Abrupt change in camera exposure

Stewart McLain · October 1, 2023

So I'm knee deep in shooting a short film right now. Last night we were shooting in an alley with intermittently spaced extant lighting. Lots of very dark areas with sparse light spots. I added light from a Godox VL200 at 100% from a rooftop pointed where the action would take place. It looked like a lot of light to the naked eye but following the meters on my camera (GH5) and monitor (Shinobi) I still wound up pushing the ISO to 1600. This is something I've never done but I just read an article called "How I learned to shoot at 1600 and not care" or something like that, so I thought, what am I so afraid of? It seemed like I was getting a good enough nighttime look on my monitor so I went with it. I figured my eye just isn't trained enough estimate the difference between what I see and what the camera sees. But after shooting four or five set ups, I decided to change lenses and BAM - with the new lens, the camera was suddenly saturated with light. Freaked out, I put the original lens back on and it's now the same thing. So much light. Same f-stop. Drastically different exposure. I finally dropped my ISO to 320 and stopped down to match the waveform readings I remembered from my first shots.

What could have caused this? The only things I can think of are:
a) There was debris on the sensor that got shaken loose when I changed lenses
b) It had something to do with the MFT to F mount adapter I'm using. It's one of those speed boosting adapters.

Has anyone else had a similar experience or maybe some insight into what might have happened? Any help is appreciated.

Stewart McLain · October 2, 2023

Correction:

From these forums. Laughing at myself a little.

Steve Switaj · October 3, 2023

Well, you mentioned 'alleyway' and 'extant lighting'

Maybe the lighting was some type of discharge lighting or LED's and was strobing at the power supply frequency. That's not uncommon in the lamps used in 'high efficiency' area-type security lighting.

If you were in a place with a 60Hz supply, that would mean that you had a 60 or, more likely, a 120Hz strobe, which is a nice multiple of a 24, 30 or 60 FPS frame rate.

A 60 Hz strobe should be noticable, but a 120Hz strobe might not be visible to the human eye. But to a camera that exposes each frame for a few millliseconds, it would make a difference.

If you were shooting with a narrow enough shutter angle (like you went up to 1600 ASA, and the camera compensated exposure by reducing the exposure time) you could find your exposure time getting smaller than one light cycle time and your image sampling falling in and out of phase with the lighting.

Sometimes you might start recording and find yourself exposing in phase with the lights while they were on, sometimes you would start recording and this time you were out of phase and exposing when the lights were off.

Edited October 3, 2023 by Steve Switaj

Karim D. Ghantous · October 4, 2023

Were both lenses used with a Speedbooster?

Stewart McLain · October 5, 2023

@Steve Switaj That's an interesting thought. I'm pretty sure the street lights were LED. I don't think the shutter angle changed though. I kept the shutter speed at 1/50. Also, it seems unlikely to me that I would have been consistently on one side of the strobe effect for 30 shots or so and then consistently on the other side of it after changing lenses. But I suppose stranger things have happened. And either way, I appreciate the info about the possibility of being affected by 120hz strobing because I didn't even know to be on the lookout for that.

@Karim D. Ghantous Yes.

Adam Froehlich · October 6, 2023

What lenses are you using? Fully manual, or electronically controlled? I've had this happen before on Canon still lenses with electronic iris control, it was a faulty EF to E adapter. Some sort of miscommunication between the adapter and lens that defaulted the aperture to around F8, even though it read different F values on the camera body.

Re-seating the lens always took care of it for a while (until it would happen again within an hour.). This seems to align with how you described your experience. I ended up just needing to clean the pin contacts on the adapter.

Stewart McLain · October 9, 2023

@Adam Froehlich Well, the lenses are fully manual so I don't think that can be the case.

Honestly, I'm starting think this must be a case of user error. The way the adapter works, the iris on the lens has to be fully stopped down for the iris control on the speed booster to work. I know I felt sure at the time that I checked to make sure that the iris control on the speed booster was engaged properly but maybe I just didn't. This has definitely been a "work as fast as possible with a tiny crew" project so it's not outside the scope of possibility that I just screwed up. On the plus side, while there's a little more grain than I'd like in some of the shots, they're cutting together just fine.