Friday, March 20, 2009

15. When is Full Power Flash not Full Power?

When you set your flash in Manual mode and full power, you expect to get a full power flash, right?

Well, you would be wrong! At least part of the time.

Many of you will be surprised to learn that even with your flash in Manual Mode you will only get full power when the shutter speed is slower than about 1/125th sec!

This is because when the flash is in Manual mode and mounted on the hot shoe, it turns ON and OFF with the Sync pulse that fires it through the center terminal.

When the flash truly fires at full power, like when you hold it in your hand, it completely discharges the capacitor, and it takes longer to do this complete discharge than the time available at the flash sync speed of 4 ms (1/250th sec).

In fact, the tail of the flash pulse goes out to about 6 - 8 ms before it is completely gone. This portion of the tail is well below the half power point that is used in the specification to define the length of the full power flash. This is how they get by with saying the flash is 1/1000th sec when at full power. That's the length of time between the half-power points on the waveform.

So, when the flash is in the hot shoe, the flash fires when the Sync signal is applied as the shutter opens, and it is quenched when the Sync signal goes away as the shutter closes. This stops the complete discharge of the flash if there is any energy left in the capacitor.

Now, when the shutter is at 1/60th (17 ms), the flash has plenty of time to completely dump all the charge on its capacitor. However, at faster shutter speeds, like 1/250th (4 ms), there is still significant charge left in the capacitor when the flash is squelched.

This remaining charge is also why the flash will recycle faster when at 1/250th shutter than when at 1/60th, even when the flash is in Manual mode and set to 1/1 (full power).

This also explains why the image from the 1/60th shutter is brighter than the image from the 1/250th shutter when the flash is fired at full power. The tail that is cut off at the higher shutter speeds but present in slower shutter speeds, contributes to the exposure.

48 comments:

Jay said...

Russ, great to see another fascinating and useful post, thanks for continuing to share what you learn.

Russ MacDonald said...

Hi Jay,

Thanks foe the nice feedback!

I'm always learning something new about this flash system!

Russ

Box of Frogs said...

Hi Russ

Great to see a post after quite some time.

Have a questions for you - when shooting at slow speeds and with say on camera mounted flash, why does the white balance go squiffy?

For example, at 1/250th and with the WB set to flash, the colour of light is as expected, a little cool; when using lower speeds either direct flash or bounced flash, the light has a tendency to yellow up somewhat and I don't know why.

Again, thanks for another great post.

Russ MacDonald said...

Hi Box of Frogs (quite a name!),

The reason the lower shutter speeds tend to warm up the image is that the ambient is having more effect.

Typically, indoor ambient is tungsten, which is much more yellow than daylight. But the light from the flash is daylight color and not tungsten, so if you allow the ambient contribution to increase the color the image will shift toward yellow.

This is exactly what gels are for!

Put a tungsten gel over your flash and select tungsten white balance on your camera, and the light from the flash will be the same color as the ambient. Now, as you change shutter or aperture, you will not see any shift in color as the ambient contribution changes.

Russ

Box of Frogs said...

Thanks Russ, I'll give that a whirl.

ktest123 said...

This is extremely helpful as I've been utterly confused in the past as to why I've been getting very inconsistent results when using full flash power: in certain outdoor situations, I use M mode on the camera and manual full flash power because I need to get a very precise depth of field, exact back-light exposure, and exact subject exposure (5-10 feet away). But when I change the exposure time slightly (to make the background darker/lighter), the subject's exposure change quite a bit as well! Your post finally explains it.

Speaking of manual mode, I've been experimenting with i-TTL as well but have been getting very inconsistent results. For example, when I use A mode (to get a more precise depth of field), then pan to the background to lock the background exposure, then pan to the subject to get a FV lock, then when finallyy snap... I'd get very inconsistent results. Any reason why this is the case? In another word, is there another more elegant, consistent way of getting the exact depth of field, background exposure, and subject exposure, short of going into fully manual mode?

