Bounce flash / fall-off - Rule of thumb?

[duncan_trebilcock]

Hi everyone,

 

My Appologies if this has been asked before, but I couldn't find

anything directly answering my question.

 

I am planning a shot with the happy couple set against a background of

an illuminated building at night. The couple will be in darkness.

 

I have a metz 45 CT-1, and I intend to use it manually at full power,

and am happy about the procedure for achieving the exposure I want,

i.e.

 

set apperture for distance to the couple, and set shutter speed to

aceive an ambient exposure for the background (on a tripod of course.)

 

I do not want to use direct flash, so I have constructed a 45 degree

card reflector which is dimensionally near identical to the off the

shelf metz item.

 

I know I am going to get light fall-off, and looking in the metz

manual, there is a simple equation to roughly calculate exposure

compensation, but this appears to refer to bouncing of a wall or

ceiling.

 

I am just about to go out shoot some bracketed tests on astia in a

broadly comparable situation, to get an idea of the compensation I

need to apply, however if anyone has a rule of thumb which would help

validate my findings I would be very grateful to know what you do in a

situation like this.

 

The final shots will be on NPS or NPH - I am using astia for the tests

to more critically reveal exposure errors.

 

Many thanks

 

D.T.

[chris_prinos]

In the situation you describe, outdoors, a 45 degree card reflector isn't useful IMO. All that your going to do is deflect a bunch of light up into the sky, and it's going to come back down onto your subjects. Indoors it's different, because you'll turn the ceiling (or a wall if the flash is oriented that way) into a big flash panel to reflect (and soften) the light. Outdoors your light source off the card will only be marginally bigger (if at all) than the flash head itself, so the quality of light is the same as direct flash, and you're just reducing the available flash output by using the card. Sometimes using a diffuser outdoors can feather the light to the edges of the frame a bit differently, but even then I don't think it's much different from direct flash.

 

 

It sounds like you already know how to calculate the flash exposure for a given distance, just use that (the bounce card will not change anything). If you were indoors using bounce, then you would have to try to calculate it based on the distance the bounce light travels (up to the ceiling and down to the subject) instead of direct to subjects. Plus factor angle of bounce, ceiling absorption, etc, etc. Absent TTL flash automation, a good flash meter would make quick work of the whole matter for you.

[picturesque]

General rule for bounce is 1 1/2-2 stops loss, but since there will be no walls/ceiling for the light to bounce off, you may lose more. Your tests will soon reveal the correct exposure, but as Chris said, a flash meter and testing in the same conditions would tell you immediately.

 

The difference in light quality between direct flash and card bounce will not be big since again, you will get no help from bounced light from walls/ceiling. If it were me, I'd just use direct flash, because the light quality from the 45 is not as harsh as small-headed shoe mount flashes, although I use my 45 CL-4 with the wide angle diffuser on all the time (I use square format 2 1/4). Or, I'd offset the flash a little for some directional light. Since the camera will be on a tripod, you can take the flash off the camera/bracket and hold it high, either to the left or right of the lens axis while tripping the shutter.

[prinosphotography]

I wish there were a way to edit previous answers you post (is there?)... In my post above,

the end of the second

sentance should have read:<p>"and it's <em><b>not</b></em> going to come back

down

onto your subjects"