From: marques@vax.oxford.ac.uk
Newsgroups: alt.folklore.computers
Subject: Re: Flash pix (was: Deutsches Museum (was: Zuse Computers))
Date: 25 Aug 92 20:33:14 GMT
Organization: Oxford University Library Automation Team

In article <1992Aug25.141057.6411@mdd.comm.mot.com>, mitchell@mdd.comm.mot.com (Bill Mitchell) writes:
> in alt.folklore.computers, berry@pei.com said:
> 
> 
>>>>>>> On Sun, 23 Aug 1992 02:04:30 GMT, jsr@dexter.mi.org (Jay S. Rouman) said:
>>Jay> This sure doesn't have much to do with computers, but I can't
>>Jay> help mentioning my favorite--watching people on the observation
>>Jay> deck of the CN Tower taking flash pictures of metro Toronto at
>>Jay> night.  
>>
>>Just to add some information to the discussion, it turns out that on
>>many cameras that use flash-cubes (and probably the newer guys that
>>have built in flashes) putting the flash-cube on switches to a slower
>>shutter speed, allowing you to take pictures in less light.
> 

Just a guess, but maybe they (the older mechanical cameras that used bulbs) did
this to make the sync between the shutter and the flash easier.

> I know nothing about these cameras, however I do recall from a past interest
> in photography that a flashbulb or flashcube burns for approx 1/30 of a
> second, and an electronic flash typically lasts under 1/100 of a second.
> So, if the normal shutter speed was something like 1/100 of a second
> without flash it would make sense to slow it down for a flash cube but
> not for an electronic flash.
> 
> -- 
> mitchell@mdd.comm.mot.com (Bill Mitchell)

Actually flash cubes have problems when used with faster shutter speeds so
I don't know how valid the above comment is.  The problem occurs with curtain 
focal plane shutters.  These consist of two curtains.  When the shutter is 
released one curtain is pulled exposing the film, a short while later the 
second curtain travels across the film covering it up again.  The delay 
between the first and second curtains setting off control the exposure.  
For very fast shutter speeds the second curtain can set off before the first 
curtain has fully exposed the film.  This means that the film is exposed by 
a 'gap' travelling across the film.  This does not cause any problems in 
daylight (or with flash bulbs) because these are continuous for the duration 
of the whole exposure.  With electronic flashes the illumination lasts for 
only 1/100th or 1/1000th of a second.  This is much shorter than the time it 
takes for the 'gap' to cross the film and so only a part of that frame is 
exposed.  With electronic flash guns one therefore has to use a slower shutter
speed so that the first curtain completely uncovers the frame before the second
curtain begins its journey.  The solution to this problem is to either make 
the curtains travel faster (as in the Nikon FM2 where the shutter is made out 
of titanium as the maximum flash shutter speed is 1/200 (1/250th?) of a 
second) or to use an iris shutter mounted in the lens (as used in some medium 
format cameras) which do not have the flash sync problem.

(PS: the above info is based on stuff I knew when I was interested in the
subject (before I discovered computers) a long time ago so all the datails
may not be 100% correct, however it captures the spirit or the thing)
-- 
Jose Marques, Systems Programmer
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