Russ MacDonald said...

Hi ktest,

I'm glad you found the information in my blog useful. It certainly was a surprise to me when I discovered that the flash power could vary in Flash Manual mode based on the shutter speed!

Regarding your question. There are a lot of things that could be causing your problem, but most probably, it is that ambient light is creeping into your subject exposure or flash is partially exposing your background. This can often happen when the ambient is medium intensity. When that happens, you have to make sure to overpower the ambient by increasing the flash power and decreasing the camera exposure.

If that is not it, let me know. There are other potential causes.

Russ

Mark said...

Russ,

Thank you so much for this great blog - it is really, really helpful. I keep recommending it to all of my friends.

I have a question I hope you could help me answer. I am an amateur and my setup is as follows:
D90 + 18-200mm + 50mm f/1.8 + SB-600 + 60" Photoflex shoot through/removable cover umbrella + 8' stand.

A good friend of mine asked me to take a photo of him for a dating website. This was my plan: Shoot head and shoulders portaits of him against the pretty brick wall next to where I live - in the shade or on an overcast date. I plan to shoot him at 1/250 for a fastest possible recycle time as you've just described, with my 50mm lens (faster lens = more flexibility) at the lowest ISO possible (i.e. 100). I would then through trial and error figure out the correct aperture. (in a fully manual mode)
The umbrella would be used turned away from him (thus bouncing light at him) at a 1/4 of a power in a manual mode (or slightly higher or lower depending on how it goes).
Shooting quality: RAW + JPEG Normal.

His body position: shoulders at a slight angle to me (not square)

What do you think about my plan? Should I use the TTL to simplify things? Any other suggestions?

Thank you in advance,
Mark

Russ MacDonald said...

Hi Mark,

That is a great question, but I'd prefer to answer you by email since your question is not really related to my blog article.

Just send your email address to me at rkmacdonald@gmail.com , and I will answer your question - no need to ask the question again.

Regards,

Russ

Bernhard said...

Russ, thanks again for your work.

I have read about flash pulse widths a couple weeks ago. Jim did a nice job measuring the pulses from a Canon speedlight (it shows a similar behaviour as it takes 7-8 ms for the 1/1 pulse to go off). For those interested you can read the whole thread at: http://www.fredmiranda.com/forum/topic/616017/0

Bernhard

Russ MacDonald said...

Hi Bernhard,

Thanks for that link to that thread.

Yes, that is exactly what I was referring to in this blog. I wish I owned the equipment to make these measurements myself.

Of course the main thing I was pointing out in this blog is that because of the tail you get a reduction in effective flash power at higher shutter speeds, even though you are still below the flash sync speed.

Thanks again,

Russ

JW Alexander MD said...

Hi Russ, thanks for the great info. Dumb questions, but here goes. In assume when you talk about "full power" and you talk about capacitor discharge that directly correlates with the amount of light delivered?

Thanks again for a great source of info.

Russ MacDonald said...

Hi JW,

No such thing as a 'dumb' question on my blog!

The charge that is stored in the capacitor is discharged through the flash tube when the flash fires. It takes approximately 8 ms to completely empty the charge from the capacitor. This is the period of time the flash is firing.

If you are at a shutter speed higher than 1/125th, then the camera will break the circuit at the hot shoe, which shuts off the flash before the entire discharge has been completed. This preserves some of the charge in the capacitor, and shortens the time that it takes the battery to recharge the capacitor.

Hope that helps,

Russ

Abe said...

Hi Rus,

Continueing to enjoy every bit of your most outstanding site.

A question though, in section 10 I read:
>In an SB800, the maximum flash lasts about 1/1050th sec.

This would be about 1ms. But here I read:
>In fact, the tail of the flash pulse goes out to about 6 - 8 ms before it is completely gone

Also not clear on what the term "tail" means, is this just another way of saying "end" ?

