Flash Photography 101, Chapter 4 - Guide Numbers and High Speed Sync

[Curtis N]

Flash photography 101, Chapter 4 – Guide Numbers and High Speed Sync

With this chapter, you get two lessons for the price of one!

First, what is this guide number thing?
The guide number for an electronic flash is a way of quantifying its maximum output in terms that a photographer can relate to – aperture and distance. The guide number (GN) is the product of the aperture (f/ stop) and distance (from flash to subject) combination that will result in enough light for proper exposure.

The basic formula is: GN = Aperture x Distance
By rearranging this formula we can also conclude that
Aperture = GN / Distance
Distance = GN / Aperture

The three most common ways to use the guide number are:
1) Determine the proper aperture for a subject at a given distance when using manual flash. Example: Your flash has a GN of 160 feet and your distance is 20 feet. The proper aperture is f/8 (160/20=8 ).
2) Determine the maximum distance a flash will properly illuminate the subject at a given aperture. Example: Your flash has a GN of 160 feet and you want to use f/8. The maximum distance is 20 feet (160/8=20).
3) Comparing the relative power of different flash units (higher guide numbers indicate more power). But make sure you’re comparing apples to apples, especially concerning point #2 below.

This all seems pretty simple, straight-forward and foolproof, right? Well it is, sort of. There are some things to keep in mind.
1) A guide number must contain a distance unit (generally meters or feet) and an ISO value. Most advertised guide numbers are in meters at ISO 100, but it’s critical to know for sure. Some times they’re listed in feet, and sometimes they’re listed at ISO 25 or something else. To convert meters to feet, multiply by 3.3.
2) The guide number changes when you zoom the flash head. For example, the 580EX Speedlite has a GN of 58 meters when zoomed to 105mm coverage, but this drops to 28 meters when zoomed to 28mm coverage.
3) The guide number increases as you increase the ISO. Doubling the ISO increases the GN by a factor of 1.4. Going from ISO 100 to 400 doubles the GN.
4) Adding any kind of light modifier (diffuser, bouncer, umbrella, etc.) to the flash unit will significantly reduce the effective guide number. Published GN specifications apply only to undiffused, direct flash.
5) The GN will be significantly less when high speed sync (FP Flash) is used. More on that later.

Since the above caveats can make these calculations a bit complicated, it’s a good idea to pay attention to the distance scale on the back of your flash unit, if it has one. The flash will do the calculations for you, factoring in the aperture, ISO and HSS, and give you a good estimate of your range. Keep in mind that the flash doesn’t know when you have a diffuser attached, so if you use one of those things, all bets are off.

Now on to part 2 of this chapter, High Speed Sync, a.k.a. FP Flash.

Every SLR camera with a mechanical shutter has a maximum flash sync shutter speed (1/200 or 1/250 on current Canon DSLRs). This has to do with the way focal plane shutters work. At slower shutter speeds, the first curtain opens, the flash fires, and after the specified time duration, the second curtain closes behind it. At shutter speeds faster than flash sync, the second curtain begins to close before the first curtain is completely open. The second curtain follows the first across the frame, exposing only a slice of the image at any given moment. Firing a flash during this process would illuminate only part of the image. Generally this limitation only becomes an issue in situations where you have a lot of ambient light and want to use a wide aperture, such as when using fill flash for outdoor portraits.

Canon’s FP Flash system enables the use of flash at high shutter speeds. With FP Flash, the flash unit fires a burst of low-powered flashes at 50 khz (that’s 50,000 flashes per second) lasting throughout the duration of shutter curtain movement. For all intents and purposes, the flash unit becomes a continuous light source that lasts a very short time. The only drawback of this approach is its inherent inefficiency and loss of range. Since the flash is firing while the shutter curtains aren’t completely open, not all of the light that goes through the lens reaches the sensor.

The illustration below is from the Canon Flashwork online brochure.


Just how much range is lost? Official Canon documentation is sorely lacking in this regard. Instruction manuals for Canon Speedlites only tell you to look at the distance scale on the back of the unit. The manual for the Sigma Super flash unit has a table to quantify things, so I’ll use that for the purpose of discussion. I don’t honestly know how closely this might match up with data for Canon Speedlites if such data existed.

In the image below, the top table shows “normal” guide numbers for the Sigma EF-500 DG Super, at various power levels and zoom settings. The lower table shows guide numbers at full power for FP Flash at various shutter speeds and zoom settings.

