Canon T4I - Unwanted Exposure Compensation when Shooting Video

[MeTheDinosaur]

I purchased a Canon T4I to shoot videos, which require a consistent exposure, but despite my best efforts the camera is automatically adjusting my exposure. It does so in the middle of shots, which looks very bad. This happens whenever I zoom in. The camera takes a few seconds to adjust the exposure, resulting in an alternating over-then-under-exposed shot before settling. This is very noticeable and I would like to eliminate it entirely.

I am using the 18-55mm kit lens. When the lens it set to an 18mm focal length, the widest apeture is 3.5. Set to 55mm, it's 5.6. I understand that if I'm shooting with an 18mm focal length, aperture set to 3.5, and then zoom to 55mm, the apeture will automatically change to 5.6, thus affecting the exposure. That is not the issue.

The issue is that if I have my aperture set to 5.6, when I go from 18mm to 55mm, the aperture remains consistent throughout, but the image still tries to compensate for the now darker frame.

I do not want it to compensate. I have the camera set to M, the aperture remains the same, the shutter speed remains the same, the ISO remains the same, and I have the Auto Lighting Optimizer set to off. There's an option for the Auto Lighting Optimizer, "disable during man expo". Whether this option is checked or not make no difference. I see the effect regardless.

Here is an example of the effect I'm talking about: example.

And here you can see my exposure settings while taking the video: example.

Here are the settings I used when shooting the example video. Note that these remained consistent. None of these values changed, as evidenced by the second video I posted.

Shooting mode: M

Auto Lighting Optimizer: Off (and the "disable during man expo" option was checked)

Shutter Speed: 30

Aperture: 6.3

ISO: 6400

White balance: Tungsten light

I have looked through the manual and cannot find anything else to resolve this issue. If anyone knows how to prevent this from happening, I would really appreciate it.

[Tim]

Greetings to you, MeTheDinosaur.

Zooming the lens during movie shooting is not recommended.  Zooming the lens can cause changes in the exposure regardless of whether the lens' maximum aperture changes or not.  Exposure changes may be recorded as a result.  The one exception noted in the manual is the EF-S 18-135mm f/3.5-5.6 IS STM lens.

This information is referenced on page 176 of the EOS Rebel T4i's instruction manual.  It's printed at the end of the Manual Exposure Shooting section, under the 'Notes on Movie Shooting' heading. 

[MeTheDinosaur]

What specifically is causing these changes in exposure, if not Aperture, ISO, or shutter speed? Does it have to do with the mechanics of the lens or the camera? Or is it a software function? Because I assume when changing to a different focal length, less light may be getting in, thus the frame gets darker. But as evidence in the first video I posted, the frame then gets too bright, before settling, which seems to me like it's the camera's software changing a value that I have no control over.

Is there any way around this, besides not zooming, or purchasing that lens? If not, what exactly is changing the exposure?

[Mykolas]

MeTheDinosaur,

Even set to f/5.6 constantly, as you zoom the lens, the aperture in the lens still needs to adjust as it moves through the zoom range.  This adjustment can lag behind zoom operation, resulting in exposure changes until the aperture is able to settle back to its set value at that focal length.  There are rare exceptions to this, the one being with the EF-S 18-135mm f/3.5-5.6 IS STM lens that Tim mentioned above.  The EF-S 18-135mm f/3.5-5.6 IS STM lens's aperture adjusts on the fly while zooming, but the aperture in the vast majority of lenses will not.