Perhaps your aluding to this and answering it with:
>This is how they get by with saying the flash is 1/1000th sec when at full power. That's the length of time between the half-power points on the waveform

But this is untintelligible to me. Is there another place on this site that this is explained?

Abe

Russ MacDonald said...

Hi Abe,

Thanks for the nice feedback!

You are putting all the information together and analyzing it! Excellent!

Yes, the half-power points are used to define the beginning and end of a pulse in engineering. Actually the term 'half-power' is a little misleading, and I will probably go back and change that. The actual measurement is made between the two 90% amplitude points on the waveform which includes about 90% of the total power of the flash pulse.

Measuring between the two 90% amplitude points on the waveform, the flash pulse is about 1/1000th sec. Then, the tail extends at a lower amplitude out to about 8 ms.

Good questions!

Russ

Abe said...

Thanks for your patients and for sharing your vast knowledge.

Basically the during of the flash I would is very significant. Since as you pointed out on this site the flash can stop action even with a slow shutter speed (and sometimes this is best way to stop action). But what speed should I assume I'm getting from the flash when I'm shooting at slow speeds, like 60 or slower?

Russ MacDonald said...

Hi again Abe,

The duration of the flash equates to the power of the flash. When you are at 1/60th, you will get the full flash whenever full power is needed for the shot.

If you are taking close pictures of people, like at a party, the flash will probably fire at 1/8th or even 1/16th power most of the time. This means the duration of the flash pulse will be very short, probably quicker than 1/3000th sec. There will also be a tail, but at lower power shots, the tail is so low that it doesn't contribute to the brightness or cause any ghosting.

However, if you are shooting a subject that is 60 feet away, the flash will be firing at full power. Then the 'effective' duration will be about 1/1000th sec. I say effective, because all the tail does is add a very small amount of brightness, not visible at 60 feet. The action will be stopped by the flash and the amount of ghosting from the tail is so small that you can't see it under normal circumstances.

The only time you can see the effect of the tail is when shooting close subjects with a very small f/ stop (f/16 or smaller) where the flash has to go to full power. Then, you can detect a brightness difference between 1/60 (which allows the full tail) and 1/250th (which chops the tail off).

Russ

Anonymous said...

Russ,

thank you very much for your explanation. I was wondering on that subject for some time - it is very obvious AFTER your explanation.

Congratulations for your work.

Regards,

O.Cristo

Russ MacDonald said...

O. Cristo,

You are most welcome!

Thanks for the compliment!

Russ

John Meyer said...

Great blog!

Doug said...

Russ,

I recently bought an SB600 speed light for my D80. Your BLOG has been a great help in understanding how the Nikon flash system works. I've now read all of your entries at least once and have gone out practicing with my flash every evening. Your information has really helped me understand what is happening and why (the why is a big thing for me, a fellow EE). I'm now going to start looking into the remote operation of the flash.

You have a very nice writing style that is easily understood and packed with information. Thanks again for taking the time to make all our lives easier with this great BLOG!

Doug

Russ MacDonald said...

Doug,

Thanks for the great feedback!

Russ

Michael Fisher said...

Ok, so let's see if I understand this. When the SB900 is in the hotshoe and I have it on M1/1 and the shutter speed is 1/250, I don't really get the full power dumped. What about an SB900 off-camera M1/1 triggered by a Pocket Wizard at 1/250? I get a full power dump from the SB900, but I don't get the full power registering on the sensor. Right? So how much registers? Half? So M1/2 on the flash at 1/250 is the most flash power I can get?

Russ MacDonald said...

Hi Michael,

I haven't experimented much on this subject with the flash in Remote mode.

However, the power loss should be only about 10% or less, so when the flash is in Remote mode and set at 1/1, you would get 90% full power (or more).

But the recycle time will be longer in Remote mode than when doing a 1/1 flash from the hot shoe, because the flash would dump its complete charge. In the hot shoe the flash pulse is quenched when the fire pulse on the center pin terminates.

