Any drawbacks to 2nd curtain sync?

I just got a 420EX for my Elan 7 and from what I've read so far, it

seems like 2nd-curtain sync is advantageous for showing the kind of

motion people expect to see. So I guess what I'm wondering is, why

would I ever use 1st-curtain sync? Are there advantages? Why is 1st

curtain the default setting? Thanks.

If you're not photographing motion, 1st curtain sync is better because it comes so close to the ETTL pre-flash.

1st curtain sync allows you to control the moment when image is captured with the flash. The flash goes off almost instantaneously after the shutter is depressed and is critical for capturing the peak of action.

 

With 2nd curtain sync, the flash goes off after a variable time period from when the shutter is depressed - by the shutter speed. As such it is very hard to capture the peak action. It is, however, valuable for capturing the sense of motion.

The difference between 1st curtain sync and 2nd curtain sync is the way motion is captured. This becomes apparent only for long exposures.

 

With 1st curtain sync, the exposure is made as soon as both shutter curtains are open. It is advantageous when you are trying to capture an expression or an instant of time with flash. Any movement or motion of the subject AFTER the flash fires will only be captured if the exposure is long enough and the ambient light level is high enough.

 

With 2nd curtain sync, given the same conditions as above, the movement BEFORE the flash fires will be captured.

 

So it boils down to the particular thing you are trying to portray in your picture. For short exposures, the picture will look the same with either 1st or 2nd curtain sync. I'm sure that when camera designers first determined how to adapt electronic flash to focal plane shutter design, it appeared logical to have the flash fire as soon as both shutter curtains were open. With more sophisticated camera design, 2nd curtain sync became another option, mainly to capture the lights on moving cars properly as trailing patterns rather than preceding ones, as well as other similar effects.

There's a blinking answer too. The E-TTL pre-flash can cause subjects to blink, spoiling the picture. For most people, 1st curtain sync means the main flash happens before they blink, whereas 2nd curtain could catch them in mid blink depending on the shutter speed. But some (and some animals and birds) have much faster reflexes, so 2nd curtain sync can be preferable, as the blink is probably already over whereas 1st curtain would show it.

On top of all the good advice already given here, 1st curtain sync can be preferable when taking pictures of moving cars at night (headlights on) from the front, as you may want the trails from the headlights to be in front of the car, not over it.

Thanks for the very helpful answers!

With 2nd curtain sync and AV mode under dim lighting, the shutter speed may be so long that with people pictures, your subjects may see the pre-flash then break their pose or blink as the exposure is being made.

Jean-Baptiste Queru's observation (1st curtain sync can be preferable when taking pictures of moving cars at night ... as you may want the trails from the headlights to be in front of the car, not over it) is well illustrated on page 29 of the Speedlite 420EX instructions (English version).

 

As Rahy Moreno noted, using 2nd curtain sync and AV mode under dim lighting, the shutter speed may be so long that in portraits, your subjects may see the pre-flash then break their pose or blink as the exposure is being made. However, with the 420EX there is no preflash, but rather a red AF assist light; less likely to be confused with flash or to cause blinking. When using Av mode to capture ambient lighting in portraits, I find 2nd curtain sync preferable, since subject are more likely to blink or move when the flash fires.

I think the pre-flash Rahy referred to is the E-TTL pre-flash that is used to gauge flash exposure. My impression is that this pre-flash (as opposed to the AF assist blinking that the Elan 7's built-in flash does) would be close enough in time to a 1st-curtain flash so that it almost looks like one flash, but if there were a longer shutter speed with a 2nd-curtain flash, then the pre-flash and the illuminating flash could be far enough apart to cause blinking or breaking pose.