How do I turn AGC (auto gain control) off on a webcam?

I want to make a DIY 35mm style digital camera from scratch out of a webcam, a Depth of Field adaptor, and a micro-pc. The only problem is that DOF adapters are very dark, and they cause a webcam to use it's auto gain control (this causes little multi-coloured patches in the dark areas known as "noise"). It's a great idea for a video chat in the dark but nothing professional.

I know that all the webcams I've researched can't have gain turned off via a PC, so surely I can detach something from the PCB of the webcam (I assume there is some sort of gain module inside it). Please Obi-wan Kenobi, you're my only hope.

What can I do to turn the gain off?

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orksecurity6 years ago
Webcams aren't intended for serious videography. They're intended, like cheap audio recorders, and telephones (which also normally use AGC) and radio broadcasts (which are often put through compressors) to capture and transmit an image which is maximally easy to interpret with minimal manual intervention, rather than one which is optimally artistic or optimally scientifically correct.

They also don't put a lot of effort into using good lenses or high-quality image capture chips or many of the other things a "professional" camera would invest in.

If you're trying to do something of "professional" quality, webcams may not be the optimal starting point. If you're trying for minimum cost and just trying to make them a bit less of a compromise, this may be worth pursuing.

tyldob (author)  orksecurity6 years ago
Don't worry about the audio quality, I'll just have a couple XLR sockets that connect to a pre-amp that connects to the mic-in on the micro pc, and the video quality only needs to be a constant frame rate as it is using a DOF adapter, which projects an image onto a 35mm focus screen (the webcam will be focused through a macro lens).

This is more of an experiment, maybe it wasn't what they were intended for, but no one has properly tested exactly what they can achieve if you throw a load of other instructables at them til you get something awesome. That's the spirit of Instructables after all isn't it? Making the unlikely something incredible.

Within the time of writing this post, and answering the replies, I have thought that there is no serious reason as to why this should not work. I think in some time I will throw up some sketches for what I plan to do, and in the near future there will be an instructable about this (providing that it is a success)!
By all means, if you think you can make it do something interesting, go for it!
On the cam's I've got, there is an AGC option in the setup for the device. Try running it in "Mplayer "and see what it says about camera options.

AGC is the camera's attempt to make recongnisable pictures: if the application is light starved, turning off the AGC won't make any difference.

tyldob (author)  steveastrouk6 years ago
In my film course at uni I have been taught differently. AGC must remain off as a rule. That is why we have been taught lighting so that pictures are recognisable and the black areas remain black.

The other problem is that I have not got a good enough quality webcam as of yet (ideally I need one that records at 1080p). Could you recommend me a webcam that does have an AGC control?

Thanks for the reply :)
You're not talking about cameras with big F numbers in "film studies"- movie camera lenses are pretty fast. MOST webcams can turn off AGC in my experience.

tyldob (author)  steveastrouk6 years ago
Duly noted. I will have to throw some other lenses into the equation I think, and maybe use a vibrating DOF adaptor. This should be interested.

I do film production as well, not studies. I'm an astronaut not an astronomer.