Russ

Michael FIsher said...

So, how would you experiment on this? A gray card target in a dark room, shoot M1/1 off camera at 1/250 and 1/60 and compare histograms?

Maybe shoot the above target at M1/2 and then M1/1, both at 1/250 and compare the histograms?

And how would you meter that? Incident meter at subject position set at shutter speed of 1/250?

My typical setup at a wedding for the altar groups is one SB800 at M1/2 thru an umbrella for main and one SB800 at M1/4 thru an umbrella for fill. I use a Quantum 2x2 to power the mainlight, so recycle time is reduced.

I have one group shot I do in the summer with the sun to their backs and I use 2-SB800 both M1/1 and powered by the Quantum 2x2 at 1/250. Am I getting all the power I paid for there?

Michael David Fisher said...

So, once again, to be clear, the main problem is when the flash is on camera and set to Manual 1/1. In that scenario we don't get full power.

Russ MacDonald said...

Hi Michael,

Try this: Be sure you are in very dim ambient so the only illumination will be the flash.

Then, set set your Remote at 1/1 power, and take two images, one at 1/250th and one at 1/60th, keeping the same aperture. Then, compare histograms, and you will see that the one at 1/60th will be farther right than the one at 1/250th, indicating that the tail of the flash pulse is being lost. By adjusting the aperture, you should be able to match the two histograms, and that will tell you how much power you are losing at 1/250th.

The best thing to do if you need maximum power, is to slow the shutter to 1/125th or so, and stop down the aperture an equal amount to maintain the same exposure. Once you are at 1/125th, the flash pulse will not be cut off.

I realize that this will widen the depth of field and reduce the separation you get between the subject and the background, but that's the only way you can get maximum flash power.

Russ

Greg said...

I am using an SB900 in commander mode with several SB600's wirelessly. I want to turn off the SB900 from firing and just use the SB600 slaves. So for the SB900 I turn the dial from TTL to AUTO to "---"... but it still fires no matter what - both in manual camera mode and Ap priority. Is there a way to shut it off completely?

Russ MacDonald said...

Hi Greg,

Yes, that is normal.


The SB-900 is acting as a Commander, so it has to communicate with the two SB-600s, and it does that with preflashes.

There is lot of handshaking that goes on between the Commander and the Remotes.

The preflash sequence tells the Remotes what power to fire at and when to fire.

My blog here describes the sequence of events that takes place: http://nikonclspracticalguide.blogspot.com/2008/02/so-what-really-happens-when-you-use.html

Russ

Greg said...

Thanks Russ. That helps! So I guess those are the pre-flashes I am seeing. Hey I went to a photo conference recently, and a pretty respected Nikon shooter said that he felt that the Nikon CLS system gets it right about 75% of the time in TTL mode, but 90% of the time in AUTO mode. From reading your post, it seems like they should give about the same result since each flash group is working independently in TTL mode anyway, and in fact, maybe the AUTO mode picks up some of the light from the other groups. What do you think of this theory?

Russ MacDonald said...

Hi Greg,

It really depends on what you mean by 'gets it right'. I like to say it 'gets it close'.

CLS TTL gets it close much more often than 75%, but you have to know the system. I have run across many pros who don't really understand the fine points of Nikon CLS. In fact, when I shoot weddings, I get more like 99% correct flash power, all because I know what mode to use for different situations, and how to recognize when I will need to add compensation.

Flash A Mode gets it close at short distances when the subject is centered in the frame. It never gets it right when the subject is on the edge of the frame. When the subject is more than six feet away, A Mode tends to fire too hot, because it is trying to light the whole frame, background included, at a standard brightness, since it can't detect the subject.

Flash A Mode never gets it right when shooting Fill. The TTL-BL is a wonderful invention that A Mode can't compete against, because it has no way to analize the subject versus background brightness.

Flash A Mode is about equal to TTL for groups, as long as the group fills the frame.

Russ

Karl @ Play said...

