Speed of light

How long does a strobe light stay on for?

Is this related to strobe power?

If a given strobe is used at half power is it on for half as long or same length of time but half as bright?

Is there a spec to look for on a strobe to see how long it is on for?

Is this related to flash sync speed?

Hi,

Flash energy is absolutely related to time on. Power is about the same and energy is regulated with time on.

Checking my YN468 simple flash, show that the duration is between about 1/800s at full power and 1/20,000s at minimum power (1/128). Modern flashes regulate enegry by extingushing the tube and thus conserve power.

About max flash sync speed I'm not sure what causes the limits...

Cheers

/OS

How long does a strobe light stay on for?

 

Is this related to strobe power?

 

If a given strobe is used at half power is it on for half as long or same length of time but half as bright?

 

Is there a spec to look for on a strobe to see how long it is on for?

 

Is this related to flash sync speed?

Is this related to flash sync speed?

Flash sync speed is more about the shutter on the camera and has little to do with the strobe and how long it is on for.

The shutter on most modern cameras is actually two parts that slide across or down over the sensor. The leading edge "opens" the shutter, exposing the sensor and the trailing edge "closes" the shutter, stopping the sensor from being exposed.

If the shutter speed is slower (longer) than the flash sync speed, the trailing edge does not start its pass until the leading edge has already completed its pass.

If the shutter speed is faster (shorter) than the flash sync speed, the trailing edge starts to "close" the shutter before the leading edge has finished "opening". At any point the whole sensor is not exposed at the same time, i.e. it is a smaller window passing over the sensor. Because the flash duration is so short (even on full power) only part of the sensor would get exposed and part of the image would be underexposed or black (depending on the ambient light).

Flash energy is absolutely related to time on. Power is about the same and energy is regulated with time on.

Yes, absolutely in the context of UW photography. To my knowledge all portable strobes work this way. The capacitor charges to full capacity and the strobe circuit quenches the flash tube, short of full discharge to reduce the total amount of light.

Worth noting for the poolside photographers among us, that studio flashes work in a different way. The quoted T.5 flash duration becomes longer with decreased power. This is because the capacitor is only charged up to the voltage set by the power control. Then lower voltage produces lower current which gives a longer decay duration.

Thanks guys.

So if I wanted to take a photo of a fast moving subject I would be stuck with 1/160th shutter speed.

I could use a strobe to capture the "instant" better but the strobe may have a long "on" time. In this case would it be better to use a larger strobe at a lower power level so it delivered more light for a shorter time?

How do you work out what the "on" time of a strobe is at different power levels?

So if I wanted to take a photo of a fast moving subject I would be stuck with 1/160th shutter speed.

I'm guessing your camera has a maximum flash sync speed of 1/160s.

You would only be "stuck" with 1/160s if you were using a flash/strobe to take the photo. If there was enough available light, you wouldn't need the flash and you'd just set the camera to whatever shutter speed you wanted.

Now, I assume you are using the flash because there isn't enough available light. There are three general situations where this would be the case. 1) The flash is pretty much the only source of light. 2) There is some available light but the flash is still the primary source. 3) You are using the flash to fill certain areas but still relying on the available light to expose most of the scene.

In 1), you can pretty much ignore the 1/160s of the camera as this will have almost no effect on the image. The effective shutter speed will be between 1/800s and 1/20,000s depending on the flash and the power setting. Note that when using iTTL/eTTL, the strobes are basically quenched when enough light has been received so it has the same end result as manually setting the right power level. Even 1/800s is going to be fast enough to freeze most action, especially under water.

For 2), you will get similar result to 1) unless there is quite a lot of available light. If you are shooting a subject relatively close to the camera, you can use a smaller aperture which will decrease the effect of the available light and so you may still be able to achieve relatively fast effective shutter speeds.

3) is the difficult situation with fast moving subjects. The 1/160s of the camera shutter is going to let enough light alone to expose an image and you're likely to get some motion blur. What you can do, however, is set the camera/strobe to rear curtain sync. Remember I was talking about the leading and trailing edges of the shutter? With rear curtain sync, the flash is synchronised with the trailing edge of the shutter (rear curtain) and so the flash effectively goes off at the end of the time the shutter is open. This will give you an image that is sharp and frozen from the flash, but with motion blur leading to the frozen image. i.e. it gives a sense of motion in the still image. You won't have frozen the motion, but you can have a pleasing image.

How do you work out what the "on" time of a strobe is at different power levels?

It is usually listed in the specifications of the strobe.

Thanks for the reply.

I have looked at a few specs on strobes but cant find this information in any of them.

FYI I have a EPL3. I also have a collection of older strobes. S&S YS300 and a YS30 plus 2 Ikelite strobes a 50 and a 35. Of course the 300 is the only one that allows reduced power levels ( in non TTL modes). WOuld all these have an on time less than 1/800th?

Are there any draw backs on using rear curtain sync for the strobe? Why not use it all the time?

The manual for my Nikon SB-800 Speedlight lists:

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

I would expect most strobes/flashes to be similar. I doubt many if any get longer than 1/800s.

The manufacturer should be able to tell you the approximate durations.

Speed of light is a constant based upon the wavelength of the light in question.

Strobe output is based upon the amount of energy put out by the strobe.

What was the question?

What was the question?

Never mind. Go back to sleep.

ATJ: Thanks again. Just the type of info I am looking for.

I have just tried doing some water drop shots using my Sea &Sea YS300 at 1/4 power. Results seem quite slow. But I am sure its quick enough for underwater use.

Speed of light is a constant based upon the wavelength of the light in question.

And the medium through which it is traveling.

(For photography, the latter is VERY important.)

