduced at the sensor stage, by combining neighboring pixels or skipping pixels, or is the full sensor read first and the image only resized later before writing to the card?
How does the Bayer color filter affect this? Does the camera still effectively build a full-color image from the full sensor data and then downscale it, or can it use a different method at 6MP?
Also, what does this mean for image quality: will a lower-resolution setting reduce visible noise or allow better high-ISO results, or does it mainly just save card space?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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Commonly cameras capture data using the full sensor which gives full-resolution RAW data which is converted into an image and then downscaled. As a matter, if you select to shoot JPEG+RAW, you do still get a full-resolution RAW file and a scaled down JPEG.
Your proposed using every 4th pixel or average of neighboring pixels is unlikely since it does not work well with Bayer filters. At best, you would get an image with heavy aliasing and strong moire. This is a technique which has been criticized when used for video where lower-bandwidth is needed.
Camera makers have tried different things. The case of quarter resolution is easy since the Bayer filter repeats a 2x2 grid of Red, Green, Green, Blue subpixels. To get exactly quarter resolution, a full-color image can be obtained by just using the Red and Blue directly, plus averaging the two Green. You still need to get the same amount of output from the sensor.
Fuji notably implemented a sensor which allowed them to get the quarter resolution image right off the sensor by using a pattern of 45°-rotated colors which they called EXR Color Array. This makes each two subpixels of the same color adjacent and can be read at the same time which reduced noise and bandwidth. The official Fuji page seems gone but here is an article about it.
Noise reduction could be applied either before or after but since reducing resolution also reduces how much noise is apparent, it will be probably done later. Sharpening needs to be done as a last step due to the fact that downscaling makes images softer.
Some cameras also offer quarter resolution RAW. I suspect they do it with a special downscaling to the RAW data as other ways would require additional circuitry on-sensor which would have impact to noise, heat and complexity.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
8y ago
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In general, cameras still read the full sensor and create the image from that data, then make a smaller JPEG afterward. A good clue is that if you shoot RAW+JPEG, the RAW file remains full resolution while only the JPEG is reduced.
Simply skipping pixels or averaging every 2×2 block is usually not ideal with a Bayer sensor, because color information is spread across the mosaic. Pixel skipping can cause aliasing and moiré. For exact quarter resolution, the repeating 2×2 Bayer pattern does make simpler reductions possible, but the camera still has to produce a proper full-color image in some form.
Practically, the lower-resolution setting mainly affects the saved JPEG size, not the sensor’s fundamental light-gathering ability. It does not turn the camera into a better high-ISO camera. However, when an image is downsampled, noise can look less visible because multiple sensor samples contribute to each output pixel. So a 6MP file may appear cleaner than the same image viewed at 24MP, but you could usually get a similar benefit by resizing a full-resolution image later yourself.
So: lower resolution mostly saves space and may make noise less noticeable, but it does not improve the sensor itself.
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