Discussion in 'Digital Photography' started by Pat, May 1, 2006.

1. ### PatGuest

This newsgroup has a few people who like physics and other things that
are more math-intensive than I am comfortable with. So I need you to
make sure I am doing this right (and am using the right formula)..

I just picked up a second flash and want to use them for night sports
photography. One is a Canon 550EX with a guide number of 180. The
other is a 580EX with a guide number of 190.

If memory serves me right, to add guide numbers, I do the following:

sqrt(180^2 + 190^2) = 262 +/-

Please let me know if I am using the right formula.

Thanks.

Pat.

Pat, May 1, 2006

2. ### Bruce HoultGuest

In article <>,
"Pat" <> wrote:

> This newsgroup has a few people who like physics and other things that
> are more math-intensive than I am comfortable with. So I need you to
> make sure I am doing this right (and am using the right formula)..
>
> I just picked up a second flash and want to use them for night sports
> photography. One is a Canon 550EX with a guide number of 180. The
> other is a 580EX with a guide number of 190.
>
> If memory serves me right, to add guide numbers, I do the following:
>
> sqrt(180^2 + 190^2) = 262 +/-
>
> Please let me know if I am using the right formula.

Flash guide numbers tell you how far away the subject can be from the
flash unit (not the camera!) using ISO100 film and an f1.0 lens and be
illuminated for a correct exposure. The guide number is therefore
proportional to the square root of the light output (because the area
illuminated is proportional to the square of the flash to subject
distance), so your formula is correct -- convert to light output, add,
convert back to guide number.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

Bruce Hoult, May 1, 2006

3. ### 2Guest

In article <>,
"Pat" <> wrote:
>
> This newsgroup has a few people who like physics and other things that
> are more math-intensive than I am comfortable with. So I need you to
> make sure I am doing this right (and am using the right formula)..
>
> I just picked up a second flash and want to use them for night sports
> photography. One is a Canon 550EX with a guide number of 180. The
> other is a 580EX with a guide number of 190.

The numbers are close enough FAPP. You gain 1 stop using two. If you want
some really depressing news, calculate how many you need to gain four stops.

2, May 1, 2006
4. ### PatGuest

I thought about that. It doesn't take very long to realize that very
quickly you need to start carrying around something of the magnitude of
the sun.

Pat, May 1, 2006
5. ### PatGuest

Thank you.

Pat, May 1, 2006
6. ### Roy GGuest

"Pat" <> wrote in message
news:...
> This newsgroup has a few people who like physics and other things that
> are more math-intensive than I am comfortable with. So I need you to
> make sure I am doing this right (and am using the right formula)..
>
> I just picked up a second flash and want to use them for night sports
> photography. One is a Canon 550EX with a guide number of 180. The
> other is a 580EX with a guide number of 190.
>
> If memory serves me right, to add guide numbers, I do the following:
>
> sqrt(180^2 + 190^2) = 262 +/-
>
> Please let me know if I am using the right formula.
>
> Thanks.
>
> Pat.

Hi.

Why bother getting into advanced maths, Square roots and things like Pie R
Squared.

The 2 guide numbers you quote are so similar that they can be treated as
identical. Using both guns together, at the same distance, gives double the
light & double the light equals one Stop less for the aperture setting.

Roy G

Roy G, May 3, 2006
7. ### All Things MoparGuest

Today Roy G commented courteously on the subject at hand

[snip]
> Why bother getting into advanced maths, Square roots and
> things like Pie R Squared.

[snip]

"cake are square, pie are round"

--
ATM, aka Jerry

"Gentleman, you can't fight in here, this the War Room!" - From
the movie 'Dr. Strangelove or How I Learned to Stop Worrying and
Love the Bomb'

All Things Mopar, May 3, 2006