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Status: obsolete. The last update was in September 2008, and it reflects my inadequate understanding of the issues at that time.
I started writing this page on 12 September 2008, when I was looking for a new flash unit. My previous one, a Mecablitz 40 CT 4, is so old that I can't remember buying it. My best guess is that it's about 30 years old.
What has changed in this time? Not the brightness. As far as I can tell, I will have difficulty finding a new unit as bright as the old one. And even then, the flash unit was able to change its light output to suit the requirements. But cameras have become digital, and they can communicate with the flash units. This offers a number of interesting possibilities. Before I read too much about what's on offer, it's interesting to consider what I'd like from a flash unit.
Flash units are used to make up for missing light. Ideally a unit would do whatever you want to get the correct lighting. Clearly that's impossible, but there are many things they can do.
Obviously you need enough light.
Always ready to flash.
Flash units require a certain recovery time between flashes, to recharge the condenser. The longer this takes, the fewer photos can be taken.
Work at any shutter speed.
Electronic flash and focal plane shutters aren't a good match for each other: if the slit in the shutter is less than the width of the shutter (i.e. not all the sensor is exposed at any one time), a single short flash will only show that part of the shutter that is open at the time of the flash.
Until recently, this meant that there was an upper limit on the shutter speed when using flash. Nowadays, flash units can emit multiple flashes, thus giving more even exposure. I'm interested to see how even it is: to do it right, the flashes should not overlap parts of the sensor, otherwise those parts will be more heavily exposed than the rest.
Flash units create a short-lived flash of light to illuminate a photo subject. Almost without exception, modern flash units are “electronic flashes” which use electronic circuits to charge a capacitor and then discharge it across a Xenon-filled flash tube.
The flash typically lasts about 1 ms (0.001 second) or less. This is ideal for catching movement which might otherwise be blurred, but it has its disadvantages when used in conjunction with a focal plane shutter, such as are typically used in SLRs. Focal plane shutters create short exposure times sequentially by exposing only a slit smaller than the picture at any one time. Under these circumstances, a flash unit will only illuminate that part of the photo which was being exposed when the flash fired. The minimum shutter speed for electronic flash is currently in the order of 5 ms (1/200 s), and older cameras can have speeds as slow as 1/30 s.
When taking a photo with a camera with a focal plane shutter (almost all SLRs, analogue and digital), you will normally set the shutter speed to the highest speed that the camera can handle. This is always described in the manual. Older SLRs had a marking x on the shutter speed dial, frequently not matching any of the “standard” speeds.
Set the iris to match the illumination that the flash unit provides. It depends on a number of factors:
Direct illumination from the flash depends on the distance from the flash unit to the subject. As light spreads, it becomes less intense. The area covered by a beam of light increases in proportion to the square of the distance, so at 10 metres the light is only ¼ as bright as at 5 metres.
The only way to compensate for this drop-off in brightness is to use a wider aperture: at 10 metres you need two stops wider aperture than at 5 metres. If you need f/8 at 5 metres, you need f/4 at 10 metres. It's fairly simple to see that you can calculate the aperture just by multiplying the aperture at one distance by the distance, and dividing by the other distance. In this case, 5 metres by f/8 give you 40; you can then divide this 40 by the distance to get the aperture you need at that distance.
This number 40 is called the guide number, and it's probably the most important measurement of light intensity from a flash unit. But take it with a pinch of salt:
The most primitive electronic flashes, no longer available, did no more than what's described in the previous section. They completely discharged the capacitor through the tube, emitting all the light they had to offer. It was up to the photographer to set his camera to expose the film correctly.
About 40 years ago, various electronic enhancements came in. The first was “automatic” exposure: electronics in the flash unit measure the light reflected from the subject and stop the flash when it has been sufficiently exposed. The following photo shows the control panel of a Mecablitz 40 CT 4, made in about 1977:
This is stuff that I started and then left. It's almost certainly not worth reading.
This is from my online diary.
I'm currently looking at the Olympus FL-50R “Electoronic Flash” (sic) and the Mecablitz 58 AF-1 digital (what a mouthful). And I can't work out what they do that's different. There are few reviews, and those that I have found don't give me the feeling that the authors really know what they're talking about. So I've started to read the instruction manuals.
The Mecablitz manual, conveniently written in 6 languages, starting with German and continuing in a sequence that doesn't obviously match the descriptions on the first page (English starts on page 84), starts by talking of the flash modes: TTL flash mode with measuring preflash, TTL - flash mode with high speed synchronisation HSS, Automatic flash mode, Manual flash mode, HSS Manual flash mode with high speed synchronisation HSS, and Stroboscope flash mode
What does that mean? I know what TTL means—“Through The Lens”, the metering system pioneered by the Pentax Spotmatic. But what does that have to do with flash units? The manual explains:
The TTL flash mode with measuring preflash is a further development of the standard TTL flash mode of analogue cameras.
That's really helpful. It also suggests that I look at the camera manual, which gives me the added information when connecting external flash units:
The external flashes communicate with the camera, allowing you to control the camera's flash modes with various available flash control modes, such as TTL-AUTO and Super FP flash.
Again, no mention anywhere in the manual of what these terms mean, though it suggests that “Super FP” is the feature I've heard about where the flash unit flashes several times during the picture to allow illumination of the complete subject at shutter speeds where the slit width is less than the full image. But TTL? Acronymfinder returns 28 definitions, including “Through The Lens”, but nothing else that relates to photography. So maybe it does mean “Through The Lens”. Maybe a description of how the camera's exposure meter cooperates with the flash? Who knows? They're certainly not telling. As usual, I had to go to Google to get some useful links. In particular, Toomas Tamm's Electronic Flash Information states what should be in all the manuals:
TTL measures light reflected off the film plane during flash exposure. Once enough light has reached the film for proper exposure, the flash pulse is cut short.
So, problem solved? No. What's “TTL-AUTO”? Are Metz's “TTL - flash mode with high speed synchronisation HSS,” and Olympus' “Super FP” the same thing? How can I choose a flash unit this way? The instructions I have are specific for Olympus, but they don't tell me whether the unit will work on my camera. Instead, I read:
This flash unit is suited for: Olympus - Digital cameras with TTL flash control and flash socket system, as well as the compatible digital cameras from Panasonic and Leica. This flash unit is not suited for other brands of cameras. Also take a look at the image page at the end of the manual.
The image page at the end of the manual shows a photo of a factory, presumably Metz. The German version is clearer: the manual is designed to have a fold-out image at the end. It's not clear what belongs there. But probably it will work on my camera, but I'd hate to be wrong.
This is a summary of what I've gleaned from the Olympus and Metz manuals. I'd be prepared to guess that other manufacturers have other terms.
Manual This is the simplest mode. The flash fires and discharges its condenser completely. There is no exposure control of any kind. Auto Olympus uses this term to describe the mode where the flash unit limits its light output to a quantity sufficient to expose the subject at a specific aperture. The camera is not involved and should have the aperture set manually to correspond to the setting on the flash. This is the oldest form of feedback, and my Mecablitz 40 CT 4 has it. One improvement on this mode is that the Olympus digital cameras can communicate the iris setting to the flash unit (assuming it understands; it seems that both the FL 50R and the Mecablitz 58 AF-1 do).You need to read the manuals carefully to understand what the flash units don't do that well. Here's my current list.
The guide numbers are given in some detail on page 64 of the manual.
I read in an advertisement by a eBay vendor that at 35 and 50 mm it should be 42, and another claims that it's 44, but I can't find any confirmation on Metz's broken web site:
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