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Re: [OM] A shutter speed mystery

Subject: Re: [OM] A shutter speed mystery
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sat, 10 Mar 2012 08:00:45 -0500
As a possible answer to my own question it occurred to me that the 
projectiles (at only about 1/300 second after firing) are probably being 
illuminated by the bright flash of the guns.  So the film was probably 
exposed by the flash of the guns and not the ambient light.  Sort of a 
built-in strobe and not requiring a fast shutter or fast film.  Sound 
reasonable?

Chuck Norcutt


On 3/10/2012 7:34 AM, Chuck Norcutt wrote:
> Yesterday we went with some friends to Fort DeSoto Park for a picnic and
> a bit of beach lounging.<http://en.wikipedia.org/wiki/Fort_Desoto>
>
> While a very large public park today, Fort DeSoto was originally
> constructed between 1898-1906 as a coastal defense battery to protect
> Tampa Bay.  If you scroll to the bottom of the page linked above you'll
> see some of the artillery and mortars originally installed here and at
> Fort Dade a short distance away.  With 12" bores the mortars are rather
> impressive beasts.  They were first fired in 1903 and were deactivated
> not later than 1917.
>
> In a long corridor of historic photographs of the guns and emplacements
> there is one very faded shot of two mortars being fired at the same
> time.  Much to my amazement the projectiles of both guns are clearly
> visible perhaps some 30 feet above the muzzles.  According to the
> details I've been able to find about these guns the muzzle velocity
> ranges from about 1,000 to 1,500 feet/second depending on the weight of
> the projectiles whose length ranges from about 3-4 feet.
>
> Now the shutter speed mystery.  How was this photo taken?  At 1/500
> second the projectiles moving at 1,000 feet/second move 2 feet or 4 feet
> at 1/250 second.  The angle of view from the camera's position probably
> creates some foreshortening of the projectiles but it *appears* to me
> that the shutter speed would have been not longer than 1/250.  I don't
> know whether shutters of the period operated that fast but, even if they
> did, I don't know how it would have been captured on a film emulsion of
> the time.
>
> So, how was this photo taken?
>
> Chuck Norcutt
>
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