what is 1st/2nd curtain for flash?

i noticed with 2nd curtain flash, when u press the button to take the pic, it flashes 1st time round then again when the shutter closes. But what does it do exactly?

If im going to be doing some low light photography in a small bar, whats the best flash method to use for the standard flash on a 20D?

From the FAQ: http://photonotes.org/​articles/eos-flash/

First flash is metering, 2nd flash is exposure.

a lot to read... but on a general consensus when would you use 1st curtain and when for 2nd?

first curtain fires the flash when the shutter opens, 2nd curtain fires the flash just before the shutter closes. Regular flash photography with 1st curtain and i would use 2nd curtain for more of a "creative" fill.. like, you wanna take a shot of a car driving by at night, using a slower shutter to get the light trails, but you also want to get the car pretty well exposed, use 2nd curtain and you will have the light trails then, just before the shutter closes, the flash fires and youll expose the car also. At least thats what I would do.... Im not sure what the other uses for 2nd curtain are.. anyone care to elaborate?
-zacker-

Zacker, you can use the same technique for photographing many sports objects in order to get a feeling of motion

What happens with the ETTL system is a pre-flash fires (shutter is still closed at this point) to do the flash metering. Then the REAL flash fires. The real flash fires when the shutter is open. In a normal shutter speed situation like say 1/60 you'll never notice that the flash was even two seperate flashes because it happens so quickly. When the shutter opens it's a focal plane shutter which means it moves across the frame in a linear fashion. With this shutter system there are two curtains. The first curtain and the second curtain. Both curtains move across the focal plane at the same speed, but there is a small gap between the two which is how the film/sensor gets exposed to what's coming through the lens. It works alot like your flat bed scanner does only a whole lot faster. When you choose a higher shutter speed, the curtains don't move faster... the gap just gets tighter. Same thing for a slower shutter... the gap just opens up more the slower you go, until you get to that point where the entire frame is expose all at once... then the second curtain is just held until it's time to close it. This makes it easier to control shutter speeds with great precision, and also helps to get those ridiculously fast shutter speeds like 1/8000 and 1/16000. When you sync your flash on the first curtain, the flash happens when the first curtain opens up the frame. When you sync on the second curtain it flashes when the second curtain moves to close it. So first sync... at the start of exposure and second sync at the end of exposure. Now, how does this effect the shots we take? Ok, consider this. The flash has strobe freezing effect on a subject. So if you take a shot at 1st curtain at say 1/4 and your subject is moving across your viewfinder from right to left. Your subject would be frozen and motionless(more or less depending on ambient light levels) at the right, and you will see the motion blur from that point on to the left until the second curtain closes. This can look very odd and almost like the subject is moving backwards. If you did the same shot with 2nd sync... the subject would be motion blur from the right until the second curtain closes and the flash goes off and then it will be frozen. This would give the subject the appearance of moving forward which would be more appealing

thanks forkball... that was informative.
-zacker-

Forkball - great explanation! (A few more paragraphs might make it easier to read though!)

I choose to think of it simply as first curtain fires the flash as early as possible, and second curtain fires it as late as possible. John's description is technically accurate, though.

Normally you use first curtain flash. If you have a long exposure and objects are moving, second curtain flash is used to ensure that you see the moving objects at the end of their trails, not the beginning.


The 'double flash' effect seen with second curtain E-TTL flash is because the preflash and the flash burst for exposure are separated - in first curtain flash they're so close as to be indistinguishable.

David

thanks guys, I think i understand, but guess best thing to do is to go out and test it myself