How does the Canon 7D generate a video image? at DVinfo.net

Bob Prichard · October 16th, 2010, 05:32 PM

I see that the Canon 7D has a pixel resolution of only 2.1 megapixels for 1080p Full HD mode, 0.9 megapixels in 720p HD mode, and just 0.3 megapixels in VGA mode.

Since the sensor is 18 MP, how does the sensor generate these lower resolution images?

The image on the rear LCD does not look smaller or more cropped than the image for a still photo.

When I shuttle back and forth between 1080p and 720p, the image size does not change.

Evan Donn · October 19th, 2010, 02:05 PM

It skips lines. I believe it uses every third line on the 5D, not certain about the 7D - but it's part of what causes the aliasing problems.

John Wiley · October 21st, 2010, 09:00 AM

As Evan said, it leaves out alot of the data when reasing the sensor. Because it is not possible to read out the full sensor 24 (or 60) times a second and down-convert on the fly, the camera only reads out some of the lines to put together the image in video mode. Because big chunks are missing from the image, we end up with aliasing.

The only cropping you will see in video mode is to remove the bottom and top parts of the 3:2 image to get a 16:9 frame - unless you are using the 550d which I believe has a standard-def crop mode that only uses the central part of the image.

October 16th, 2010, 05:32 PM	#1
Bob Prichard Regular Crew Join Date: Jan 2005 Location: Tiburon CA Posts: 123	How does the Canon 7D generate a video image? I see that the Canon 7D has a pixel resolution of only 2.1 megapixels for 1080p Full HD mode, 0.9 megapixels in 720p HD mode, and just 0.3 megapixels in VGA mode. Since the sensor is 18 MP, how does the sensor generate these lower resolution images? The image on the rear LCD does not look smaller or more cropped than the image for a still photo. When I shuttle back and forth between 1080p and 720p, the image size does not change.

October 19th, 2010, 02:05 PM	#2
Evan Donn Major Player Join Date: Feb 2006 Location: Philadelphia Posts: 795	It skips lines. I believe it uses every third line on the 5D, not certain about the 7D - but it's part of what causes the aliasing problems. __________________ My latest short documentary: "Four Pauls: Bring the Hat Back!"

October 21st, 2010, 09:00 AM	#3
John Wiley Trustee Join Date: Oct 2008 Location: Byron Bay, Australia Posts: 1,155	As Evan said, it leaves out alot of the data when reasing the sensor. Because it is not possible to read out the full sensor 24 (or 60) times a second and down-convert on the fly, the camera only reads out some of the lines to put together the image in video mode. Because big chunks are missing from the image, we end up with aliasing. The only cropping you will see in video mode is to remove the bottom and top parts of the 3:2 image to get a 16:9 frame - unless you are using the 550d which I believe has a standard-def crop mode that only uses the central part of the image.