Freezing Motion with Flash

I have often read that you can freeze camera or subject motion with a flash. I had always assumed that meant the flash would allow you to use a higher shutter speed. Or is there something inherent in the flash so that if I shoot a restless child, at say 1/100 shutter speed, one with and one without flash, the flash picture wil be sharper?

Let's say that you shoot three frames at a hummingbird. The first one is without flash, and you fire at 1/60th of a second. The camera sees all of the ambient light at 1/60th, and there is a lot of blur in the bird's movement.

Now, you shoot the second one with a "big" 550EX external flash set up for normal sync, and the camera is set at its maximum sync speed, i.e. 1/200th or so. The actual burst of light is much faster than 1/200th, and it hits sometime during that 1/200th shutter time. You will see much less blur in the bird's movements, and ambient light won't figure in.

Now, you shoot the third one with the 550EX set for high speed sync, and the camera is set to 1/1000th shutter. The actual light will be a short series of light bursts that are spread out in time so that one light pop will be present at any point in the shutter travel. Ambient light won't figure in.

Which one of these three will give the best results?

I vote for the second one. The single flash is about 1/10,000th of a second, and that will freeze motion better than a short series of light bursts that are spread out over 1/1000th of a second.

For a child, you should not have to shoot faster than 1/1000th or so.

---Bob Gross---

Despite your prompt and complete answer I am not sure my feeble brain has got it.
Taking your second example, are you saying that since the single flash is 1/10000 of a second, the 1/200 shutter speed has, in effect, been increased to a shutter speed of 1/10000 because the flash has eliminated the effect of ambient light so that the hummingbird is only exposed for 1/10000 a second?

Precisely. Although, we should note that the sensor will still get the full 1/200 sec. of illumination, so, depending on the conditions, the hummingbird might show some ghosting.

Think about it this way. Which do you think will get a cleaner image?

(A) A standard sync flash shot. One flash pop at 1/10,000th of a second within a shutter time of 1/200th.

(B) A high speed flash shot. Let's say five flash pops, each at 1/10,000th of a second, but spread out over 1/1000th total.

---Bob Gross---

Some excellent reading on the subject....

http://webs.lanset.com​/rcochran/flash/hss.ht​ml

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Think about it this way:

You have two light sources: flash and ambient light.

If flash is stronger than ambient light, it will freeze motion. The stronger the flash and smaller the amount of ambient light the sharper the image will be, but it also means ambient light is lost in exposure (you get e.g. dark backgrounds).

If flash is weaker than ambient light, it will freeze motion but the ambient exposure will continue to govern. So, the weaker the flash and stronger the amount of ambient light the blurrier the image will be, but it also means ambient light governs the exposure (more natural exposure).

This means that to get a desired effect, you should first meter only ambient light (Manual mode). The more you want flash to fill in, the darker you set the ambient exposure. This ratio governs the look of the image.

Just a little different way to look at it:
Consider flash pictures as the combination of two pictures taken simultaneously.
1. The ambient light picture, with its exposure determined by the aperture and shutter speed.
2. The flash picture, with it's exposure determined by the burst of light from the flash and the aperture(limits distance of flash). The flash burst freezes the action.
The resultant image is a combination of the two exposures.

A good example of this is to take a flash picture with a long ambient light exposure of someone walking. You will get a sharp picture of the person at one point with a blurr moving in front of it from the ambient light. Now do the same thing with 2nd Curtain sync. The blurry trail will be behind the subject.

In High Speed Sync, the flash gives a series of pulses approximating a contiouous light source, so the shutter speed controls the stopping action. Even a maximum shutter speed of 1/8000 is slower then a flash on regular mode.
Regards,
Scott

The actual speed of the shutter past the sensor is probably no faster than 1/200 (to travel the total distance). If the high speed sync puts out bursts of light at 1/10,000 each then they would be spaced out to evenly cover the amount of the sensor that is exposed at any given time.

For instance, if a shutter speed of 1/800 is selected then the actual shutter opening would be about 25% of the total width, to get an effective shutter speed of 1/800 out of an actual speed of 1/200. In this case four flashes could cover the entire sensor (although more might be used to reduce overlap issues).

In any case, the actual time for the entire exposure would still be about 1/200 and any rapid subject movement would show up as multiple images.

That's how I understand it works, any corrections will always be welcome.


boB

boBquincy wrote:
The actual speed of the shutter past the sensor is probably no faster than 1/200 (to travel the total distance). If the high speed sync puts out bursts of light at 1/10,000 each then they would be spaced out to evenly cover the amount of the sensor that is exposed at any given time.

For instance, if a shutter speed of 1/800 is selected then the actual shutter opening would be about 25% of the total width, to get an effective shutter speed of 1/800 out of an actual speed of 1/200. In this case four flashes could cover the entire sensor (although more might be used to reduce overlap issues).

In any case, the actual time for the entire exposure would still be about 1/200 and any rapid subject movement would show up as multiple images.

That's how I understand it works, any corrections will always be welcome.


boB

My understanding of High Speed Sync is that the bursts of light from the flash occur in such a fashion that they appear as one continuous light source. This results in the shutter speed contolling the freezing of movement and not the flash. So you get blurry shots of fast moving objects rather then distinct individual shots. High Sync Flash actually slows down the exposure time rather then facillitates capture of fast moving objects. An ordinary flash has an exposure time of about 1/20,000 sec. The fastest shutters are at best 1/8000 sec. High Speed Sync is most useful for using flash fill on bright days with large apertures.
Regards,
Scott

This was a very informative thread... a concept that makes sense and a subject I did not understand.

Thanks to all for the information.

Dan

How fast are the shutter curtains? Like, how long does it take for them to cross the sensor?

For X-sync, the shutter curtains are 1/200th sec. apart, and that's the fastest speed at which the entire sensor is uncovered at one time. So the shutters have to cross the sensor in 1/200th second. Any slower and you wouldn't have X sync at 1/200th (since the first curtain wouldn't have crossed before the second started). Any faster and X sync could be faster, since if they crossed at, say, 1/400th sec., the second curtain would be hanging around for 2/400th sec, or 1/400th after the first curtain finished moving. before closing at a shutter speed of 1/400th.

It's a vertical-moving shutter, 15 mm in 1/200th sec. or 3 m/sec, if my mental back-of-envelope calculation are right.