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03/29/2007 09:44:29 AM · #1 |
DSLR newbie question....
Just mucking around with flash, and I tried a few things... On full auto, I took a shot at 1/60s, f3.5. I then switched to full manual and set to 1/200s, f8, and the result looked exactly the same. OK, so the time probably doesn't make much difference, as I've heard the flash is only 1/200s long anyway, but surely the aperture would make a difference, no?
I also tried diffusing/bouncing the flash off a sheet of paper. Why don't all DSLRs allow the onboard flash to rotate? That would be really nice. But I digress... I took the same shot twice, once with just normal flash, and once with a sheet of paper above the lens. Both images were nicely exposed, but the bounce flash one looked much nicer - the paper idea is very cool. Anyway, surely the flash was diminished by the diffusing, yet the exposure was still fine - both shots were 1/60s at f3.5.
So, what's going on here? The flash must be adjusting it's power automatically. I've got a feeling that this is TTL flash at work here, which is really cool, but how is it working?
I wondered if the flash was looking at the shutter/aperture settings and the lens focus distance, but this wouldn't work for my bounce flash test. How did the camera know I was holding a piece of paper there?
I read something about detecting the light coming back through the lens and closing the shutter when it has enough light, but I checked the shutter settings on the shot afterwards, and they weren't changed.
Anyone? I'm a computer engineer, so feel free to go into gory details on how it works. :)
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03/29/2007 10:14:49 AM · #2 |
Well, I should have looked for the answer first before posting.
Wikipedia has entries for TTL and ETTL, and there's plenty of good external links from there. It's amazing what there is to learn if I just look in the right places... :)
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03/29/2007 10:17:38 AM · #3 |
You don't see it, but there is a pre-flash that allows the camera to look at what is coming back and adjust flash power. Since the pre-flash is traveling the same path as the flash used to record the image, it automatically takes into account whether you've bounced or diffused it. This is TTL at work.
Your shutter speed has very little to do with flash exposure. The flash duration is much shorter than even 1/200s. The only function that the shutter has is to control how much ambient light is recorded with the flash. Slower shutter, more ambient light. Shooting with flash but an intentionally slow shutter speed to include more room light is called "dragging the shutter." |
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03/29/2007 10:23:22 AM · #4 |
There must not have been much ambient light to see such little difference in the outcome of a shutter speed of 1/60th versus 1/200th.
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03/30/2007 09:28:15 AM · #5 |
Shot was taken in my kitchen, with the light on. It's not a great light, but still was some. Obviously the flash was doing most of the work in that particular test though, certainly for the foreground. Played around with some more bounce flash today. After having a P&S for many years, the TTL flash is just awesome! I can actually use the flash without my photos completely washing out! The flash performance alone is worth getting an SLR.
Kirbic, yes, I read about the pre-flash. The web-reports said that if you look real carefully, you might be able to notice it, but I couldn't. Too fast for me.
Also, I discovered a new piece of wise advice for myself. When you're bouncing or diffusing the flash with a bit of paper, particularly if it's really dark, before you press the shutter, CLOSE YOUR EYES!! One particular flash shot, taken basically in darkness, I couldn't see properly for about 5 mins afterwards, with the body of the camera burnt into my retina. Most of the flash comes right back at point blank range! You live and learn. :)
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