Diffusion test - what went wrong?

I was testing the effects from home-made diffusers with an off-

camera, hot-shoe flash (550EX) in manual mode using slide film

(RDPIII.)

 

One shot, where flash light was diffused with a white pillow case,

turned out slightly (maybe 1/2 stop) underexposed and, most

importantly, looking like a print from an underexposed, flash-lit,

night shot on negative film where correction was done upon printing

(grainy, muddy, greyish, washed out, etc.)

 

Flash power was calculated from GN opening up 2 stops to compensate

for the diffuser light loss. Flash-to-diffuser distance was 25cm and

diffuser-to-subject distance 1.75m.

 

The subject (small souvenir items) was placed on a non-reflective,

black cloth, and the background was a non-reflective black cardboard

sheet at 40cm from the subject.

 

The shoot was in a dark room with off-white coloured walls and

ceiling. The subject was at least 1.5m away from any wall.

 

What do you think caused this effect? Other shots taken in the same

environment don't show this problem.

 

Sorry, but I have no access to a film scanner to show you the anomaly.

 

BTW, I noticed the OmniBounce adds a slight yellowish cast...do you

agree?

If you are going to work in manual mode, particularly with light modifiers, you need a flash meter. Until you meter your flash, you don't know what its actual GN is. You certainly don't know how much light is being lost with the diffusers. Unless you want to run endless science fair experiments with lots of film, shoot TTL or get a meter.

The GN is optimistic and based upon new batteries, and taken at the hot spot in the light. You ruined the hot spot by diffusing it. Now the truth be seen, this unit isn't as powerful as advertised. You need a incident domed flashmeter, like a Minolta iii flash meter, used. Once purchased, you will be dead on.

I'm not concerned here about the light loss; the whole test was about getting a feel of the amount of power loss when a diffusion material is added.

 

The exposure itself was not much off, as I mentioned. I have other slides from the same shoot that are underexposed more, but don't show this anomaly. What I want to know is the cause of the described quality; i.e. the underexposed negative film effect.

 

Could the fabric have scattered the light in such a manner to cause the smokey-like loss of contrast?

 

I know that the use of a lightmeter is better, etc. The whole this is, well, a test. I'm simply seeking an explanation......

I'm trying hard to visualise what you're describing and the thing that comes to mind is flare. Is there any chance that light from the difuser was striking the lens surface? That could, together with underexposure, produce an effect such as you describe.<div></div>

Although the diffuser was above the camera, it was behind the lens, and there were no reflective surfaces in front of the lens. In addition, the effect is evenly spread across the image; flare usually creates "patches."

 

It's frustrating not to be able to show you the slide.

 

BTW, nice photo, Harvey; in this case, the flare adds nicely to the retro ambience.

Harvey that picture is absolutely adorable.