Flash Doesn't Freeze Motion

That's right, flash does not freeze motion. Well, it can freeze some motion, but not a lot.

If you remember from Flash Exposure you actually have two exposures occuring simultaneously when using flash. And the flash exposure is actually a combination of both. 

Here is a photo of a hummingbird using flash at higher power (lower ambient). Note how much wing blur has been recorded. The wing blur is recorded by the ambient exposure and slower flash duration caused by being at a higher power setting.


Photo by Bill Gracey

A flash's ability to freeze motion is based upon it's T.1 time (10% illumination). The T.1 time is effectively the "flash shutter speed." This information is not commonly available as T.5 (50% illumination) times are generally what are given. T.1 is going to be approximately 1/3 of T.5 for a typical studio strobe, or it will be closer to the T.5 time for a speedlight/IGBT strobe.

 

These are the published T.5 times for the Nikon SB 800, one of the fastest flashes available:

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

 

These are the measured T.1 T.5 times

1/1 -- 257.73 -- 825.08
1/2 -- 759.3 -- 821.02
1/4 -- 1615 -- 1797.3
1/8 -- 2979.7 -- 3620.6
1/16 -- 5112.5 -- 7363.8
1/32 -- 7874 -- 13351
1/64 -- 11338 -- 22371
1/128 -- 13661 -- 29674

(**these were not measured by me, and the T.5 times do not match the rated/reported by Nikon. This could be due to differences in methodology or some other factor)


For an SB800 at full power we have an effective "flash shutter speed" of 1/250 which is enough to freeze some motion. But lets say we are using a 200mm lens handheld and we want a "critically sharp" image. For "critically sharp" (as opposed to "acceptably sharp") we need a shutter speed (SS) approximately four times the focal length... so 1/800. At full power the T.1 of 1/250th is not enough. Or maybe we have a subject moving fast enough that 1/250th isn't fast enough to stop motion blur; something like kids running around or a hummingbird.

Ok, but at 1/4 power the T.1 time is 1/1600 which *is* enough...so lets use that right? Well, no.

Another way to look at those fractions is as the amount of flash vs the amount of ambient light contributing to the image for a "correct" exposure. So at 1/4 power it's 1:4; we are four times more dependent on the ambient exposure, and the ambient exposure's ability to freeze motion is limited by the x-sync SS. The highest x-sync SS I am aware of in a modern DSLR is 1/320 for the D800 (2013). Again, not fast enough.

So how can we make 1/4 power "enough" for a correct exposure while not being dependent on the ambient exposure? First, we have to "kill the ambient exposure" by reversing the equation...the flash has to be our primary light source. Seems easy enough, just reduce the aperture or ISO right? Again, no. The problem is, as we reduce the ambient exposure we become more dependent on the flash and that means we need *more* flash power, the opposite of what we want to achieve.

There are a couple of ways to make the flash more powerful without increasing it's output... One is by placing the flash closer to the subject.

Alternatively we could add more flash units to the exposure with all of them set at 1/4 power. That works if you have enough units, or you combine the two; multiple flashes closer to the subject until you get a combination with an acceptably fast T.1 time. This is how most high speed photography (i.e. water drops) is accomplished.

Here is a photo of a hummingbird using two flashes at a lower power setting. Note how much less wing blur has been recorded even though a slower SS was used (180th vs 200th). 


Photo by Bill Gracey

Here are two water drop photos taken at different power settings by moving the flash closer.


Photo by Jeff McBride

And here is an article with excellent examples of water drop photography and flash T.1 times.


But neither option is very feasible with many moving subjects.


For many moving subjects the only option we really have that will allow us to use flash and also freeze motion is high speed sync (HSS, "FP" in Nikon speak). Once again we are dependent on the ambient light and we have to have enough of it that it allows us to use a SS fast enough to freeze the motion...a SS above x-sync. In order to get a proper flash exposure above x-sync the flash fires many smaller/faster outputs. This means it's not as powerful and it won't go as far... HSS eats flash power.

There is one other option and that is to use a camera with a "Leaf" or "Diaphram" shutter. These shutters open entirely exposing the sensor/film all at once instead of the "moving window opening" of a DSLR's focal plane shutter. Some film cameras use lenses with leaf shutters and they are very expensive. But here's the good news: many compact cameras use some form of leaf/diaphram shutter and will sync up to 1/4000th (some even higher).

I suppose I should also mention cameras with an electronic shutter. This is not very common with todays CMOS sensors as it requires a lot of extra electronics and processing, or it results in a "rolling" shutter which reduces image quality. A CCD sensor with an electronic shutter does not have these issues...the old Nikon D70 is an example of one of these.

 

If you want to learn more about flash sync you can watch this excellent video by Paul Duncan.

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