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Introduction
Today, marketing and advertising use numeric performance indicators for everything. And if there aren't any, they make up their own. This is quite wise from their point of view, because you can use these to show or at least make people believe, that a) any other or newer or more expensive item is better than the other one and b) make them want and spend money on it. In digital photography such numbers are resolution in Megapixels, which is great, because even a 33% increase in real resolution doubles the amount of Megapixels (because it is a square function), maximum ISO, which is useless, because a crappy looking ISO - value is not of any use at all, or zoom-ratio, which doesn't say anything, because a 5x zoom-ratio from 50-250mm is something completely different than a 5x zoom-ratio from 15-75mm.
In flash-photography, there are only two such indicators: Guide-No., an indicator for the stregth or reach of a flash, and maximum sync-speed. And while you can "fake" a Guide-No. for marketing - purposes, max. sync-speed, sadly, is very hard to enhance significantly, or in fact sometimes is decreased in newer models.
The terms
Guide-No.: The Guide-No. is quite an easy term: It simply indicates the reach of a flash. It says, how far away an object can be and still be lit by the flash. But it's called "Guide-No.", not "distance", so, of course, there is one caveat: GN = distance x f-stop. As always when it's about exposure,the aperture plays a role here. It's simple: When a flash has a GN of 40, it has a reach of 20m at f/2 or 10m at f/4 and so on. But... well, can you already guess? Otherwise, read on at "The dependencies".
Maximum sync-speed: This is the second very important term in flash-photography, but something completely different. It even isn't a characteristics of the same piece of equipment: The max. sync-speed is something in your CAMERA, not your flash. But it still is absolutely essential to know about it. This term simply means the shortest shutter-speed you can shoot at, while still using your flash to enlight your picture. It's really that simple. When a photo is taken, the shutter opens the film / sensor to light (or the sensor is read-out in the case of an so called electronic-shutter) for a certain time. This is the shutter speed. Together with the aperture, it makes the exposure-value. When the shutter is open for, let's say, 1/100th of a second, the flash has to be started / fired (the light has to be "switched on"), power up, light and stop (be "turned off") again. There are several points costing time in that process: The shutter has to be completely opened in order to light the whole sensor to the same amount, the electronics / mechanics of the camera have to get the signal "shutter open" and send it to the flash-trigger, the flash's electronics have to start the light, the light has to get to it's full power, the light has to burn for it's complete time, the "shutter closing now"-signal has to do all the way above, ... you get the point. So, as all this has to happen in this short period of time, it's not possible to use any imaginable, all-so-short shutter speed, of course, it's simply a matter of time. Electronics don't work in light-speed yet.
Focal length: This is quite a common term in photography, of course, but it's also a point in flash-photography. But here it's not a matter of your lens alone, but also of your flash. As well as you bundle ("magnify") the image with your lens, the light getting into your camera from the outside, you can of course also bundle the light emitting from your flash, being "shot" at your subject! So, when you bundle the light, it can travel a lot further, but will light a smaller area, like your lens magnifies a smaller area.
ISO: Higher ISO means a higher sensitivity, so less light is needed to light an object to the same amount - be it your flash or ambient light, it's no difference.
The dependencies
This is the section I really want to share. This is, what only few people understand. And even more important: Many people don't realize, that these dependencies can make a whole camera completely useless. The Panasonic GM1 is such an example.
Let's begin with the Guide-No.:
That the meaning, the impact on pictures, of the GN depends on the speed or aperture of the lens used, lies within it's definition itself, as it is "distance x f-stop".
But of course, when you think further, what's the other thing relevant for exposure in photography, NOT talking about shutter-speed now? Right: ISO. The dependency seems to be quite easy: When you double the ISO - sensitivity, the Guide-No. rises, that's clear. The real question is, when a manufacturer of a flash tells us it's GN, at what ISO does he mean it? Normally, every GN is meant to be at ISO 100. But the manufacturers cheat here: For example, the Panasonic GM1 (or the Olympus E-PM2) have a BASE ISO of 200, even if they still can go down to 100. So... right: The GN of the flashes of both cameras are given at ISO 200! If you want to compare it to a Canon flash, you have to subtract about one third to have the value at ISO 100!
