Please check my calculations for flash photography

"zxcvar" wrote in message
m...
Greetings! I shall be grateful if some one checks my calculations for
an external flash. A flash has a guide number of 92 at ISO 100. My
Dx6490 has f number of 3.7 at telephoto end. So the flash can light up
to 92 divided by 3.7, which is equal to 24.86 feet. If I use ISO 400,
then it will illuminate upto 24.86 times 2 =49.72 feet. Is this
calculation seems right to you? I need to use the flash to photograph
my daughter at school functions. She will be 30 to 40 feet away from
me. With thanks.

Is this "alt.photo.retro"?? Who's bothered with guide numbers since the
70's??

"mcgyverjones" wrote in message
.. .

"zxcvar" wrote in message
m...
Greetings! I shall be grateful if some one checks my calculations for
an external flash. A flash has a guide number of 92 at ISO 100. My
Dx6490 has f number of 3.7 at telephoto end. So the flash can light up
to 92 divided by 3.7, which is equal to 24.86 feet. If I use ISO 400,
then it will illuminate upto 24.86 times 2 =49.72 feet. Is this
calculation seems right to you? I need to use the flash to photograph
my daughter at school functions. She will be 30 to 40 feet away from
me. With thanks.

Is this "alt.photo.retro"?? Who's bothered with guide numbers since the
70's??

If you are using a flash in a manual mode you need to know the guide number.
Bet you got a real fancy camera?

Jim

zxcvar wrote:
Greetings! I shall be grateful if some one checks my calculations for
an external flash. A flash has a guide number of 92 at ISO 100. My
Dx6490 has f number of 3.7 at telephoto end. So the flash can light up
to 92 divided by 3.7, which is equal to 24.86 feet. If I use ISO 400,
then it will illuminate upto 24.86 times 2 =49.72 feet. Is this
calculation seems right to you? I need to use the flash to photograph
my daughter at school functions. She will be 30 to 40 feet away from
me. With thanks.

Be careful. Some guide numbers are in meters and some are in feet. That will
throw off your calculations. Most are for ISO 100.

---Bob Gross---

(zxcvar) wrote in
m:

Greetings! I shall be grateful if some one checks my calculations for
an external flash. A flash has a guide number of 92 at ISO 100. My
Dx6490 has f number of 3.7 at telephoto end. So the flash can light up
to 92 divided by 3.7, which is equal to 24.86 feet. If I use ISO 400,
then it will illuminate upto 24.86 times 2 =49.72 feet. Is this
calculation seems right to you? I need to use the flash to photograph
my daughter at school functions. She will be 30 to 40 feet away from
me. With thanks.

You're correct, the calculations have been done right. It should be
noted, however, that flashes don't always put out what the GN claims,
especially if they're older, so testing it would be better.

You might also consider changing the ISO, rather than the aperture,
as she gets closer to you - this will improve quality. At 35 feet you can
switch to ISO 200, f3.7.

And if you want to get really creative, there are little credit-card
sized magnifiers available at office supply stores. They are flat plastic
fresnel lenses - the same kind you have in front of the flash tube. Rig one
about six inches in front of the flash head, and you've just extended the
flash reach (but a much narrower beam) by a considerable margin. With the
DX6490, this could help a lot, and the narrower field of view at the
telephoto end shouldn't be affected by the narrow flash coverage. But you'd
need to do some tests to determine how much light increase you get.

Good luck!


- Al.

--
To reply, insert dash in address to separate G and I in the domain

(zxcvar) wrote in
m:

Greetings! I shall be grateful if some one checks my calculations for
an external flash. A flash has a guide number of 92 at ISO 100. My
Dx6490 has f number of 3.7 at telephoto end. So the flash can light up
to 92 divided by 3.7, which is equal to 24.86 feet. If I use ISO 400,
then it will illuminate upto 24.86 times 2 =49.72 feet. Is this
calculation seems right to you? I need to use the flash to photograph
my daughter at school functions. She will be 30 to 40 feet away from
me. With thanks.

You're correct, the calculations have been done right. It should be
noted, however, that flashes don't always put out what the GN claims,
especially if they're older, so testing it would be better.

You might also consider changing the ISO, rather than the aperture,
as she gets closer to you - this will improve quality. At 35 feet you can
switch to ISO 200, f3.7.

And if you want to get really creative, there are little credit-card
sized magnifiers available at office supply stores. They are flat plastic
fresnel lenses - the same kind you have in front of the flash tube. Rig one
about six inches in front of the flash head, and you've just extended the
flash reach (but a much narrower beam) by a considerable margin. With the
DX6490, this could help a lot, and the narrower field of view at the
telephoto end shouldn't be affected by the narrow flash coverage. But you'd
need to do some tests to determine how much light increase you get.

Good luck!


- Al.

--
To reply, insert dash in address to separate G and I in the domain

(zxcvar) writes:

Greetings! I shall be grateful if some one checks my calculations for
an external flash. A flash has a guide number of 92 at ISO 100. My
Dx6490 has f number of 3.7 at telephoto end. So the flash can light up
to 92 divided by 3.7, which is equal to 24.86 feet. If I use ISO 400,
then it will illuminate upto 24.86 times 2 =49.72 feet. Is this
calculation seems right to you? I need to use the flash to photograph
my daughter at school functions. She will be 30 to 40 feet away from
me. With thanks.

Its in the ballpark. I was able to take pictures of my daughter's junior high
school graduation at 40-45' with a flash with a guide number of 80 at ISO 100.
By using ISO 400, the flash in manual mode, manual focusing, and my Olymspus
C-2100UZ at f/3.5, I was able to get the shot. Obviously under those
conditions, the picture will be noisy, and you will have a horrible case of
red-eye, but at you should be able to get the shot.

--
Michael Meissner
email:
http://www.the-meissners.org

(zxcvar) writes:

Greetings! I shall be grateful if some one checks my calculations for
an external flash. A flash has a guide number of 92 at ISO 100. My
Dx6490 has f number of 3.7 at telephoto end. So the flash can light up
to 92 divided by 3.7, which is equal to 24.86 feet. If I use ISO 400,
then it will illuminate upto 24.86 times 2 =49.72 feet. Is this
calculation seems right to you? I need to use the flash to photograph
my daughter at school functions. She will be 30 to 40 feet away from
me. With thanks.

Its in the ballpark. I was able to take pictures of my daughter's junior high
school graduation at 40-45' with a flash with a guide number of 80 at ISO 100.
By using ISO 400, the flash in manual mode, manual focusing, and my Olymspus
C-2100UZ at f/3.5, I was able to get the shot. Obviously under those
conditions, the picture will be noisy, and you will have a horrible case of
red-eye, but at you should be able to get the shot.

--
Michael Meissner
email:
http://www.the-meissners.org