You learn something new everyday. Great Blog, thanks Russ.

Greg said...

Thanks Russ. Nikon should hire you to explain their processes! They put the time into developing this system but then don't have much to communicate it.

So any time I use any kind of wireless CLS system, its going to cause a slight delay from when I click the shutter release, and the shutter actually opens, so that these flashes can communicate?

Also, I have noticed that the CLS system works fine in doors, or maybe at close range outdoors, but then it fails outdoors with more distance. Have anyone been able to keep it working using small reflectors near the sensors on the slave flashes?

Lastly, has anyone testing the new pocketwizards that are going to be released for Nikon some day (already 3 months behind schedule)? That seems very promising.

Russ MacDonald said...

Hey Greg,

This Q&A has drifted quite far from the topic of this blog 'When is Full Power Flash not Full Power?' So, I wonder if you could email me your question at russ@russmacdonaldphotos.com. Then, I can answer you by email.

Thanks,

Russ

Anonymous said...

I still have problem understanding the "tail" concept.

By the way, here what one author writes: "the bottom line is that the shutter speed will have no affect on the exposure of the area of your image that’s lit by the flash."

http://www.sublime-light.com/index.php/2007/07/18/portrait-lighting-for-beginners-metering-for-flash/

How true is that?

Russ MacDonald said...

To anonymous,

That's exactly what I was taught, as well. That is also what you will read in all the books on photography. The problem is it is not true, and it can be easily shown. This is one of those things that has persisted from one author to the next with no one ever questioning it.

It is true that most of the time, when the flash is not set near its maximum, that shutter speeds have very essentially no effect on the flash portion of the image.

However, as soon as you set the flash power at maximum, you can clearly see the difference that shutter speeds make. Just try it for yourself. It's plain as day.

When I first discovered this, I didn't even believe it myself. I thought I was doing something wrong, or that my equipment was faulty. So, I tried it with other cameras and other flashes and the same thing happened. The flash contribution to the image went down as the shutter speed approached flash sync speed when the flash was near maimum power.

I have a background as a design engineer. I used to work as an IC designer in the camera department at a large Semiconductor company, and I still have contacts there. So, I went back to them and asked the question as to why the shutter DID affect the brightness of the flash image when at full power, and it was there that I found out about the tail. That is something I did not know prior to my research. But now that I know about it, everything falls into place. This is why the flash will recycle faster when on the camera than when off the camera, for instance, because on the camera the tail is squelched by the end of flash strobe.

Just try it for yourself and you will see. The authors you have read simply have not considered all the possible situations.

Russ

Joe said...

Hi Russ,

I have not read all the questions on your full power post, so I hope this has not already been asked.

Is the flash firing full power, no matter the shutter speed is up to 250s, if you are using the CLS system and the flash is on a lightstand, being triggered by the SU-800? Or maybe another flash?

Russ MacDonald said...

Hi Joe,

You have similar problems when using the flash in Remote mode.

When your flash is being controlled by a Commander like the SU-800 in TTL mode, the flash will automatically set its power based on the scene. It could be anything from minimum power to maximum power depending on the distance from the flash to the subject plus the color and reflectivity of the subject.

However, if you set the shutter speed at 1/128th or higher, you begin to cut off and waste part of the tail. At 1/250th you may begin to notice this with slightly darker images.

Russ

Joe said...

Hi again Russ,

In my last post I was referring to Manual mode only. If you use a higher speed than 60s (up to 250s) is it possiable to get full power with the SU-800?
Thanks

Joe said...

Hi again Russ,

In my last post I was referring to Manual mode only. If you use a higher speed than 60s (up to 250s) is it possiable to get full power with the SU-800?
Thanks

Russ MacDonald said...

Hi again Joe,

In Commander Manual Mode, you can set the flash for full power, and it will flash at full power.

However, fast shutter speeds will close the shutter before the entire tail is finished.

