Flash Freeze

<p>OK. I feel a little stupid asking this question. However, why is it that flash freezes the motion. Also, how much flash is needed to freeze it. Will items less affected behind the main subject also be frozen? <br>

Thanks,<br>

Dan</p>

<p>It is possible to freeze motion using flash because the <em>flash duration</em> is very short. The burst of light from a shoe mount flash could be <em>very, very fast,</em> along the lines of 1/30,000th of a second at it's lowest power or 1/800th of a second at it's highest power (roughly). The trick to freezing motion using flash is to <em>underexpose</em> the ambient allowing the flash to properly expose your subject(s). The more underexposed the ambient is, the more the flash will freeze motion. You need to understand that the shutter speed does not effect flash exposure (you need to say with you Xsync). Again, you could have a shutter speed of 1/15th or 1/200th- that will only effect the <em>ambient </em>light since the flash duration in either case is much, much faster. As to how the "background" motion renders is dependent upon the position of the flash, how close to "correct" exposure you are to the ambient, and the speed of the motion in the background.</p>

I'm not sure how to explain it in technical terms but I'll try for a general explanation. When you use a flash, you have at least 2 light sources in your image, the ambient light and the flash. The ambient light will basically setup the general brightness of your capture. If you add a flash to that, it will add brightness to your subject. That additional brightness is what gives the flash the ability to freeze the action. If you use rear curtain sync, the flash goes off after the ambient light has effected the photo. In other words, it's the last thing that happens before the shutter closes. If you use front curtain sync (I think this is generally the default), the flash goes off first then the ambient light starts to bleed into the image. The bleeding in of the ambient light can muddy the photo (which could be desirable) and NOT freeze the subject. I hope this helps. Like I said, this is not a really technical explanation.

Good luck!

-- Wade

<p>Flash freezes motion because its duration is quite short...depending on the flash usually from 1/1000 second to 1/10,000 second. So little movement occurs during that period that it appears as if it is frozen (most subjects that is). The intensity of the flash isn't what freezes motion, the duration does. The intensity, however, has an impact on how your camera's sensor or film records the image...too little intensity and everything is black....too much and everything is washed out. Your last question is much more complex, because the answer depends on the distance behind the main subject, the relative speed and luminance of the object, and whether or not the flash intensity reaching the background is such that it is recorded by the sensor or film. Many photographers want the background to show motion, and the tricks to do so are too numerous to go into detail here. If you pick up a good book on flash, you will get a lot of information on those techniques.</p>

<p>deleted.</p>

<p>Flash freezes motion because the light produced doesn't last very long. if the objects behind or in front of the subject are also primarily lit by the flash they will be "frozen" as well. </p>

<p>As to how much flash you need to freeze a subject that will depend on the existing ambient light level (and that can be controlled by your choice of shutter speed, ISO and aperture combination) and the duration of the flash. At full power most flashes produce light that lasts anywhere from 1/300 - 1/1000 of a second.</p>

<p>Be aware that there are two different official standards for measuring flash duration : t0.5 and t0.1.</p>

<p>For most flashes the t0.1 duration measurement is about three times longer than the t0.5 measurement but more accurately indicates how well la flash freezes motion. Manufacturers generally like to use the t0.5 measurement because it makes their flash look like it has a short duration. The difference is that that t0.1 measures all of the photographically significant light the flash is producing while the t0.5 measurement is only the length of time that the flash is producing 50% or greater of the maximum energy release in the flash. <br>

Both Paul C. Buff, inc. and Broncolor have good information describing flash duration and the differences between the t0.1 and t0.5 measurement methods: </p>

<p>http://www.paulcbuff.com/pcb2009/flashduration.html</p>

<p>http://www.bronimaging.com/faq/index.php?action=artikel&cat=1&id=3&artlang=en</p>

<p>Last year I did a "freeze motion" video experiment with one of my Paul C. Buff Einstein heads: http://vimeo.com/11533678</p>

<p>Photonet member Jeff Lieberman has a series in his portfolio called <a href="../photodb/folder?folder_id=488067">Frozen</a> where he experimented with 1µ second flash duration which essentially gave him a "shutter speed" of 1/1,000,000th of a second. If you were curious about flash freezing action you should check out his gallery. He is kind enough to delve into his technique a bit so it's not just fascinating photographs but an interesting read as well.</p>