Although the question came up on serveral different M7 threads, no one ever explained the reason why you can't get auto-exposure flash and high speed sync at the same time - so here goes.

The High-speed flash sync (HSS) requires that the flash stay ON for the entire time the shutter curtains are traversing the film.

Most auto-flash systems set correct exposure by turning the flash OFF once the right amount of light has reached the sensor (whether it's TTL/off the film, or on the flash itself).

These are mutually incompatable:

If the autoflash 'reaches' the right amount of light while the shutter is still only part way across the film gate, it will 'quench' the flash and leave the rest of the frame unexposed.

In addition, the HSS flash will be putting out some constant amount of light during the 1/48th second it takes the shutter to cross the whole frame. But the EXPOSURE will vary depending on which shutter speed is used, just as with ambient light exposures. In other words, if the flash puts out a constant 1000 units of light per second, the film gets 1 unit of light at 1/1000th second, 2 units at 1/500th, and 4 units at 1/250th. So how does the poor flash unit know how to 'expose' for the shutter speed in use?

Time to refamiliarize ourselves with guide numbers, compadres! Which, with off-center subjects, may be more accurate anyway!

(Although - also little noted in passing - the new Metz flash will ALSO offer rear-curtain sync with the M7 only - something that comes close to a "Hexar killer app" for me - Looking forward to seeing if the M7 'in the brasss' lives up to specifications.)

-- Andy Piper (apidens@denver.infi.net), February 25, 2002

Answers

Actually the high speed flash is based on multiple low power flashes as the slit crosses the film gate. It is manual and you use a guide number to figure out the desired aperture. Shutter speed has no effect on the flash exposure. The number and timing of the low power flashes is governed by the shutter speed selected.

-- John Collier (jbcollier@powersurfr.com), February 25, 2002.

Hmmm... You're right, the flash puts out a string of overlapping short bursts - but the effect is as though it was putting out one long 1/50th second burst - otherwise you'd get flicker stripes on the film.

I don't know about whether the flash senses shutter speed and changes light output accordingly - neither Erwin or anyone else has been authoritative on that point yet. It could just work like an old FP flashbulb exposure - you open up a stop for each step up in shutter speed. Maybe I just missed that part of the specifications...

-- Andy Piper (apidens@denver.infi.net), February 25, 2002.

The way the "FP" flash settings work on a Nikon the flash to subject distance range for correct exposure does change with the shutter speed, but not because of a shutter speed change. Because the F stop must change for ranges the shutter must also change.

OK lets say I've got a shot with my 80-200 2.8 and I want little depth of field. I want to shoot at F4. I dial in the FP mode on the flash and I get a distance scale for FP1 and FP2. So if my subject is on the distance range for say 10'-15' at F4 I have to use the shutter speed to give me an F4 exposure, that is why you'd adjust the shutter speed. The flash then "strobes " to imitate a slow flash bulb burn. So you may be given say 2 choices of 1/1000 at 5.6 for 6-10 feet or 1/2000 at F4 for 10-15'.

What I've found in use is that in FP mode with 2 choices I often chose the weaker of the 2 FP mode flash fills. Too much flash fill bothers me, and is more of a problem than too little. The system on the Nikon really does work quite well but being manual you do have to tweak it a bit to get what you want.

-- Neil Swanson (neilsphoto@yahoo.com), February 25, 2002.

But, but, but . . . isn't the FP flash function on Canon EOS cameras (EOS 3, EOS 1V, Elan 7, D30) TTL controlled?

-- Robert Schneider (rolopix@yahoo.com), February 25, 2002.

I answered my own question today. Took my friend's Canon D30 and 550EX flash, set the flash to ETTL, HS; set the camera to 1/4000 at f/2; took a picture. Perfect exposure, perfect fill flash.

And it occurred to me that the new D60 will probably cost about the same (or less) than an M7.

-- Robert Schneider (rolopix@yahoo.com), February 26, 2002.