Issues with 70d video. Video Brightness changing during shoot, camera not in auto

[MarkStache]

This is a tough one to explain. I shoot cooking videos and I let the camera run throughout the shoot. I keep the 70d in manual mode. My settings never change as I am in my own studio. I shoot 1080p30 All I ISO 400 f5 60. I shoot 2 videos a week. My lighting doesnt change everything is stationary. 
The last 2 months when I import the footage half of the footage is fine but almost always on the 10th or 11th clip the rest of the clips look like they were shot 1 - 2 stops brighter, but the camera settigns dont change. I have been fixing this in post because this is my full time job and I don't have time for gear repairs, but it is driving me crazy! I used to do IT support for a media school so I did all the basic trouble shooting.  Checked all settings to make sure nothing is on auto, checked the batteries, tried different batteries, tried different SD cards, reset all settigns on the camera? Anyone have and ideas? 

[Waddizzle]

It could light flicker at the power line frequency. I would try recording at a different time of day, just as a test.

But first and foremost, buy a backup camera.

[MarkStache]

I would say that is a possiblitliy but I don't have any one specific time of day I shoot. This last shoot was at night, but prior to that was a morning shoot. 

[wq9nsc]

Depending upon the lighting in your studio, it may actually be getting brighter because incandescent lighting responds quickly to changes in voltage and depending upon driver type the same is true of LED.  If you power company is doing various forms of load sharing it is quite possible that the voltage to your house is different at different times and a heavy load in your own home can easily cause a couple percent voltage change and more if there is a poor connection or undersized feed.  As a first diagnosis, if you don't have a volt meter get to Harbor Freight or one of "big box" home stores and log the voltage during your shoots from one of the outlets in the room, preferably on the same branch circuit that feeds your lighting. 

Are you using supplemental studio lighting?  Any chance that your lighting is going into reduced power mode to protect the equipment during longer shoots?  This is a safety feature built into the driver module for high output LED devices.

This is a case where an old fashioned exposure meter would provide a nice check on whether it is the camera or the actual lighting conditions that are creating the fault.

Rodger

[MarkStache]

Thank you for your input. I could totally see how this could be belived to be a lighting issue but the change always occrus on a new clip, never in the middle of a clip or seconds in... it changes when a new clip is made. The 70d shoots for 30 minutes then stops recording. In those 30 minute segments it splits the shot into 4 clips. I think the likely hood that my lights suddenly got brighter the second the camera creats a new clip rules that out. 

[Waddizzle]

It could be light flicker. Is your frame rate a multiple of you line frequency?

Buy a backup camera, and not another 70D.

[MarkStache]

yes I am in the US 60hz I shoot at 30p 

[MarkStache]

so if we compleltly rule out Lighting as an issue. 100% what could be causing this issue in the camera? Sencor malfunction? Over heating? Also I hav ebeen shooting with these exact settings and lights for about 2 years. This is a recent issue, within the last few months. 

[wq9nsc]

If it is happening exactly with clip changes, then it does sound like a camera issue.  

Although it would be tough to see from the clips, you MIGHT be able to closely examine to see if the depth of field is any different between the light and dark clips.  Maybe there is a camera/lens communication malfunction (probably related to heat) where the lens isn't being properly stepped down to your manual setting when you start the new clip.

But the 70D (and pretty much any DSLR or Mirrorless camera) is a poor choice for these long continuous video sessions because they simply aren't designed to reject the level of heat that is involved with sustained periods of video recording.  Still image camera bodies are designed to provide some level of weather resistance which isn't conducive to good heat management while video cameras are designed with much better cooling, generally forced air, and give up some weather resistance in the process.  I use a Canon XF-400 for some video work and I do as Canon suggests and leave the cooling system set to default mode which runs the fan continuously.  The image sensor itself plus the electronics involved in A/D conversion and storage are running at near 100% duty cycle in video mode (higher resolution increases loading) and generate a lot of heat.  You are using a body designed as a still camera with some video capability in a role best suited for a true video camera.

Keep the camera away from any lighting and supply some air flow if possible but realistically given the level of usage you have the wrong tool for the job.

Rodger

[MarkStache]

I just looked at a bunch of clips to see if the DOF changed. From what I can tell it doest appear to be affecting the dof at all. That actually made sence to me as being a possibility, but it doesnt seem to be so. I could see the heat being an issue but this is in an air conditioned house shooting video. I have seen these cameras shoot outside in the dirrect sun for hours without issue. I mean just about an sports photographer propably has a hotter camera than me haha but again that could very well be the issue, def not discounting it.