Xref: utzoo sci.electronics:4768 rec.photo:4257
Path: utzoo!utgpu!attcan!uunet!tektronix!sequent!brian
From: brian@sequent.UUCP (Brian Godfrey)
Newsgroups: sci.electronics,rec.photo
Subject: Re: Flashbulbs (was something stupid and illegal...)
Message-ID: <9844@sequent.UUCP>
Date: 18 Jan 89 01:13:18 GMT
Reply-To: brian@crg1.UUCP (Brian Godfrey)
Organization: Sequent Computer Systems, Inc
Lines: 59

>> 	.015 + .025 = 0.04 = 1/25
>> Much slower than most cameras are capable of?
>> I thought flash sync was limited by the shutter action??

   To sync to a flash you must make sure the shutter is all the way open
before the flash fires (begins emitting light.) To get reproducible
exposures you must ensure that the flash is finished firing before the
shutter closes. (Actually the filaments in flash bulbs glow for a while,
but that doesn't count.)
   There are three basic syncing problems: electronic flash, bulbs & leaf
shutters, and bulbs & focal plane shutters.
   Electronic flash is extremely fast, ranging generally from 1/5000 to
1/25000. With electronic flash you have to make sure and wait until the
shutter is all the way open before triggering the flash. This takes from
1/250 sec on some cameras to 1/60 sec on others. The actual exposure 
happens very quickly. Then the shutter closes at its leisurely pace.
   With flash bulbs (the above equation is for flash bulbs) and leaf
shutters you have to trigger the flash, then wait for a bit while its 
filament is heating up before tripping the shutter, the object being to 
have the shutter fully open at the same time as the light arrives from 
the bulb. In the above case, the bulb will be emitting light for 1/25 of
a second. If you wanted truly accurate and reproducible exposures you would 
have to use 1/25 sec shutter speed. Actually, I think you can cut off some 
of the rise and fall time without affecting the exposure appreciably. 
   Focal plane shutters and flash bulbs have a little bit different timing.
Leaf shutters open very quickly, so you triggered the flash then waited
for most of the 15 milliseconds before opening the shutter. Focal plane
shutters open very slowly. If your sync speed on a focal plane camera is
1/60 sec, then that is how long it takes for the shutter to open. 1/60
sec is about 16 ms which is very close to the 15 ms it takes for the flash
to warm up. In this case your camera will have to trip the shutter and 
trigger the flash at the same time in order for the light to begin striking
the film at the same time as the shutter is fully open. But at 1/60 sec
(16 ms) you will be closing the shutter long before the bulb has fully 
discharged (40 ms). Your sync speed for this bulb would still be 1/25 sec
on a camera with a focal plane shutter, though you would probably have no 
problem with 1/30 sec which is more common on focal plane shutters. With
different bulbs you will probably have different sync speeds.
   Cameras and shutters before electronic flash probably didn't have to 
worry about these different syncing schemes (although they may have had
flash powder to worry about.) Modern cameras probably don't have provision
to sync to bulbs. Cameras in the transition period (1950s to 1980s) did make
provision for both schemes while applied to them. My old OM-1 (1975) has 
a switch for "FP" or "X". "FP" is for bulbs (stands for Flash Powder??)
and "X" is for electronic flash. My old Linhoff (I think it is 1950s
vintage) has settings on the shutter for "M" and "X" sync. "M" is also 
for bulbs, though I have no idea what it means. Maybe a loremaster knows
the meanings of the abbreviations. I have never looked to see if there is
a special setting on my OM-4 for bulbs. I vaguely remember reading some-
thing about it in the manual, but it may have just said to use an extremely
slow shutter speed to give the bulb time to warm up while the shutter is
open.
   Hope this was helpful.

-- 

--Brian M. Godfrey
  Sequent Computer Systems Inc.
  !tektronix!sequent!brian