1st and 2nd curtain flash

Rowee

Rowee

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Mark
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Hi all

I was playing with a my new (used but new for me) Sigma 105mm f2.8 Macro. I do have a ring flash (or light) but it's on loan at the moment, so I thought I'd just go with on board for the sake of trying it out.

I found an egg timer that I thought could be interesting. Below are 3 shots, the first is long exposure (1/4sec) and PP'd. 2 is 1/250sec flash sync 1st curtain, and 3 is 1/20sec 2nd curtain. 2 and 3 are SOOC.

What I am confused about is, why in shots 2 and 3 do the grains of sand look like they are going up. I have used 2nd curtain in the past with relative success. NB - the 2rd shot was taken at 1/20 sec deliberately so I could make sure the flash was definitely firing at the end of the shutter time.

Any Ideas?


#1
sand timer by rowee2, on Flickr


#2
First Curtain Flash by rowee2, on Flickr


#3
Rear Curtain Flash by rowee2, on Flickr
 
Hi Mark,

What you are seeing on each grain of sand is the light output curve from the flash. Even though it looks like an instantaneous event to us, flash light fades over a period of time - a small period of time (say 1/1000 of a second) but long enough for the drops to have moved as the light faded. In fact, if you were to take one of those grains, and rotate it counter-clockwise 90 degrees, and just look at the top half, the curve pretty much *is* the light output graph over time. It makes no difference to the apparent direction of the grains if you use first or second curtain as the light still outputs in the same way: ie it grows very quickly to a peak and then falls off on a curve that gets flatter and flatter before finally tailing off to nothing. First or second curtain sync merely determines when the light starts, and on rear curtain it starts with enough time to peak and tail off before the shutter closes. When the flash light peaked, each grain was in certain position, but then as it fell, the light faded. These grains are a perfect illustration of the *tail* off of light :) It may clips the tails on second curtain.

The easiest way to make them appear to fall, is to just flip the picture. Your image is pretty abstract so you could get away with that. For water, some photographers spray the drops upward from a hose so they look like they are falling. To do it with light (well get flat drops anyway - you won't get a tail-off of any kind as the light is constant) you'll need to use a continuous light and a fast shutter speed (you'll need a very powerful light). It may be possible to design a flash that has a very slow build up, and then and cut-off using an electronic switch, or a mechanical shutter over the light, but I suspect the need is a bit too niche to make it worthwhile :)

Owen
 
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I think Own is right. The flash must have been at full power to have a 'tail' and create blurring. At lower power settings, the flash pulse is cut off sharply, chopping off the tail and eliminating the blurring. You would also get a similar effect if the ambient light was bright, but other evidence doesn't fit with that.

Second curtain sync doesn't work at all above 1/30sec, regardless of camera settings, and to be effective shutter speeds need to be much longer than that, say 1/8sec or 1/4sec, for there to be a significant and visible difference between first and second curtain timings. Plus there needs to be some ambient light to create the blur.

Edit: the usual description of first and second curtain sync is that the flash fireds at the beginning of the exposure (first curtain) and at the end of the exposure (second curtain). That's a good enough shorthand for most purposes, but not strictly true. First curtain sync, the flash fires when the first curtain has reached the bottom of the frame; second curtain, the flash fires just before the second curtain begins its downward travel. Because of this, at marginal second curtain sync speeds, like 1/25sec - 1/15sec or so, it often looks more like 'middle curtain sync' if you follow (but this doesn't apply here).
 
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Good point Richard, and if you go even faster on the shutter - above sync speed where you're getting a moving slit, "2nd curtain sync" (just before second curtain starts to travel) happens *before* 1st curtain sync (just after 1st curtain completes travel).. (so it's pretty meaningless by then) :)
 
Thanks for that guys, that makes complete sense, I'd not thought of the actual flash fade.
I might make this a bit of a project to see how it reacts to different forms of light.

Interesting thought to flip the image, but it might look odd with the shape of the timer.

I'd also like to see how fast the SS needs to be too freeze the sand.

Looks like that timer might get more use than originally planned :)
 
Rowee said:
Thanks for that guys, that makes complete sense, I'd not thought of the actual flash fade.
I might make this a bit of a project to see how it reacts to different forms of light.

Interesting thought to flip the image, but it might look odd with the shape of the timer.

I'd also like to see how fast the SS needs to be too freeze the sand.

Looks like that timer might get more use than originally planned :)
Click to expand...
Could use flash to freeze the sand by shooting in a darker environment.
 
I know what you mean but based on the above theory the ambient light isn't the issue I think.
 
Rowee said:
I'd also like to see how fast the SS needs to be too freeze the sand.

Looks like that timer might get more use than originally planned :)
Click to expand...
What you'd need to do is dial down the flash to get it to burn shorter.
 
As above. However, it's worth pointing out that if you had used a conventional technology (non IGBT) studio flash instead, then turning down the power would have made the fall off of light more obvious, not less.
 
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