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As an example, canon 6d full frame has the highest resolution of 5472 x 3648 with a pixel aspect ratio of 3:2 . As this is the highest I assume this is utilising the full size of the sensor.

If the canon 6d is recording video the highest resolution is 1920 x 1080, with a diffrent aspect ratio of 16:9. Does this mean it is cropping the full frame sensor(in this case cropping the top and bottom for a more rectangular sized sensor field of view)?

if this is the case, then will using a low resolution with the same pixel aspect ratio (3:2) as the highest resolution be a way to utilise the full size (field of view) of the sensor?

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    Yes it is. Yes you can. – Michael C May 15 '14 at 10:44
  • Thank you very much Micheal Clark! I thought this may be the case but is is very helpful to know for the purposes of a current project. – sen May 15 '14 at 10:45
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    note that aspect ratios are not measured in pixels. The term "pixel aspect ratio" is reserved for describing non-square pixels. – Matt Grum May 15 '14 at 10:59
  • Oh I was wondering about that. I thought I may have used the term unnecessarily/incorrectly but was not sure. Thanks Matt Grum! – sen May 15 '14 at 11:22
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"As an example, canon 6d full frame has the highest resolution of 5472 x 3648 with a pixel aspect ratio of 3:2 . As this is the highest I assume this is utilising the full size of the sensor."

Yes, and no. The actual sensor is slightly larger than the effective highest resolution. The sensor is 21 MP while the used area is 20 MB (source). That means that almost the full size is used.

There are a few pixels at the edges that are not used. There can be several reasons for that, for example that the sensor doesn't have the exact 3:2 aspect ratio, or that the extra pixels are used to adjust the used area to better fit what you see in the viewfinder.

"If the canon 6d is recording video the highest resolution is 1920 x 1080, with a diffrent aspect ratio of 16:9. Does this mean it is cropping the full frame sensor(in this case cropping the top and bottom for a more rectangular sized sensor field of view)?"

Yes.

Although there might be a few more pixels available on the sides of the 3:2 area, they are far from enough to reach the 16:9 ratio. It's likely that the camera doesn't use the pixels outside the 3:2 area and just crops the image for other aspect ratios.

"if this is the case, then will using a low resolution with the same pixel aspect ratio (3:2) as the highest resolution be a way to utilise the full size (field of view) of the sensor?"

Yes.

  • This makes everything very clear. Thank you for the additional information! – sen May 15 '14 at 11:30
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    Some of the "unused" pixels are masked and used to help set the black level and others are use to provide data for demosaicing the adjacent pixels that are "used" when converting the monochromatic luminance values form the sensor readout into colors. – Michael C May 15 '14 at 11:57
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Yes, only a portion of the sensor is used for recording video (or photos) at a different aspect ratio than that of the sensor. This is done so that you can use the same lenses for video as for photos. By video standards, 1080p should be 1920 by 1080 with square pixels. The physical pixels on the sensor are square, so we can't change the pixel aspect ratio (which refers to the actual shape of the pixels.

This means our only options to get a 1920 by 1080 image are either to crop our sensor or use an anamorphic lens. Anamorphic lenses are a class of lens that do not resolve an image equally in both direction, but instead squeeze a wider width in to a smaller space. This was originally used for shooting anamorphic wide screen on traditional film, but there is no reason the same technique wouldn't work on a DSLR if you get it to make use of the extra sensor space.

There isn't a whole lot of advantage to doing this though. You might be able to catch a little tiny bit more light for low light performance, but you'd probably lose more quality in the anamorphic optics and having to use custom firmware to handle shooting anamorphic video. You couldn't simply capture all the data and post process as the data rate that the camera can maintain for video is not high enough to record non-stop full resolution images from the sensor, so it would have to be processed to a 1920 by 1080 image (or nearly that) in camera.

Since the sensor is still much higher resolution than the finished video resolution, the cropping is really not a significant impact on final quality.

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