Then there's focal length. As explained above, you can bundle light emitting from the flash like you can do with the light entering the lens. So you can enlarge the reach of your flash, but decrease the area lit by it. And because GN = distance x f-stop, when you increase the distance, you increase the GN! The question, again, is, when a manufacturer tells us the GN, at what setting does he mean it? Simple flashes, like the built-in ones, have only one setting: These usually cover an angle of lighting matching the widest angle of the most common (KIT) lens: It's 18mm on Canon APS-C or 14mm on mFT, so this is comparable. The cheating begins with big and expensive accessory-flashguns. My very old Metz CT-1 has a GN of 45. But it only has one single setting, it lights an angle about as wide as the angle of view of a 35mm - lens on 35mm film or "fullframe". My new Sigma EF-500 DG Super even has a GN of 50! Wow. But wait... it has a so called "zoom-head", which adjusts the bundling of light matching the focal length setting on the lens, which is electronically transmitted by the camera to the flash. It's "longest" or most bundled setting is the 105mm - setting. The GN of 50, of course, is given at this longest setting by marketing! It has a reach of 25m with a 105mm lens at f/2! At 35mm, there only is a GN of 35 left.
So, to make it short: The Guide-No. is a marketing term like MP or ISO. Manufacturers use it in a way, that is not always comparable between models and they cheat whereever they can using and calculating it. You can deal with this by asking or looking for the ISO and focal length it is given at.
And now that I have explained the GN and with it the dependencies that have influence on the reach of the flash, we get to one of the most underrated technical characteristics of a camera:X-Sync (sync-speed). While technically, this term also is quite easy to understand, it's dependencies with other things in flash-photography and impacts on your real-life-photos is bigger than most people expect. There is no limitation for flash sync-time on the "long" end. When you do a long-time-exposure for, let's say, 2 minutes, you can fire your flash within this time as often as you want and the light is recorded. But that's not the case on the "short" end. When the shutter-speed is shorter than the maximum (or more correct: minimum) flash-sync-speed of your camera, what you really can do with some cameras when you use it in M-Mode, you'll get an ugly dark bar across, on top or the bottom of your image. What happens then, is simply, that your shutter already rolls across your picture, while the flash still lights the scene. So the part of the film / sensor, the shutter has already covered while the flash was still lit, simply is darker, because it "got" less light than the rest.You have to avoid this. In practical photography, this is mostly irrelevant in dark surroundings, if and when the flash is the main light-source. The flash itself is short enough to stop action in the dark and the sync-speed mostly is, too. That's also the point, why X-Sync isn't relevant for beginners or point-and-shooters - they often don't even realize. But when you start caring about flash-photography, you soon start to learn, that very dark environments, the situations, where every beginner thinks that he needs his flash for, are the only situations, in which you often really want to avoid using a flash. You mostly want to use as much ambient light as possible, in order to not have one single lit object in front of black or at least to avoid to harsh shadows. This is not a static rule, of course, you can surely use such effects as a stylistic device, but often you won't want to. When there's only a bit of ambient light and you suport it by a flash, or light your main-object to isolate and make it "pop out", sync-speed of any camera is OK, too, because you'll use longer shutter-speeds in order to use that ambient light. All this is not a problem.