The flash pulse including the full tail is 6 - 8 ms long. If you use a shutter of 1/250th, the shutter is only open for 4 ms. Obviously the last 2 - 4 ms of the tail will occur after the shutter is closed, so it cannot contribute to the image.

1/128th shutter is open for 8 ms, so it should be able to use the entire pulse including the tail.

Russ

Joe said...

Thanks Russ. That clears the air on that one for me.

jeremy said...

Hello Russ,

Thanks once again for the great post, I have ammased lots of useful knowledge from your practical guide.

I was hoping you could clarify the following for me:-

1) When considering the flash firing @100% power vs 50% power does that mean the maximum height of the waveform is the same for both, and that the time between 2 points of 90% amplitude is reduced (ie a fatter waveform vs skinnier waveform)?

2) Could you consider the area under the waveform for the flash as the amount of illumination (excluding ambient exposure)? Therefore, if the area or illumination increases by x2 for a given distance from the flash then the aperture must be closed by 1 stop to achieve the same exposure?

3) Lastly, if we consider the flash waveform on a x-y graph and the horizontal axis (x) is time, then what is the unit for the vertical axis(y)? Is it Watts?

Thank you in advance.

Kind Regards,

Jeremy

Russ MacDonald said...

Hi Jeremy,

Sounds like you might be an engineer or maybe studying engineering.

This is very confusing to an engineer, because, photographers use the term 'power' incorrectly. They call it 'flash power' when it's really flash energy. In my blog I just ignore this and use common photography usage.

1. For analysis purposes, the waveform of the flash pulse is considered constant amplitude, ie a flat top. It is also considered to be constant brightness over the waveform. The 'power' of the flash pulse is controlled by the length of time the waveform is present. The measurement is inconsistent however, but is usually measured from 80% point to 80% point and considered vertical at the ends.

2. Yes, if you measure the area under the curve (ie perform an integration) you will come up with the 'power' (as defined by photographers). You can see that the tail portion of the waveform is usually ignored. But that tail amount is what causes the changes in power of the full power flash, which is the subject and the point of this blog.

3. As I am sure you are aware, the units are very complex and confusing, and way beyond the scope of a photographers blog. As an engineer, you will find all sorts of unit errors and inconsistancies in flash documentation. Only the flash tube designers can keep the units straight!

Russ

Jan said...

Hi Russ,
i also need to thank you for your posts. Until I find this blog I though I understand most of the CLS. :)

One note (D90): when I change from Matrix to Spot, my FV lock is reset. I wanted to use it for a little bit different purpose than you describe and I am quite sad about.

Question/suggestion: Me and probably even other could be interested in your answer to Mark (umbrella with SB-600). I know that it is not directly relevant however can please you post it or send by mail?

Cheers

Jan

Russ MacDonald said...

Hi Jan,

FV Lock will normally stay locked until the light meter times out. That time out setting is found in the camera menu.

I cannot imagine why you would want to change from Matrix to Spot metering after firing the monitor preflash.

Are you aware, that the metering mode has absolutely no effect on the flash power?

The meter in the camera only measures ambient light.

And when you are shooting flash pictures and using FV lock, normally you are indoors where you don't want the ambient to contribute to the image. You use a high shutter speed to block the ambient. And if ambient is not contributing to the image, then the metering mode has zero effect on the image.

Hope that helps,

Russ

Anonymous said...

Thanks for sharing in your blog.

Would the effect of the full power tail be different when using second curtain sync (as opposed to using first curtain sync) at shutter speeds slower than 1/125 second? My thinking is that on 2nd curtain sync, the flash would fire at a fixed arbitrary (maybe 1/1000s?) time just before the curtain closes, and therefore the tail is lost either by the curtain closing or the flash power being squelched.

Thanks

Yoon Lee

Russ MacDonald said...

Hi Yoon,

Yes, I think you are right, except, I don't think the time is fixed. It is possible that this time changes as shutter speed changes, but I don't know that for a fact.

Russ