I did an event shoot in a fairly dark room, and various attending took a good bit of flash photos. I'm noticing that, when the flash goes off, it looks rather odd, and when looking at it frame by frame, the flash only lights half or so of the image, and sometimes followed by the other half in the next frame, but rarely does it light the full frame at once. It's not a big issue, I just find it odd. Any idea why this is?
Weird flash thing with T3i
- Thread starter pulp
- Start date
Normally this is caused by the shutter speed being faster than the flash synch speed, which is 1/200 sec on the T3i. So you have to keep the shutter speed at that speed or lower. Lower than, or at the synch speed the last curtain of the shutter doesn't start travelling until the first curtain has completed its journey. After that it the whole sensor is never fully exposed at any point in time. The flash burst is much briefer than the shutter speed so captures only the part of the sensor exposed at that point in time. With dedicated flashes there is high speed synch (FP), where there is a long burn flash, and it can be used at shutter speeds faster than the synch speed. I assume you were using a non-dedicated flash. Normally I use the camera on manual to stop the shutter speed going faster than the sync speed with manual non-dedicated flash.
Michael Carter
Veteran
To clarify - SteB's reply is all about shooting still images. SteB apparently missed the mention that the flashes you were observing were from other cameras, not your own. And that you're referring to flashes looking odd frame-by-frame, which is a video issue, not still (nothin' against SteB, I think he just read it too fast...)
Qaz's reply is on the money, however. The way the camera records video is much like how a scanner scans a piece of paper... hummmmmmm and something moves across the paper.
In a DSLR sensor, the data being recorded is fed of the sensor in rows, from top to bottom (or maybe bottom to top, who knows). However, this "scan" is very fast; faster than the duration of some flash gear. So, when you look frame by frame and see one frame that's half (or a third, or a quarter) flash-illuminated... that is the range the data was scanned from the sensor while the flash was firing.
You can "fix" this in post - make a white solid, and align it to cover the frame with the partial flash visible; then fade it out across 2 - 4 frames. This looks like a "movie" flash, and makes it very clear to the viewer that someone just took a flash picture. It doesn't remove the flash, it enhances it to something a viewer will instantly (and subconsciously) understand.
My bad, I missed the "various attending" bit. It' looks obvious when I re-read it. However, it's the same sort of effect because the sensor is read just like it is with a mechanical shutter, so no part of the sensor is being read at any one point in time. Electronic flash can be very brief, as little as 1/30,000 sec. Typically it will probably be about 1/5,000-1/10,000 sec. That's why it only appears on a small bit of the scan. The flash duration looks a lot longer to our eye because it leaves a bit of an after image. A good suggestion with the post fix that I'll have to remember, because that's the way we see flash.