It was kinda humiliating that after almost 5 years of photography (point&shoot camera for 2 years,advanced point&shoot 2 n half years, 1 month DSLR) that I discover about this function - perphaps previous P&S camera couldnt perform significant difference worthy of notice.

Basically there are 2 types of sequence in flash photography:
1) First curtain (normal) flash
2) Seconf curtain flash

In first curtain (normal), below is the sequence of events:
Shutter opens --> Flash shot --> Shutter opens for amount of time determined (i.e 3 seconds) --> Shutter Close

In second curtain:
Shutter opens --> pre-flash --> shutter opens for amount of time determined --> Flash shot --> Shutter Close

The major difference is when the Flash is shot, in the case of second curtain the flash is shot right before the shutter closes.

Situations you might find useful (or in pharma terms = indication):
Ever experienced where you and your friends are in a low light cosy environment where you wanted to capture the ambience but at the same time have your subject staying sharp? Normally under such circumstance, there are several solutions that most photographer would use:
(a) Set longer shutter speed
- but that would mean you are prone to blur and need to keep your camera and subjects still.
(b) Set higher ISO
-You compensate the quality of the image (high noise level), even though the best DSLR/lens can provide significantly still image at very high ISO with little noise, but you may have problem freezing moving subjects under very low light condition.
(c) Set bigger Aperture (smallest possible F value, i.e F2.8)
-It helps that with bigger aperture, the film (or photodiode to be precise) absorbs more light hence minimise blur with faster shutter speed, that that would mean compromising depth of field (imagine taking photos in large group with varying distance from camera).

Don't have L lens therefore biggest Aperture was Av3.5, even though ambience of background is captured with relatively still subjects, the resulting small depth of field (DOF) costed people from behind Joon Hun's distance to be blur even though they were standing still. This is because the focal point was at the girls (evaluative focus-not I ham sap purposely focus them) and DOF starts from there - resulting people further from them to be blur.

(d) Tripod
-Definitely the best solution, but presents mobility restrictions
(e) External Flash
-This is also the best solution and offers mobility. Point the flash towards the ceiling and let it bounce off your subject/ diffuser/ reflector to sufficiently light your subject but at the same time not overexposing your background with flash flight. Cost is the main problem-not everyone can afford a good external flash.

The cheaper solution? 2nd curtain flash - it gives you the ambience of the background and a sharp image of your subject (i.e good when photographing dancing couples under low light condition). I will show you what I meant:


2nd curtain flash. Tv 4 seconds, Av 4.0, ISO 100.

Notice I moved my camera AND finger around a lot (background blur) but the subject (finger) remains frozen.


2nd curtain flash. Tv 4 seconds, AV 4.0, ISO100. Notice that things that emit/reflect light has a trailing effect while things that don't stay still and sharp (because the flash shoots just before shutter closes, the strong flash light had masked any handshake/trailing/movement pre-flash that would otherwise result in a blur).


Kelantan mango sticky rice "fast" delivery.
Tv 6 seconds, AV5.0 (approx), ISO100.


Tv 6 seconds, AV5.0, ISO100

Notice my hand is sharp but lack opacity (like ghost) because when the shutter opens there were few seconds that my hand wasn't at where it is at the end of the shutter.


Metamorphosis-from small to big


Shutter speed was opened for 3-4 seconds with me spinning 360 degree.

Notice the subject is still very sharp, the faint background relatively sharp with ambience intact.

This is useful when photographing moving subjects in low light condition with the ambience retained.


No tripod. AV3.5, Tv 0.6 sec, ISO1600 (max variables settings). Photo taken with the photographer raising the camera to the higest point her hands could reach (whom you can see is relatively short :p), but the subjects are still blur - a situation that could be addressed with 2nd curtain flash (even though the background would still result in slight trailing lights)

I was in Zhu Hai 2 weeks ago (which essentially was the worst trip in my entire life due to lack of research&circumstances). What made the trip "worser" is the unfriendliness and over sensitvity amongst the locals towards foreigners. Everywhere I went they forbid taking photos- from the supposedly globalised outlets of KFC to local businesses of a 珍珠奶茶 stall.

People would just come to you warning you rudely that photography is not allowed, I mean come on I am not even taking photos of their designs/products/stall interior ok - even just from the streets they would have been giving you the sacarstic stare like discriminative Aussies.

Soon enough along the trip, I found out the reason they forbid photography is because they fear people with dslr make potential kenny sia like reporters/media that defames/ make a joke out of the way they operate their business - wtf ...-_- It doesn't make sense because this is such a negative approach towards improvement. If people were to defame you, then all the more more caution should be taken to eliminate unhealthy practices, besides when a tourist takes photo most of the time it probably meant they found something interesting/ memorable in the locals! 

I was also sick of the tour guide that spat saliva when he was speaking -whom also conned us into going dodgy acupuncture outlets (cuz they don't seem genuine), silk and pearl outlets - all too typical of china trips, despite us demanding just to rent a van to tour around the city. If the tour guide was more honest and the culture more foreginer and photography friendly, I would have sang much praises about it -but their very core culture of not wanting to defame themselves have turned against itself and sickened me.

Was very glad that I finally made it to Macau eventually after that, where photography is allowed (except in casinos of course) and people don't give tourist with dslr weird unfriendly looks. the only regret is while in Macau we experienced scale 8 typhoon. Overall Macau is very tour friendly (more pictures to come), everywhere is pedesterian friendly and within walking distances, one weakness though - a number of Macau people are just as rude and impolite at similar rate as in HK. No offense to my HK friends of course ;)