Digital zoom, aliasing, and lense choices for film

[Roman]

Hey,

Using a 600D with the APS-C sensor.

My understanding is that the sensor in the camera is 18 megapixels, 5184x3456.

However when shooting video at say 720p, the video resolution is 1280x720. Just under a quarter of the width across.

In order for the sensor to produce the smaller sized image, instead of using 5184 'receptors' across, it might use say every 4th one instead to give an image that's a smaller width, and downwards for height. (Sorry not sure what the right word is for the individual elements on the sensor)

An 18 megapixel photo will be taken wih the sensor operating like this:

(where an asterisk represents part of the sensor in use)

_____________________
|********************|
|********************|
|********************|
|********************|
|********************|
|********************|
|********************|
|----------------------------|

However for video it's using the sensor like this:
_____________________
|*  *  *  *  *  *  *  *  *  *|
|                                    |
|*  *  *  *  *  *  *  *  *  *|
|                                    |
|*  *  *  *  *  *  *  *  *  *|
|                                    |
|*  *  *  *  *  *  *  *  *  *|
|----------------------------|
The problem being that the spacing of the receptors being used is one of the contributors of the aliasing issue.

However, when you use digital zoom, is it effectively working like this instead?

_____________________
|                                    |
|                                    |
|           *******            |
|           *******            |
|           *******            |
|           *******            |
|                                    |
|----------------------------|

If so, would this reduce the aliasing issue?

Does this also mean that there's no quality loss in buying say a 20mm lense and using it with digital zoom instead of an 80mm lense?
Obviously you cant make the depth of field etc work the same.
However it is true, it's definitely worth considering when picking lenses for filming, vs how you'd pick lenses for photos where the digital zoom isnt worthwhile.

Although it would obviously be nicer to have say a 35mm and an 80mm lense, I'd rather spend $$$ on a good 35mm (or there abouts) prime and fake it as an 80, rather than the same money on two lesser equivilents if the 35mm is what I'd be using 90% of the time.

My favourite lense that I have  is a Sigma F1.8 28mm prime, thinking I'll have a play with it tomorrow using the digital zoom and see how it works out, might be able to wring some extra milage out of its usefulness.

I wonder what a particular lense becomes an equivilent field of view angle to, when using 5x or 10x zoom.

[JasonATL]

Roman,

You are correct. This is why I so strongly advocate a 600D over a 650D or even a 60D or 7D, if video is the primary use of the camera. The digital zoom feature on the 600D is simply great to have when you run into a situation in which moire will be a disaster. It does, as you point out, also tilt in favor of having a very good wide lens. However, as you point out, there is the depth of field cost here.

See this video:

[Roman]

Okay, great! That's awesome.

So yeah, although depth of field increases compared to a longer lense, it means it could work well shooting with say an F1.2 or F1.4 wide open, as at a distance the depth of field opens up slightly beyond the width of a piece of paper.

So a bit more ammo towards my planned wide/fast lense purchase. 

One other question, what contributes to the rolling shutter effect?

It cant be that one receptor starts exposure, finishes exposure then moves to the next one across, then down to the next row...

As if so it would be impossible to set exposure times to say 340 degrees.

So would a smaller or larger exposure time affect it at all?

Just trying to get my head around what it's actually doing.