Background intrusion

[philthejuggler]

Hiya,

I took a load of shots today for this week's 52 entry where I lit a subject with flash and wanted the background to be black.

I had my 30D + 580EXII (on camera) + 24-105 set at the long end on a tripod. I set flash sync (1/250) and used f8 & f11 settings on ETTL.

The room was long & dark, the subject only 4 feet away, the background 25 feet or so away.

Given the ambient reading of the background was 1/6 s @ f5.6, why wasn't it pitch black when I took the shots? All the subjects were nicely exposed but there was some detail in the background.

I would have thought the flash would have dropped off long before the background.

I got round the issue with a black surface as an improvised backdrop, but I'm interested to know why it wasn't pitch black anyway!

Many thanks

Phil

 

[HoppyUK]

Inverse square law, double the distance, quarter the light.

Could be that the background was only four or five stops down, which is very dark but not black. You need at least seven stops to be certain.

Anything reflective will pick up a lot, and white things will also be visible. Plus a bit of ambient that was maybe brighter than you thought, and maybe the background was a bit less than 25ft away.

 

[philthejuggler]

Inverse square law, double the distance, quarter the light.

Could be that the background was only four or five stops down, which is very dark but not black. You need at least seven stops to be certain.

Anything reflective will pick up a lot, and white things will also be visible. Plus a bit of ambient that was maybe brighter than you thought, and maybe the background was a bit less than 25ft away.

Oh ok - that explains it - I thought a couple of stops might have done it!

Many thanks

Phil

 

[alpina]

Inverse square law, double the distance, quarter the light.

Could be that the background was only four or five stops down, which is very dark but not black. You need at least seven stops to be certain.

Anything reflective will pick up a lot, and white things will also be visible. Plus a bit of ambient that was maybe brighter than you thought, and maybe the background was a bit less than 25ft away.

Great answer, copied into the keep foldet

 

[philthejuggler]

Great answer, copied into the keep foldet

Yeah - you can count on Hoppy for an intelligent and helpful response.

 

[HoppyUK]

Yeah - you can count on Hoppy for an intelligent and helpful response.

Cheers guys

Rule of thumb in the days of film was about seven stops from brightest white to darkest black - 128x ratio - and that's still a decent yardstick.

Normal caucasian skin is roughly 1.5 stops down from pure white, and 18% grey should be about in the middle, around 3.5 stops.

The best digital can go a bit further, say eight stops which is 256x, or even a bit more.