Freezing action

Trying to stop all motion of a Karate student breaking bricks. I have been

reading all the old search topics. I understand the principle of shooting

manual and using my flash manual. Here's an<a

href=thread,http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00N5P7/>Old

thread</a>. </p>

What I am trying to do is to use the flash or the sync speed to freeze the

action, while not as fast as a water drop I would like to freeze the hand as it

stikes the wood and be able to have some DOF to have the face in focus. I

understand that by opening up or raising the ISO will help, but I would like to

keep the noise to a minimum.<p/>

I'll have some practice time tomorrow. My questions are many but how would you

shoot the shoot if you wanted the DOF of say F8, is it obtainable by going past

the sync speed,Fv or more flash power?<p/>

Equiptment: I have a D2h and would like to use it, I also have a D70s and two

Sb0800 and using a 85 1.8 for glass.<p/>

Ambient: For discussion sake, let's say it's 250 at f/4.

How would you do it?,</p>

Thanks in advance.</p>

Jim

You want your flashes up close for a short motion stopping duration, the power level set for f11 exposure for lots of DOF and to suppress ambient, and lots of luck for pushing the button at the right time.

You can't go past the sync speed,1/250 I assume, you just need lots of short duration flash power.

Thanks Bob, I also have axcess to 2 pocket wizards, if needed. I would like to stay on camera, for fear of flying rocks.

I think its possible with the equipment listed, but I am not 100% sure.

 

first, if you go over 1/250, you'll end up with multiple flashes from the high speed sync, so you'll end up with not great results... 1/250 is a good idea.

 

second, if you just put the flashes in manual at 1/16 power and point them right at the subject you might be okay. You may point one at the face, one at the brick and see how that looks.

 

According to this website: http://www.scantips.com/speed2.html

here are the sb-800 durations:

1/1050 sec. at M1/1 (full) output

1/1100 sec. at M1/2 output

1/2700 sec. at M1/4 output

1/5900 sec. at M1/8 output

1/10900 sec. at M1/16 output

1/17800 sec. at M1/32 output

1/32300 sec. at M1/64 output

1/41600 sec. at M1/128 output

 

The trick to the whole thing is to make sure the flash exposure is at least three stops hotter than the ambient exposure so that you don't mix in 1/250 exposure to the whole thing.

 

According to the pictures on that website, 1/128 power was significantly better for stopping motion on a milk drop than even 1/64 power, so the less ambient light the better, the higher the ISO within reason the better, and the closer you can get to the subjects with the flashes the better.

 

I hope that helps and good luck!

(1) If having enough depth of field is a major issue, then light fall-off may well be a big issue too. In other words, you may have everything in focus, but the near stuff will be substantially brighter than the far stuff.<P>

 

(2) You will never stop fast action with the x-sync speed of the shutter. The fastest x-sync on any shutter (Hasselblad H-series, I think) is IIRC 1/800 s. From shooting little league baseball, I can tell you that a ball shot at 1/1000 s still shows appreciable motion effects (although for baseball, I like some motion effects, and usually shoot between 1/500 s and 1/1000 s).<P>

 

(3) You <I>may</I> stop action with the flash duration. The x-sync may be, say (I don't know) 1/250 s in your D2h, but the flash duration may be, depending on how much power is needed, say, 1/10,000 s. So the shutter is open 1/250 s but the flash is firing only 1/10,000 s, or 1/40th of the time the shutter is open. Under those circumstancs, <I>if the contribution of ambient light is negligible</I>, then the flash duration can stop motion.<P>

 

(4) Various high-speed "sync" features (i.e., flash with shutter speeds faster than X-sync) on many modern dedicated flash systems are rarely the answer to stopping motion. Because the shutter is never fully open (instead a slit runs across the sensor or film), the flash actually fires a series of pulses, tending to create multiple ghosts of fast-moving subjects. Also, high-speed sync greatly reduces the maximum flash output.<P>

 

So to your example: If ambient is 1/250 s at f/4, and that isn't close to fast enough to stop the action, you need to use the flash duraction to freeze the action. To avoid ghosts, you have to make the ambient light's exposure relatively inconsequential--try maybe 3 stops below full exposure. So if your camera will x-sync at 1/250 s max, manually set it to 1/250 s at f/11 (which is three stops below 1/250 s at f/4)--at this point your flash would provide about 89% of the illumination. Remember that, as a practical matter, only aperture affects the flash exposure, while shutter speed does not.<P>

 

Here's where things can get a tad tricky. If you have to set the flash's exposure manually, then you need to use the old Guide Number approach. Suppose your ambient exposure of 1/250 s at f/4 was at ISO 800, and your flash has an actual Guide Number of 36 m / 118 ft. That's at ISO 100 (assuming the standard way of rating guide numbers), so you get an effective guide number at ISO 800 of 36 m x square root(800/100) = 36 x 2.82 = 102 m (or 334 ft). You will be shooting at f/11 (actually 11.3, so divide the effective Guide Number (102 m / 334 ft) by the aperture (11.3) and you get a subject-to-flash distance of 9 m / 30 ft. If you want to get closer, then you need to throttle-down the flash. How much? Divide the full-power distance by the desired distance, then square the result, then set the flash to that power. If you are shooting at, say, 10 ft / 3 m, take (30/10)^2 = 9, which means you need about 1/9th power--many flashes offer 1/8, which should be close enough.<P>

 

Part of the beauty of digital is that you can experiement and learn almost instantly what works and what doesn't. Try it!<P>

 

By the way, if you don't get the squares and square root parts, and want to learn more instead of mechanistically applying the system, look on the inverse square law.

Dave, thanks for your detailed explanation, now it makes sense. The 3 stops gets rid of the ambient. I'm working on the math and I'll look into the inverse square law.

Happy holidays

Jim

Jim,

 

Simple test - just take a shot without the flash and see it it's "dark as" - if it is, then the ambient light is under control.

 

Stopping down (increasing numbers) will increase your DOF - increasing the ISO will help your flash (but also work against your ambient light issue).

 

I don't know about Nikon, but in the Canon world a normal flash is in the region of 1/1000th to 1/2000th which has pretty good stopping power. Shoot at your X-Sync speed - if you go over it the flash duration increases, and the effective range decreases.

 

Remote triggering is ideal if you can arrange it.

 

Cheers,

 

Colin

Good advice above.<p>

I would fire your second flash bounced into the ceiling which could make up for the light fall-off for the ambient light. Maybe even snoot the flash on your main subject.<p>

Regards,<br>

Kris.