Recently, on some of our safaris people have been confused, or
completely misguided, as to how High Speed Flash works, and also
on how the + exposure compensation actually works. Let's address
the second part of this question first.
First off, TTL exposures are based upon middle tone tonalities. Depending upon the camera system, the metering mode involved may be an evaluative or matrix system, or perhaps a spot-metering system, but in any case the flash metering sensor is basing the exposure upon a middle tone. Consequently, a white subject would be grayish or slightly underexposed with a 0 flash compensation, and a black subject would be grayish or slightly overexposed with the same compensation, as the flash meter controls output to produce a middle tone-ish value.
Most folks understand that if you want less flash, as you might if you are filling in light, you would dial the flash to a minus (-) compensation. Exactly how much depends upon the camera system, so you might need -1, -2, or -1.7 or just -.7. Back in film days the default fill flash compensation was around -1.7, but with digital cameras I've seen good results, and consistent results, all over the place. Fortunately, the LCD monitor allows you to see what you've done, and to compensate accordingly.
On a recent Africa trip one participant was photographing a subject that was beyond the reach of his flash, so he dialed the flash to +3 to give it more power. Well, that just doesn't work, because a flash has a finite amount of power and if it has used all that power, even at 0 compensation, and can't expose the subject properly, dialing it to +3 won't make a difference. To explain this, let me use an analogy.
If a flash's stored energy, the energy stored in the capacitor that will eventually fire out light, could be compared to water in a glass I think you'll get the idea. Imagine a glassed filled to the brim with water. This represents a full charge, full capacitor with your flash. Now suppose you wanted to wet something, let's say a 6x6 inch piece of cloth, with that glass of water, and to do so you would have to toss the entire contents of the glass at the cloth. Let's say that the cloth is 10 feet away, but enough of the contents of the glass reach the cloth to wet it to your satisfaction. Remember, as you toss the glass some of the water spreads out and misses the cloth, some falls to the ground before it hits the cloth. Light functions similarly -- not falling, mind you, but spreading out, so the farther you are from something, the less concentrated the light will be. Understand? Hold a flashlight 12" from a wall and see how concentrated it is, then move 12 feet from the wall and see how much more spread out the beam is, and how it is also less intense. OK, get the idea?
If you held the cloth 20 feet away, not ten, I think it's obvious that even less water will reach the cloth. You can't +3 the glass of water because the size of the glass dictates the contents, and you can't add to something that is already full. A full capacitor on a flash is full, period. If it dumps all of its energy trying to achieve middle tone and the distance is beyond the flash's reach, or effective range at the f-stop you're using, dialing in +3 does nothing.
If the subject is within its working range for that f-stop, dialing in a +, whether that's +1, 2, or 3, will of course make a difference as the flash will now add more light, sufficient to over-expose a middle tone by 1, 2, or 3 stops, respectively. If it is within the working range. If it is beyond the range, it won't make a difference.
If you don't believe me .... try this for yourself. Look at your TTL distance range on your flash, move to about mid-range for that distance, and take an exposure. It should look correct, if you're shooting a middle tone value. Keeping the same distance, dial the flash to +1, shoot, then +2, shoot, and then +3, and shoot. You will find that your middle tone subject gets progressively lighter or over-exposed.
Now, pick a subject that is far beyond the distance range of your flash at the aperture you selected. For example, if your flash range for an aperture is 4' to 40', pick a subject 80 or 100' away. Do this at night so that when the flash doesn't illuminate the subject you will see the results quite obviously. Take a shot at 0 compensation. Note, underexposure. Now, dial the flash to +3, and shoot again. You'll still have the same underexposure because the flash used all of its power in both cases, and going to + does not magically imbue the flash with more juice!
Now let's address the first question - how does high speed flash synch affect Guide Number (GN). To make a generalization, the GN is like the wattage on a light bulb -- the higher the GN, the brighter or more powerful the flash. The larger the GN, the farther a flash's light will be cast. But using HiSpd Flash Synch (HSFS) lowers the GN. Why?
Most digital SLRs employ a focal plane shutter, which is actually two curtains that pass across the sensor or, in the old days, the film. The first curtain covers the sensor until the moment of the exposure when that curtain sweeps across the sensor, revealing the sensor to light. The second curtain follows behind, and the lag time involved in this follow up depends upon the shutter speed chosen. At normal synch speeds, there is a brief time when the sensor is completely revealed before the second curtain begins its chase of the first sensor, following behind and shutting the sensor off from light. With most cameras this fastest synch speed is 1/250th sec., although in some cameras it's a bit higher or lower.
In HSFS the second curtain begins to follow the first curtain before the first curtain has completely traveled across the sensor. At speeds just slightly faster than the fastest synch that head-start is almost as much as the normal synch, but not quite, and if a flash fired 'normally' you'd see that a small portion of the image was underexposed, or black, because the second curtain has blocked off light as it began its passage. At faster shutter speeds the second curtain starts following the first curtain with an even shorter lag time, so what actually happens is a slit or partial opening passes across the sensor -- the first curtain revealing the opening, the second curtain traveling right behind. How large that opening or slit is depends upon the shutter speed, and as the shutter speeds increase this opening gets smaller and smaller.
Can you see how one flash firing at these faster shutter speeds could not possibly illuminate the entire sensor, since the entire sensor is never completely revealed during the exposure? How then can the entire sensor be illuminated, when only a small portion is exposed at any one time? Confused -- remember, this small portion is the slit or opening that is travelling across the sensor, not just one static, immobile slit.
In HSFS the entire sensor is illuminated by flash, but not by just ONE flash. Instead, your TTL must fire two or more bursts during the time of the exposure. When we're talking about durations of time shorter than the normal synch speed of 1/250th sec, you can imagine you won't be able to discern there two, or more, bursts. It happens just too quickly.
At very fast shutter speeds the flash may fire almost stroboscopicly, firing several flashes in the time span of 1/1,000th, 1/2,000th, or even 1/5,000th of a second. Imagine, five flash bursts in the span of 1/5,000th of a second! I'm guessing at 5, it might be 4, it might be more, but you get the idea. Now, if you think back to the finite amount of power or 'juice' in a flash, the flash must obviously ration out each burst so that there is enough energy to equally supply each flash burst during that time span. To do so, the power of each burst must be reduced, and that's why the GN lowers, and gets smaller and smaller the higher the shutter speed you use in HSFS.
This isn't theory. You can see this for yourself if you look at the back of your TTL flash and read the distance scale. You'll notice that the furthest distance decreases as you increase your shutter speed, as the following photos illustrate.
As you can see from the illustrations, the effective distance the flash reaches drops considerably. If you photograph birds, can you see how ineffective a fill flash will be if you're subject is some distance away if you're using a fast shutter speed? Dialing a flash down, going to -1, -2, etc. will increase its distance, but not by much. So, if you want to have a chance at reaching a distant subject you need to project your flash further, and that's where a tele-flash extender comes in.
Sold as a Tele-Flash or Better-Beamer, these fresnel lens attachments to your flash really work, but even at their best a tele-flash will only increase the maximum working distance by as much as 2.5X the normal distance. But in the 1/5000th sec. example above, that would be around 12 feet, a considerable improvement. We think a tele-flash is a vital accessory to carry afield, and it is why it is one of the few products we actually sell here at our office. If you'd like to order a tele-flash extender (aka Better-Beamer), contact our office at 717 543-6423 or via email. The price is $....., shipping included.
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