Re: Re: pin hole and flash

I literally agonized on the decision of whether to comment further or not,
the truth is when topics like this come up, the tips of my fingers itch
badly with the urge to write, after all, we could be on the verge of , well
forget it I better not go there!

> ----- Original Message -----
> From: heyseus@xtra.co.nz
>
> I used this theory to start with, (the difference between f-stops = number
> of flashes) but i found it to be insufficient. i found that i got good
> results from multiplying the difference by 1.5. (So; exposure = the
> difference of meter reading and camera aperture x 1.5)
> EG; meter = f-64, camera = f-283 'pops' = 18.

Well, there are 4.75 stops between f/64 and f/283, let's say 5 stops, which
is the same as approximating f/283 to f/360. 5 times 1.5 is 7.5, let's make
it 8, therefore, if you are giving 18 pops, you are giving 10 pops more than
your theory mandate.

On the other hand, physical laws say that an immediate full stop smaller
aperture needs twice the exposure that the previous one. When we are using
flash as primary and maybe the only source of light the exposure time is
irrelevant as long as the exposure time is equal or longer than the duration
of the flash "pop", let's assume our flash pop last 1/1000 sec, then let's
also assume you are making an exposure for which our flash meter is giving
us an aperture of f/64 that in fact means f/64 @ 1/1000 secs. So if f/64
needs 1/1000, what exposure would f/360 need?

f/64 @ 1/1000
f/90 @ 1/500
f/128 @ 1/250
f/180 @ 1/125
f/256 @ 1/60
f/360 @ 1/30

Then, f/360 needs 1/30 , nothing xtrange about that, right? Well, let's see
how many 1/1000 of a seconds there are in 1/30 of a seconds, to find that
out we just divide 1000 by 30 and that gives us 33, that tells us that if I
am going to split an exposure of 1/30 second in multiple exposures with a
duration of 1/1000 second we would have to give 33 exposures. In
conclusion, if a pop of flash that last 1/1000 is what f/64 needs, an
aperture of f/360 would need 33 pops of flash (32 actually) and that is the
same as consecutively doubling the number of pops for each f/stop separating
the f/64 and f/360.

There is also another physical phenomena working against us called
INTERMITTENCY EFFECT which tell us 33 exposures of 1/1000 (33 pops) will
provide less effect on the film that a single exposure of 1/30 second, to
correct for this effect, we need to increase the number of pops, a book I
read sometime ago, recommends 5 or 6 (I believe) pops extra for 5 stops
difference, that would make it some 38 pops in total or so!

> I have to run a workshop on my theories as part of my
> course so i will get back on the flash formula, and see if it accurate for
> more than just my camera.

I like it!! An artist interested in his theories complying with the
scientific method ;-)
Let the results be known to the list please.

My guess is that formula is just accurate for your camera :-)

> thanks for the replys, i'll check out the archives ;)

Good for you! ;-)

Guillermo

PS: for some type of flash photography, like "low key" photography and if
using negative film, a less number of pops may gives us enough exposure in
our negative for very printable results, after all, in low key photo we are
interested in some highlights and lots of shadow and/or black areas.
Received on Sat Jul 7 11:40:14 2001