T3i 3x Crop Zoom Comparison

[Tom Hardwick]

I'm a bit late to this discussion, but can someone tell me what this 3x crop factor is all about? I have a 60D BTW.

[Taky Cheung]

You don't have that feature on 60D. Sorry =)

It's the camera using the inside 1920x1080 pixel from the entire 18MP sensor. In that case, it's a digital zoom by cropping but there is no loss in image quality.

[Lee Ying]

Originally Posted by Taky Cheung

...It's the camera using the inside 1920x1080 pixel from the entire 18MP sensor. In that case, it's a digital zoom by cropping but there is no loss in image quality.

That is probably not true.
I have yet to see people mention this--if 3x digital zoom just crops the center portion then it implies that you are getting one nineth of the amount of light going into your video. The IQ is bound to suffer. Sure, the resolution remains the same but you lose more than three stops of light, which means poorer light sensitivity and less dynamic range.
3x zoom feature is nice to have in good lighting conditions, but there is no free lunch here.

[Taky Cheung]

I have been using this features for the last 2 weddings. If it loses 3 stops of light, I'm sure I will notice. You can go back to the first post to see the pictures. They were shot at the same settings in the camera. The crop zoom picture doesn't look darker.

Or you can think of, in still photos situation. You took a picture with 18 megapixel. Then crop to the center 1920x1080 pixels. The picture you cropped out has the same brightness.

[James Donnelly]

Originally Posted by Lee Ying

That is probably not true.
I have yet to see people mention this--if 3x digital zoom just crops the center portion then it implies that you are getting one nineth of the amount of light going into your video. The IQ is bound to suffer. Sure, the resolution remains the same but you lose more than three stops of light, which means poorer light sensitivity and less dynamic range.
3x zoom feature is nice to have in good lighting conditions, but there is no free lunch here.

I don't think this is right Lee. In zoomed mode, the camera is receiving the same number of lumens per pixel as in non zoomed mode, and the number of pixels contributing light to the image remains the same, or there abouts.

[Lee Ying]

If the cropping doesn't affect light sensitivity there is no point in having a larger sensor beyond the DOF control. The fact that full sensor cams have much better low light performance is well documented. The same has to be true here, at least to some extent.

Originally Posted by Taky Cheung

..Or you can think of, in still photos situation. You took a picture with 18 megapixel. Then crop to the center 1920x1080 pixels. The picture you cropped out has the same brightness.

That's exactly my point, Taky. The picture cropped out is much smaller now, so stretched out to the same size of the video frame, it will have less light per pixel.
Of course, we know that Canon throws away many pixels in video to begin with (under limited processing power). But some day--and I am sure that day will come when chip processing power is up to task---Canon will use all the pixels on the sensor to form video frames. Then the 3x cropping will indeed reduce the light by a factor of 9. My guess is that current Canon algorithm reduces the light by a factor of three, assuming that chip uses line skipping.

[Taky Cheung]

We all know camcorders with smaller sensors perform poorly in low light. It's a well known fact. However, the way this 3X crop doesn't mean it turns the camera into a smaller sensor. In standard mode, the entire 18MP sensor was used then scaled down to 1920x1080 (thus causing the moire pattern). In the crop zoom mode the same 18MP sensor was used too but only the center 1920x1080 is outputted.

Your thinking of the cropped picture is "stretched" to the same size of video frame. It is incorrect. The video frame is 1920x1080. There isn't any stretching at all. It is cropped to 1920x1080 (the video frame size). That's 1:1 and no stretching.

Again, you can go back to the first post to see the screen grabs. The crop zoomed pictures are not darker than the regular pics.

[John Wiley]

Originally Posted by Lee Ying

If the cropping doesn't affect light sensitivity there is no point in having a larger sensor beyond the DOF control. The fact that full sensor cams have much better low light performance is well documented. The same has to be true here, at least to some extent.

There is no light loss with the 3x crop. What it comes down to is each photosite on the sensor having the same sensitivity as the others. No matter which 2,000,000 pixels you use, it is still going to add up to the same amount of light being sampled.

The DSLR's have better lowlight sensitivity not a result of the larger sensor size, but because of the lower pixel density. The balance of sensor size vs resolution will determine the pixel density, and a lower pixel density allows larger individual photosites - this is where the better sensitivity comes from.

Saying a larger sensor has better light sensitivity is only true on an apples to apples comparison - ie if there is always the same amount of pixels and the pixel size stays in proportion to the sensor size.

[Lee Ying]

I understand your points, Taky and John, that is T3i takes 1920x1080 pixels from the 18mp sensor and throws away rest of it no matter what. Only if that is true that 3x crop doesn't affect the light sensitivity. And it seems that this may indeed be the case with Canon DSLRs.
Now it is a very worrisome fact with T3i, and other current Canon DSLRs as well, because it implies that their video only utilizes one nineth of the available signal coming out the sensor. No wonder Panasonic GH2 with only half of the sensor size trounces T3i in terms of noise in its video, since it average all pixels to get the signal. And in GH2 you do see light loss with 2.6x crop.
I say worrisome because it means T3i is not worth buying new at this point as Canon can easily implement what Panasonic has done and improve IQ in T4i many folds in terms of S/N ratio (which means better low light, less noise and more dynamic range).

[Taky Cheung]

Lee, you worry too much. =)