I've had this T2i for a couple of years now, with Magic Lantern installed almost a year ago. Just got a Tamron 28-75 f2.8 lens for it, was shooting second camera in a big, produced studio interview, and watching the footage now I see that my locked down, fully manual camera is fluctuating in its exposure. That is, every so often, the image slowly gets a little brighter, then darker, then brighter again. It's seemingly random and it's subtle- it's never a jump down a stop or anything, it's always a smooth ramp up or down, but it's definitely there, as if I had some auto gain turned on somewhere. Cam A which was a P2 Varicam does not have this, so it's definitely not a light on set causing the problem. This, of course, renders my camera pretty useless, but I can't figure out where the problem is. The lens is new, everything else is the same, but could it really be the lens? Is there a magic lantern setting I am skipping over that does this? I actually noticed this on the previous shoot as well (with the same lens) on the actual camera screen and had thought it was something to do with the LCD screen, not the video itself (I don't have a reference monitor). Which, of course, is clearly not the case.

I know I need to do some more tests with different lenses, etc. but would love to hear if anyone has experienced this...

Any thoughts? ISO, f-stop and shutter were all fixed- shooting 1080 / 24p @ 50th shutter speed...

Hi David,

Try turning off the "Auto Lighting Optimizer" and "Highlight Tone Priority" if they are on.

I've had this exact same issue with my 60D.

I'm shooting with a lowel light kit, basic three light interview setups. I've shot with the 50mm Canon, and the Sigma 24-70mm EX. My ISO has been between 100 and 320, mostly at 160 or 320.

There is a very very slight but noticeable shift in the exposure. I shot the first interview with a ceiling fan on low speed, so I thought maybe it was just the white blades reflecting some stray light. But I had the same problem with an interview I just shot.

I hope shutting off the Auto Lighting Optimizer and Highlight Tone Priority options works, but I won't be able to give it a whirl until my next interview.

Hi David,

Try turning off the "Auto Lighting Optimizer" and "Highlight Tone Priority" if they are on.
This, if you haven't already. EVERYTHING that can potentially fiddle with exposure behind your back must be disabled. This can be a pain if you switch back and forth between video and still.