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.