It get's a problem in situations, most beginners don't associate with flash-photography: In bright daylight. The brighter, the more problematic. One of the most common and most important situations to use a flash is, when everything is nicely lit and bright, but throws shadows, you want to light. When you shoot your girlfried during your holidays in the bright sun at the beach, her hat or the tree she's standing or lying below throws shadows on her face. It has to, otherwise she would get blind or at least not look as good as she can, when closing her eyes to narrow slits to avoid this. Now you have two possibilities without flash: You can adjust exposure to her face, then it will be evenlyand nicely lit, but the beach in the background will be severely overexposed, sometimes white. Or you expose the beach right, then your girlfriend will be black. The only possibility to take a good photo in this situation is, to adjust exposure to the beach and light her with your flash to match the background. "Fill-flash" is the word for this. So what happens when you adjust exposure to a bright environment? Right: You have to lower the ISO, choose a narrower aperture and a shorter shutter-speed. ISO is limited at the lower end, mostly to 100, sometimes 50. OK, then you choose a shorter shutter-speed. But wait, what was this point with flash? Right: The shortest shutter-speed is limited by your camera! Think about it: There's hardly any camera that has a sync-speed shorter than 1/250 of a second, only my E-PM2 works at 1/320, which is really great. Some old Nikons go to 1/500, but that's the absolute maximum, I don't know of any actual camera going this far, actual Canons do 1/200. It's not easy to expose an environment well with 1/320th of a second in bright Australien sun at noon! This is the point, why the Panasonic GM1 is useless, simply because of it' sync-speed... of 1/50th of a second! 1/50th is way too short for daylight fill-flash. But you are limited with every camera here and every all-so-little improvement helps!
Let's think this through a bit further: When you are limited to a shutter speed of 1/200th and ISO 100, what do you have to do in order to expose the beach right? Right: You have to close-down the aperture (or use an ND-filter, what's the same in my example, as you'll see). OK, that's simple, just use f/22 and loose a bit of irrelevant sharpness due to diffraction or screw-in an ND-filter. Well, now we close the circle and the Guide-No. becomes relevant again. Remember? GN = distance x f-stop. Of course, noone needs to light an object 25m away with his flash at f/2. But now suddenly you have to use f/22! So, my big fat Sigma EF-500 DG Super flashgun has a GN of 50 at 105mm, but I want the beach in the picture, too, so I'll use 35mm, where it has a GN of 35 left, but that's 35 reach at f/1 and I have to use f/22, so I have a maximum distance of 35/22=1.6m or 5ft left! With my GN 50 flashgun! With a Canon Speedlite 580 EX you end up with 1.8m or 6ft! And now let's get back to the Panasonic GM1. It's flash has a GN of 7... at ISO 200, that's 4 at ISO 100, that would leave us with 4/22=0.18m, or 18cm of maximum distance to my girlfriend's face. That's a bit less then the distance between the tip of my thumb and the tip of my little finger, when I spread my hand... and useless.
So, the shortest sync-speed gets even more important: The difference between the GM1 and the E-PM2 is 1/50 to 1/320, which is 2.6 stops, which makes the difference between f/22 and f/9 and enables you to shoot from 2.6x farer away with the same flash, it's 4.2m to 1.6m! Or 18cm to 47cm, which is half a meter and getting in a range maybe possible to achieve a usable result with.
To show you how important all this is, here are three samples showing you possibilities. No, this is not my girlfriend:
Left: Exposed to Ant; Middle: Compromise towards background; Right: fill-flash
Verdict
The Guide-No. is a marketing term like MP or ISO. Manufacturers use it in a way, that is not always comparable between models and they cheat whereever they can using or calculating it. You can deal with this by asking or looking for the ISO and focal length it is given at. Old flashes are often even better than new ones, because they have their GN at 35mm, not at a tele-setting like newer ones.
You don't need a high GN, a powerful flash, for it's maximum distance in the dark. You need it for as much reach as possible in bright daylight! Remember my girlfriend at the beach.
The shortest possible sync-speed of a camera is an extremely important characteristics! It severely effects the need for a high Guide-No. and with it the maximum distance you can use your flash at in daylight. Remember, that 1/50 vs. 1/320 is the factor 2.6!
The reason, why the slower sync-speed of former days (most mechanical cameras sync at 1/60) haven't been as much of a problem as shown above, is, that the external flashes used back then simply were more powerful than the built-in ones today: Even my old Braun-flash has a GN of 40 at 35mm, while the built-in flash of my EOS 40D has 14 and the GM1 has 4 (7 at ISO 200) - what's 10%, to call it by it's name.
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