Never mind. Go back to sleep.

 

 

ATJ: Thanks again. Just the type of info I am looking for.

 

I have just tried doing some water drop shots using my Sea &Sea YS300 at 1/4 power. Results seem quite slow. But I am sure its quick enough for underwater use.

Ha!, I deserved that. Posting from the local microbrewery is always fraught with danger.

Ha!, I deserved that. Posting from the local microbrewery is always fraught with danger.

No, you deserved the first reply I typed out.

You would only be "stuck" with 1/160s if you were using a flash/strobe to take the photo. If there was enough available light, you wouldn't need the flash and you'd just set the camera to whatever shutter speed you wanted.

I don't want to be incredibly rude, but what about "balanced light", the single most used technique in serious wide-angle underwater photography? Strobe power, f-stop and shutter speed are varied to create a colourful foreground and a blue or green background. It's the main way to get colour into a wide-angle image...

Tim

I don't want to be incredibly rude, but what about "balanced light", the single most used technique in serious wide-angle underwater photography? Strobe power, f-stop and shutter speed are varied to create a colourful foreground and a blue or green background. It's the main way to get colour into a wide-angle image...

Read the rest of that post. The part about the 3 conditions....

Additionally, the OP was worried about freezing action...

Thanks for the reply.

I have looked at a few specs on strobes but cant find this information in any of them.

 

FYI I have a EPL3. I also have a collection of older strobes. S&S YS300 and a YS30 plus 2 Ikelite strobes a 50 and a 35. Of course the 300 is the only one that allows reduced power levels ( in non TTL modes). WOuld all these have an on time less than 1/800th?

 

Are there any draw backs on using rear curtain sync for the strobe? Why not use it all the time?

IMHO, you should not worry about strobe working time for UWP because they will always be fast enough to freeze the action and much faster than 1/160 secs. Subjects should move very very fast (I don´t think that is possible underwater) in order not to be frozen by any strobe light.

There are no drawbacks on using rear curtain sync except that some iTTL/eTTL strobes don´t work well on this setting. Some people prefer front curtain to first freeze the action with the strobe light and then move the camera to create that motion look.

Speed of light is a constant based upon the wavelength of the light in question.

 

Strobe output is based upon the amount of energy put out by the strobe.

 

What was the question?

HUH? I somehow missed the idea that speed of light was related to wavelength (frequency yes, but speed not so many). As for the speed in the medium I don't understand the practical applications of the small difference a few nanoseconds would make to any picture

Bill

HUH? I somehow missed the idea that speed of light was related to wavelength (frequency yes, but speed not so many). As for the speed in the medium I don't understand the practical applications of the small difference a few nanoseconds would make to any picture

Wavelength is inversely proportional to frequency (and vice versa). If frequency affects speed, so does wavelength.

In a vacuum, wavelength/frequency do not affect speed. In other media, they do affect speed depending on the refractive index of the media. The refractive index of a medium varies with wavelength which causes dispersion and is why we see rainbows (refraction and dispersion of light through water moisture in the air) and also chromatic aberration in lenses.

As different media have different refractive indexes, the speed that light passes through those media changes. If the speed of light in a vacuum is c, the speed in a specific medium is v = c/n, where n is the refractive index of the medium. Air has a refractive index slightly higher than 1 (1.000293 at 0ºC and 1 atm according to Wikipedia). For water is is around 1.333.

Temperature and pressure can also affect the refractive index which is how we see thermoclines.

Lens (cameras, spectacles, eyes, etc.) work because of the variation in speed of light as it moves from a medium of one refractive index to a medium of another. If the light passes through perpendicular to the interface, the light follows a straight line. If it passes through at an angle, it is bent (refracted) and the amount of bending (refraction) depends on the refractive index. Even dome ports work because of refraction, mainly from the difference between the refractive index of water outside the port to that of the air inside the dome port.

So... if the speed didn't vary with media, refraction would not occur and we wouldn't be able to see the way we do and pictures wouldn't work.

Wavelength is inversely proportional to frequency (and vice versa). If frequency affects speed, so does wavelength.

 

In a vacuum, wavelength/frequency do not affect speed. In other media, they do affect speed depending on the refractive index of the media. The refractive index of a medium varies with wavelength which causes dispersion and is why we see rainbows (refraction and dispersion of light through water moisture in the air) and also chromatic aberration in lenses.

 

As different media have different refractive indexes, the speed that light passes through those media changes. If the speed of light in a vacuum is c, the speed in a specific medium is v = c/n, where n is the refractive index of the medium. Air has a refractive index slightly higher than 1 (1.000293 at 0ºC and 1 atm according to Wikipedia). For water is is around 1.333.

 

Temperature and pressure can also affect the refractive index which is how we see thermoclines.

 

Lens (cameras, spectacles, eyes, etc.) work because of the variation in speed of light as it moves from a medium of one refractive index to a medium of another. If the light passes through perpendicular to the interface, the light follows a straight line. If it passes through at an angle, it is bent (refracted) and the amount of bending (refraction) depends on the refractive index. Even dome ports work because of refraction, mainly from the difference between the refractive index of water outside the port to that of the air inside the dome port.

 

So... if the speed didn't vary with media, refraction would not occur and we wouldn't be able to see the way we do and pictures wouldn't work.

I should have put a smiley face here. I understand refraction and dispersion and chromatic aberration as well. My initial comment was that wavelength varied with frequency not that speed varied with either wavelength or frequency in a vacuum.

I still don't understand the worry about the slowing down with index as being important to a strobe.

Bill

I still don't understand the worry about the slowing down with index as being important to a strobe.

There is no worry at all. This side discussion stemmed from Dave being smart with the thread title...