Suppose that you have “sunny 16” ambient light (1/100 shutter speed & f/16 at ISO 100). With normal flash, zoomed to 105mm coverage, depending on your camera’s sync speed, you could use:
1/200 & f/11 for a range of 4.55 meters (50/11). This is your maximum range with a 5D, 10D, 300D or 350D.
1/250 and f/10 for a range of 5 meters (50/10). This is your maximum range with a 20D or 30D.
1/500 and f/7.1 for a range of 7 meters (50/7.1).This is your maximum range with a Nikon D70s.
I threw in the 1/500 example because Nikon has a DSLR with a 1/500 flash sync speed, and I really wish Canon would step up to the plate in this regard. But I digress. The point here is to show the advantage of a faster sync speed.

But when you study the numbers for FP Flash, the first thing you realize is that the guide number is cut in half each time the shutter speed is quadrupled. So your effective distance doesn’t change as you shift exposure settings in a given ambient light situation, matching faster shutter speeds with wider apertures. This makes sense when you think of FP Flash as a continuous light source. For example:
Using the guide numbers for FP Flash and 105mm zoom (right side of the lower chart)
1/250 and f/10 gives you 2.5 meters (25.0/10)
1/1000 and f/5 gives you 2.5 meters (12.5/5)
1/4000 and f/2.5 gives you 2.5 meters (6.3/2.5)

So there’s no single thumbrule that says how much distance you lose with FP Flash. It depends on the flash sync speed you start with, at least with the Sigma unit. But the loss is significant enough to conclude that using FP Flash when you don’t need it will drain your batteries faster and make recycle times longer, so it makes sense to use normal flash whenever you don’t need the background-blurring effects of a wide aperture. Since FP Flash is less efficient and gives you less range, it is advantageous to choose exposure settings that do not require it unless you specifically need it for the reasons mentioned. In other words, in bright conditions it's better to stop down the aperture rather than using a shutter speed that requires FP Flash.

[JaertX]

ah...now I see why I shouldn't leave my 580EX on high-speed sync mode all the time....thanks!

[Jim G]

Hang on... doesn't high speed sync only kick in when your shutter speed is over the max sync speed? I have mine on all the time, I thought it didn't have any effect when I was shooting around 1/100.

Judging by what you've said I could be wrong and I should be turning it on only when I need to

[Curtis N]

Ok, hang on ... apparently I have confused some folks.

1) If you enable HSS on your Speedlite, it will automatically use HSS for shutter speeds faster than sync speed, and normal flash for shutter speeds at or below sync speed.
2) So there is nothing inherently bad about keeping HSS enabled - it only kicks in when it needs to.
3) Since it is inherently less efficient and has less range, it makes sense to choose exposure settings that do not require HSS unless you specifically need it for the reasons mentioned. In other words, in bright conditions it's better to stop down the aperture rather than using a shutter speed that requires HSS.

Now maybe I'll edit the text to make that more clear. Thanks, Jason & Jim, for commenting.

[JaertX]

got it....I was worried it wasn't working as efficient when I was using lower shutter speeds. I normally use flash in "M" mode only....sometimes "P" mode (did I admit that in public?) But I like leaving the flash set on high speed sync for those times I want to use it as fill in AV mode.

Thanks again Curtis!

[Jim G]

Thanks very much Curtis for this, it's useful information

[goatee]

Originally Posted by JaertX

got it....I was worried it wasn't working as efficient when I was using lower shutter speeds. I normally use flash in "M" mode only....sometimes "P" mode (did I admit that in public?) But I like leaving the flash set on high speed sync for those times I want to use it as fill in AV mode.

Thanks again Curtis!

Nothing wrong admitting using P when using flash in dim lighting - it's a sensible route to take - when you want to expose for the background leave it in Av or M, but P & M are the only sensible modes if you don't want to expose for the background.

[jrsforums]

Curtis, nice piece of work...again...

Just a personal experience view...

High Speed (a.k.a. FP) flash (shutter above max. sync speed) will require add'l flash power vs. regular flash (at or below sync speed). However, I find it invaluable for outdoor flash fill and would not want to leave fellow forum readers that it should be something to avoid.

I do not know the characteristics of the Sigma, but the 580EX has plenty of power, very fast recycle times, and a very high number of flashes per set of NiMh batteries. In normal flash fill conditions I rarely notice any significant delay in shot to shot transitions, even when shooting many close together.

The only time that one needs to be concerned about sync speed (i.e. FP or regular flash) is when the flash requirement is at or past the light capability (range) of the flash.

Actually, it is fairly eay to calculate the general rule of the max. distance difference between FP and regular flash. This difference is dependent on the camera's sync speed. For the data you provide, it is the guide number of regular flash divided by the guide number of FP flash at that speed. So for a 20D with 1/250 sync at 105mm it is 50/25=2...or 2 times the distance. Again using your numbers, the distance distance would be 1.8x for 1/200 and 2.8 for 1/500 (which is why shooters like Nikon's sync speeds).

