[sci.electronics] Strobes and shutter speeds

johnw@astroatc.UUCP (John F. Wardale) (01/21/89)

In article <1818@maccs.McMaster.CA> nusip@maccs.UUCP (Mike Borza) writes:
>Pardon my ignorance, but... doesn't this mean that shutter speed is
>essentially irrelevant for a flash picture taken using an electronic
>flash?  If so, how does one control the exposure?

Yes!

You can change F-stops on the lens.
You can control the light-output from the strobe.
Automatic strobes have *FAST* sensors that shut the strobe
off when enuf light has come out.

You can NOT control it in the time-domain.

You must set your camera at or slower than it's "X" sync speed.
[slower lets you blend in more of the available light.
 (3 stops down is better than 5 or 6) and it softens the lighting
 effect slightly]

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The Rest of This Discussion *ASSUMES* a FLAT (ie FOCAL-PLANE) Shutter
---------------------------------------------------------------------

Most cameras have X at one of {1/60   1/100   1/125}

At speeds FASTER than the X sync, the the closing blade of the
shutter starts closing *BEFORE* the opening blade completes its travel.
This is fine for stoping action because each point [grain, pixel, region
or ???] on the film is exposed for a shorter amount of time!

For a shutter speed of 1/1000 (for an X @ 1/100 system) the shutter
is a moving slot, 1/10th the width of travel

*YES, if you *PAN* with a moving object you can extend or compress
it with these "apparently fast" shutter speeds.
NOTE:  Some shutters move hoizontally, some move vertically.
Watch *YOUR* shutter at 1/4 or slower with NO film, and NO lens.

-------------------------------

On my camera If I use a strobe at 1/125 I get about 1/3 or 1/4
of the frame black.

If *I* designed a camera, I would rig it to *NOT* fire the strobe when
the shutter was set faster than the X speed.  [Thus you'd ruin *ONE*
frame, not the whole damn roll!!!  :-(  :-(]

-- 
					John Wardale
... {seismo | harvard | rutgers | ames} ! {uwvax | cs.wisc.edu} ! astroatc!johnw
                                            or   astroatc!johnw@cs.wisc.edu
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