Locking Aperture Setting in Movie Mode - Canon T4i

[MeTheDinosaur]

I recently bought a T4i to use for shooting short videos, some of which will require me to zoom in during filming. I noticed that if the scene is properly exposed and I zoom in, the image becomes darker. This is not much of a problem sinceonly a short amount of footage will be seen before the zoom starts, so I can just expose the image so that it looks fine after the zoom.

The issue is the when I zoom in, I noticed the camera is automatically adjusting the aperture. I'm using the Manual mode, not Shutter Speed Priority. The ISO doesn't change and the Shutter Speed doesn't change, so why is it compensating by adjusting the aperture? Is there any way to lock the aperture setting in Manual mode? Is there is an auto compensation feature that needs to be turned off? As far as I can tell, Scene Intelligent Auto is off, as is the Auto Lighting Optimizer. What am I missing?

[TCampbell]

This depends on the f-stop and lens being used.  

You didn't mention which lens, but the "kit" lens that comes with the camera (if you buy the camera as a kit) is the EF-S 18-55mm f/3.5-5.6.  

A lens normally lists it's WIDEST possible aperture, but notice that the focal ratio for this lens (f/3.5-5.6) is listed as a range.  

This means that the widest possible focal ratio is variable and depends on the focal length.  At the 18mm end, the lens is able to provide an f/3.5 focal ratio.  But as you zoom in, this ratio increases to f/5.6.  

If are using a low focal ratio such as f/3.5, then yes... the focal ratio will increase and the image will get darker as you zoom in.  That's the physics of the lens.  There are better lenses such as the EF-S 17-55mm f/2.8 IS USM.  That lens is able to provide an f/2.8 focal ratio (which is even brighter) and can do so at all focal lengths.  The f/2.8 focal ratio collects considerably more light (the lens is physically larger).

If you set the aperture on your variable focal length lens to the higher f-stop in the range (e.g. f/5.6) then the lens CAN provide that focal ratio at all focal lengths... hence it will not get darker as you zoom... this assumes you have enough light.  

[MeTheDinosaur]

That makes sense. Thanks! The lens is the 18-55mm that came with the kit I purchased. If I set the f-stop to 5.6, it no longers changes when I move between 18mm and 55mm, however, the camera is still adjusting itself. This time however, it doesn't appearing to be affecting the shutter speed, ISO, or aperture.