# How good is flash at “freezing” a scene? [duplicate]

I've heard that flash could "freeze" a scene. Let's say that we shoot a scene where people moving a lot, is flash able to even "freeze" it at slow shutter speed, i.e. 1/30th ? Assuming that people are in the flash range, i.e. flash could reach them easily. Is flash very effective at eliminating the motion blur ?

On a side note, is flash also good at reducing camera shake ?

Thank you.

• – mattdm Jul 20 '17 at 14:40
• – mattdm Jul 20 '17 at 14:41
• – scottbb Jul 20 '17 at 17:01
• Just a note for people that are just as confused as I was when I clicked on the `hot network questions` link and read trough the question... They are talking about a photo camera flash, not about Barry Alan, better known as The Flash (or red streak?) – Mathlight Jul 20 '17 at 18:26
• @Mathlight Hopefully immediately obvious once you see the site. :) – mattdm Jul 20 '17 at 19:00

## 3 Answers

The flash is created when a high electrical voltage is discharged into a slender glass tube filled with xenon gas. The electricity excites the gas and it outputs a blitz similar to lightning. This blitz is extremely short. The duration of the flash can be from 1/500 to 1/100,000 of a second. Most likely your unit operates around 1/2000 of a second. Because the blitz is fast, it is able to freeze fast action. High speed flash can freeze and thus capture such things as hummingbird wings. It is unlikely that your flash will accomplish this super feat, but it likely will freeze common sports activity.

The key to success is: The flash must synchronize with your camera’s shutter. Check your camera manual. There will be one shutter speed, usually marked with an icon of a lightning bolt. This marks the fastest shutter speed that is compatible with electronic flash. Likely, slower shutter speeds are also compatible.

One problem is: The designated max shutter speed might only be 1/125 of a second. This is not terribly fast. What can happen is: The flash fires and freezes the action, but the scene is also being illuminated by ambient light, so both light sources record the image. The result is much like a double exposure. The image of the frozen subject illuminated by the flash records as does an image illuminated by existing light. When this happens we sometimes get a frozen image superimposed atop a streaky image.

We get blur from subject motion and from camera motion. A fast shutter or a rapid flash mitigates. Combine a fast flash with a slow shutter and likely you will get both the streaky image combined with camera shake.

So the bottom line is: An electronic flash can and will freeze subject motion. You need to experiment and find out how subjects are rendered using this wonderful tool.

• You should probably include the fact that the flash duration is linked to the flash power setting - lower is shorter - full power is longest. – Digital Lightcraft Jul 20 '17 at 15:33
• @ Digital Lightcraft --- You are spot on -- Modern flash units emit a pre-flash or otherwise meter the subject using data gleaned from the camera’s chip logic. The idea is to automatically adjust the power of the flash so an over or under exposure is circumvented. Because the exposure is an accumulation of light energy over time, one of the best ways to control the impact of the flash is to custom alter the length of the blitz. Thus the photographer is not in full control, the hardware and software of the flash take command. – Alan Marcus Jul 20 '17 at 16:48
• Is this the reason people flash, even when it's really bright? To "freeze" the moment, for a clearer picture? – Mathlight Jul 20 '17 at 18:29
• @Mathlight No, that is probably Fill flash. – AndreKR Jul 20 '17 at 19:47
• @Mathlight -- Sunlight will cast deep shadows. Sometimes these shadows are so void of light that reproduce super dark or even black. The areas in shadow are void of detail so they contribute little. We use flash in daylight to soften shadows. This technique is called "fill flash", because the light from the flash fills in the otherwise dark shadows. – Alan Marcus Jul 20 '17 at 20:05

Camera speedlights are called speedlights because they are incredibly fast at lower power levels. Used at low power to stop extreme motion like water drop splashes or hummingbird wings. Here is a Nikon SB-800 speedlight duration chart from its manual:

1/1050 sec. at M1/1 Full output (t.5)
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

Yes, this would include freezing camera shake. Flash can be a big help for hand-held macro work.

And as mattdm pointed out, ambient light level should be dim (not dark, but certainly not bright) to prevent the continuous ambient light exposure from blurring the motion that the speedlight already stopped.

Full power is an exception measured to half power points called t.5, and the actual full power duration is more like 3x or 1/350 second. But the other levels are chopped off short to be low power, which makes them fast, and these numbers would be actual durations.

Note this is about camera speedlights. Studio monolights are typically voltage controlled, which work oppositely in a couple of ways, and their lowest power might be twice slower than their maximum power.

More info about it: http://www.scantips.com/speed.html

• This is good but a complete answer should mention that in order to stop motion the flash should be the primary source of light (and that it might not be at 1/30th). – mattdm Jul 20 '17 at 16:18

Yes, a flash can freeze a scene - the pulse is brief enough to freeze most action.

Counterintuitively, the shutter speed has to be relatively slow, probably 1/125s or slower1. Otherwise only part of the frame will be exposed (or frozen).

It's actually really hard to build a shutter that can completely open (exposing the entire frame) and close again in less than 1/125s or so, depending on the shutter style and format2. Most cameras with focal plane shutters use two shutter curtains - the first curtain opens to expose the frame, then the second curtain closes behind it. Up to a certain shutter speed, the first curtain is able to open completely before the second curtain closes. At higher speeds, the second curtain starts to close before the first curtain has completely opened, so the entire frame isn't exposed at the same time - you're basically dragging a narrow opening across the frame. The narrower the opening, the less time any part of the frame is exposed, but it still takes 1/125s or so to expose the entire frame. Thus, if you set your shutter speed to 1/1000, only the first part of the frame will be exposed to the flash pulse; by the time the last part of the frame is exposed, the pulse has already finished.

Many cameras with focal plane shutters (except for maybe the most bare-bones entry level models) have what's called a "second curtain sync", where the flash doesn't fire until the second curtain begins to close. That allows you to capture cool effects like these.

1. Depends on the camera - check your user's manual.
2. IINM, leaf shutters in really expensive MF lenses can open and shut in 1/500s.

• Most modern DSLR cameras with focal plane shutters have X-sync speeds in the 1/160-1/250 second range. My FF Canon 5DIII has an X-sync speed of 1/200 second. My APS-C Canon 7DII is 1/250 second. Even entry level APS-C DSLRs are usually around 1/200. It's been a long time since I've seen a DSLR with an X-sync of 1/125 or slower. – Michael C Jul 21 '17 at 0:16