Turn on camera light but not camera

I'm fairly certain the green light is hard-wired to the camera, there's no way to activate the camera without activating the light, precisely for privacy reasons. If you're really concerned about it, you can always just use a piece of electrical tape to cover the camera (or fold a piece of cardboard over the top of the lid for a non-sticky solution).

Frankly I wouldn't worry too much about the risk of malware. That particular piece of malware (MacDefender) was contained fairly quickly by Apple, it's very unlikely that you could be infected by it now, unless you maintain an unpatched system. There have been a few OS X security issues lately (such as Flashback), and they get a lot of press, but the actual amount of harmful software out there for a Mac is extremely low.

My advice is to keep your system up to date and avoid downloading files from questionable sources. But other than that, don't worry too much, unless you have a highly sensitive job (in which case there should be other people whose job it is to worry about it).

There must have been a group of designers and engineers who created the camera and camera light system – they would surely know and have told others.

John Gruber received an email from one such former Apple engineer:

https://daringfireball.net/2019/02/on_covering_webcams

Security researchers at Johns Hopkins released a paper in 2013 revealing that the indicator lights on Macs released prior to 2008 could be circumvented by software. I linked to this in 2016, wondering if the same exploit was possible on more recent Macs. Here’s an answer I received from a former engineer at Apple who was intimately familiar with the software drivers for Mac webcams:

The original cameras had the problem that the JHU researchers detailed in the article that your linked to. Problem was that the firmware was downloaded on every boot and there was no security/encryption mechanism for verifying it. The part used was fairly common and the firmware was just in RAM (hence the loading after a cold boot), as oppose to flashed.

All cameras after that one were different: The hardware team tied the LED to a hardware signal from the sensor: If the (I believe) vertical sync was active, the LED would light up. There is NO firmware control to disable/enable the LED. The actual firmware is indeed flashable, but the part is not a generic part and there are mechanisms in place to verify the image being flashed. […]

So, no, I don’t believe that malware could be installed to enable the camera without lighting the LED. My concern would be a situation where a frame is captured so the LED is lit only for a very brief period of time.

The still photo problem — where the light only turns on for the instant the image is being captured — is interesting. But I would wager real money that the camera indicator light cannot be circumvented by software on any Mac released this decade.

^{emphasis mine}