However, this notion is heavily contradicted in fiction. Many sub-light characters including Superman still appear invisible to human eyes even though the observers are quite far away from him.
While light experiment are true, there are still other factor to consider. Human's reflex response to visual stimuli is 18-300 millisecond. The camera is 100,000 times faster, let say most human have 150 millisecond reaction time, then the camera can see event happening in 0.0015 millisecond, that's 1.5 microsecond, and we know to observe something traveling at the speed of light required femto photography, this require few nanosecond to picosecond of precision to see light moving. That alien camera is far thousands of times slower than what it takes to see something moving at light speed.
And by the way, human can process about 12 images per second, the alien's camera can process 1.2 million images per second. But surprise, the fastest camera our real world have can process an astonishing trillion images per second! That camera is in MIT by the way. We can see photon wave moving with that camera, but the wave still move relatively fast. Alien's camera is 833,333 slower than MIT's camera, so it can't observe light moving. That's pretty good enough to infer that Goku can't react faster than light, let alone close to light speed.
I'm in hurry typing this, so expect that my wording can be poorly done, but you can discuss it and I'll break it down.
But Superman doesn't fight and become invisible, he's just invisible when he zips past people. I already addressed this.
And @limpoyzloan: wasn't the camera in the Cell Saga supposed to be fast enough to 'see a solar flare'? Or am I just high? You remember the one I'm talking about, right? The one that was 100,000 times better at seeing than Tien?
Actually, if I recall in the anime the cameraman explicitly said the camera was faster than light, but not fast enough to catch them. Indeed, Goku was fast enough to escape the Imprisonment Light Ball from Frieza in the manga at SS1, which used light that paralyzed, and I believe Piccolo stated that Goku managed to escape from the light before it paralyzed him, i.e. faster than light.
Of course this is all more or less irrelevant, since Toriyama blatantly stated that at SS1 Goku was ftl so could catch the ftl Dragonballs. Maybe not exactly like that, but you get the point. It's a feat. He can catch FTL objects by reacting and putting his hand up, i.e. he's FTL.
I would have posted tons of things like this if CS had done more than compare my calcs to GT, buut he didn't.
Superman can already be truly invisible by vibrating himself, and again, he isn't faster than light either.
There you go, the writer (or the translator) shamelessly give off the wrong science again. 100,000 times faster than human eyes are not enough to see even something moving near light speed. You need to be over billions of times faster than that. MIT's camera is even more powerful than alien's camera, as you can see, and the camera is over 800 thousand times slower than MIT's.
And someone with superhuman durability can catch something moving faster than light without themselves having to be as fast as light. How far is the Dragonballs from Goku? With enough distance, he can react first and catch the ball in time.