However...that all said, I take a different view. I believe you should only concern yourself about this when you (rarely) experience the need for more power. Most outdoor situations would have you shooting at inordinately small apertures to stay within the sync speed...and there is normally no need to be concerned about this.

Net...this (among other considerations) is why, when people ask "...580EX or 430EX..." I usually suggest the 580EX. You may not normally need the add'l power, but it is nice to have it there.

[Curtis N]

John, I don't disagree with your views at all. And it should be noted that, even though all these tables and formulae can be confusing, determining your range with the 580EX or Sigma Super is as simple as looking at the distance scale on the back of the flash unit. I suppose this would be another selling point for these units vs. the 430EX which I don't believe has such a scale.

About once every other week, someone will complain about overexposed shots when they try to use fill flash in the sun (the result of the flash forcing the shutter to sync speed in Av mode). And often the first advice they get is to turn on HSS. While this may be a viable answer, it's not always the best answer. Sometimes the best answer is a smaller aperture.

Most outdoor situations would have you shooting at inordinately small apertures to stay within the sync speed.

As noted in my example above, 1/200 and f/11 is about as bright as it gets. Whether f/11 is an inordinately small aperture depends on whether you're trying to blur an ugly background into oblivion or preserve the beauty of a majestic landscape behind your subject.

Hopefully this information will help people understand the concepts involved well enough to make the right decision when the situation requires it.

[Wilt]

Originally Posted by Curtis N

Just how much range is lost? Official Canon documentation is sorely lacking in this regard. Instruction manuals for Canon Speedlites only tell you to look at the distance scale on the back of the unit. The manual for the Sigma Super flash unit has a table to quantify things, so I’ll use that for the purpose of discussion.

Now you ruined it, Curtis, by not telling us you made all the measurements yourself one evening...whattaya trying to do, lose your Master Measurebator title?!?!?! Oh...I just noticed you already lost it, you're only King Measurebator.

It is different what a decade plus makes...when Olympus introduced the F280 full synchro flash, the press yawned because of the loss of power when that was used in that mode. Now, when Canon offers HSS, everyone gets excited. Glad you are setting the world straight that there is no free flash lunch!

[Curtis N]

I didn't get serious about photography until recently (couldn't afford the film & processing required to learn), but I can't help but think that fill flash in the sun must have been a lost cause back when a good flash unit had a 100 ft. GN and flash sync speed was 1/60.

Outdoor fill flash can make such a difference, I can't imagine shooting people without it.

[jrsforums]

I have a question on how the intensity of high speed sync (HSS), a.k.a. FP flash is regulated....assuming it can be.

In regular sync flash, the intensity can be requlated from 1/1 (full power) to 1/128 (as shown in Curtis' charts above) by changing the duration of the burst.

In HSS, there is a 'stutter flash' which evenly spreads the flash light across the sensor as the focal plane shutter 'slit' moves across it. Can the intensity of this light be modified by either reducing the strength of each stutter burst or increasing the time between bursts?

[Curtis N]

Originally Posted by jrsforums

Can the intensity of this light be modified by either reducing the strength of each stutter burst or increasing the time between bursts?

I don't know how the light output is controlled, but obviously it's controlled somehow. Otherwise, E-TTL metering or adjustable power with FP flash would not be possible. According to Canon's Flashwork online brochure, the flash fires at "roughly 50 khz", so I would guess it's varying the length of each little burst.

But it's just a guess.

[prime80]

Curtis, thank you so much for these posts! I just stumbled across them, and they are a GREAT source of info.

[imhotep]

First off, thanks for putting all of this together.

I'm struggling a little with the tables you show above. It seems strange to me that you should be able to just multiply the aperture you are using at any given time by the distance you are from you subject at that time, and get the GN of your flash. I'm probably just misunderstanding. Does it work that way, or is the GN something that you have to already know about your flash in order to use the formula to solve for either the appropriate aperture or distance?

Another question:

In your example above,

1/250 and f/10 gives you 2.5 meters (25.0/10)

Does it still hold true if just adjust the aperture and recalculate. For example, if I stop down to f/16, does that then mean that if I keep the same shutter speed of 1/250, I will have a proper shooting distance of 25.0/16 = 1.6 meters? It makes sense to me because your are darkening the exposure by only stopping down and not adjusting shutter speed, so it makes sense that you would need to be closer to the subject that is getting illuminated by the flash.

Sorry for the lenght of this one. Am I atleast on the right track here?