Compensation for flash bounce

How do I compensate for bouncing the flash? Is there any formula/method
to establish a relationship between angle of bounce and additional
power required from the flash?

Thanks,

- Siddhartha

"Siddhartha Jain" wrote in message
oups.com...
How do I compensate for bouncing the flash? Is there any formula/method
to establish a relationship between angle of bounce and additional
power required from the flash?

Thanks,

- Siddhartha

No, the angle has little to do with it.

What matters is the total distance involved, Flash - Ceiling - Subject, and
the reflectivity of the ceiling.

Using Guide numbers divide by Total distance and then allow one extra stop
of exposure.

If using an Auto flash like the Vivitar 283, have the sensor pointed
towards the subject, with the head angled up, and check output using a Test
Flash.

But do try it out in advance of any serious work.

Roy G

Siddhartha Jain wrote:
How do I compensate for bouncing the flash? Is there any
formula/method to establish a relationship between angle of bounce
and additional power required from the flash?

Thanks,

- Siddhartha

No too many variables. Best is a combination of experience and testing.

--
Joseph Meehan

Dia duit

"Siddhartha Jain" wrote in message
oups.com...
How do I compensate for bouncing the flash? Is there any formula/method
to establish a relationship between angle of bounce and additional
power required from the flash?

Thanks,

- Siddhartha

I use either the traditional method suggested by Tesco News or TTL metering.
Jim

On 14 Feb 2006 02:33:06 -0800, Siddhartha Jain wrote:

How do I compensate for bouncing the flash? Is there any
formula/method to establish a relationship between angle of bounce and
additional power required from the flash?

As others have said, there are variables involved that won't be
amenable to a simple formula, but you can use one as a starting
point, although it may not be worth the complexity and some testing
will still be needed. You could use the angle of the flash, but
it's simpler and probably easier to base the calculation on the
ceiling's height. Assuming that the subject is at roughly the same
height as the camera/flash, a simple trig. formula (Pythagorean)
will give you the distance the light travels. Where D is the
distance from the camera to the subject and H is the height from the
flash to the ceiling, LD (the distance traveled by the light) will
be calculated as

LD == 2 * square root( (H * H) + (D/2 * D/2) )

As an example, using a camera to subject distance of 20 feet and
camera to ceiling height of 5 feet, you'd get

LD = 2 * sqrt( 25 + 100 ) == 22.36 feet. Not much greater than the
distance to the subject, so for a simple rule of thumb, just
increase the camera/flash to subject distance by 10% to 15% unless
the ceiling height is unusually great. Unless I'm making a silly
mistake, to make a difference of 1 f/stop based on distance alone,
the distance (LD) would have to be increased to a bit more than 28
feet. This would give 28 = 2 * sqrt( H*H + 100 ) so 14 * 14 == (H*H
+ 100) and H == 10 feet. Assuming the camera is 6 feet above the
floor, the ceiling height would be 16 feet.

But this would be what you'd use if the ceiling was a good mirror,
reflecting without loss. A real ceiling will not be a perfectly
bright white, and will also diffuse the light, reducing the amount
of it that reaches the subject, which is why testing is needed if
you don't have enough experience to accurately estimate the
exposure. I think it would be much simpler to forego the
calculations and to increase the exposure by 1.5 stops compared to
what you'd use if the flash was aimed directly at the subject. If
you're calculating the exposure based on a guide number and distance
to subject, multiply the distance by about 1.6. Either method
should get you close, unless you're dealing with a very high ceiling
or its color/reflectance is atypical, so only one or two test shots
should be needed from there to get the correct exposure.

"Siddhartha Jain" writes:

How do I compensate for bouncing the flash? Is there any formula/method
to establish a relationship between angle of bounce and additional
power required from the flash?

Best is to use a modern flash that cooperates with your camera to do
TTL auto-exposure.

But I first learned to do bounce flash with a completely manual flash
(a Braun RL-515, to be specific), so let me tell you about it.

First, you have to figure the distance flash to ceiling and ceiling to
subject, and figure your exposure based on that (guide number /
distance = aperture). Then you have to compensate for the absorption
and diffusion of the ceiling (the beam is less focused coming off the
celing). My rule of thumb was to open up one full stop for a normal
white ceiling. For beamed ceilings or darker paint, open up more --
you learn this through experience. (I was shooting B&W negatives, so
the color didn't matter too much, only the density; for color of
course you'll have problems bouncing off seriously non-white
ceilings).

Good luck! Testing with a flash meter makes this a more precise
science, but I didn't have one then, and completed many assignments
successfully.
--
David Dyer-Bennet, , http://www.dd-b.net/dd-b/
RKBA: http://noguns-nomoney.com/ http://www.dd-b.net/carry/
Pics: http://dd-b.lighthunters.net/ http://www.dd-b.net/dd-b/SnapshotAlbum/
Dragaera/Steven Brust: http://dragaera.info/

The formula for other newsgroups would be different but the formula for
digital is the same as is for so many other situations.
Shootinspectadjustshootinspectadjust....just do it before you have to
take "the shot"

--
Thanks,
Gene Palmiter
(visit my photo gallery at http://palmiter.dotphoto.com)
freebridge design group

"Siddhartha Jain" wrote in message
oups.com...
How do I compensate for bouncing the flash? Is there any formula/method
to establish a relationship between angle of bounce and additional
power required from the flash?

Thanks,

- Siddhartha

Its funny I never knew old time photographers doing all this
calculation. Open two stops from normal, evaluate, adjust. A TTL or
even a thyristor flash should do that for you. Just don't try to bounce
off a 20ft, (6m) ceiling and wonder where all the light went.

Tom

On 14 Feb 2006 18:42:01 -0800, tomm42 wrote:

Its funny I never knew old time photographers doing all this
calculation. Open two stops from normal, evaluate, adjust. A TTL or
even a thyristor flash should do that for you. Just don't try to bounce
off a 20ft, (6m) ceiling and wonder where all the light went.

But they were photographers long enough to know better. Don't you
think that some of them might have early on tried to see what
calculations would get them? It would only take a little experience
for them to work out their own rule such as your "open two stops" to
avoid tedious calculations that wouldn't provide much of an
advantage over quick rules of thumb. Also, old time photographers
would be able to evaluate and adjust, but not nearly as quickly as
digital cameras allow.