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04-12-2012, 01:48 PM   #1
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technical obstacle using monitor as background - strange shadow. Help!

Hello,

I have a curious technical obstacle that I was hoping someone more experienced than me might have some insight into, namely odd shadows. I use a smallhd field monitor as background (I pipe PC output onto it, ie a photoshop gradient) and use 3 PC synced flashes to illuminate my subject. Typical exposure is 1s which gets both the light falling onto the subject from the flash pop and the light on the monitor. This can give very nice results.

But.

In some of my shots, when I look at individual hairs on my subject, often there is a dark shadow to one sde of the hair and a corresponding light area on the other. The weird thing is it's not on the hair itself but beside, on some virtual 'plane' parallel to the hair (ie not on the monitor plane either).

The strength of the effect seems to vary, it isn't related to monitor orientation and actually becomes more extreme on shorter exposures (the shadow is further removed from the hair). I was wondering if anyone had any insight into what might be going on, and recommendations how to avoid it. This linked photo is an example: it is a 100% shot at Flickr :



My gut feeling is it's something to do with light falling on the subject from the monitor, not so much the flashes themselves. But has anyone seen this before?

Thank you very much for your advice!



-Johan

04-12-2012, 01:49 PM   #2
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Is the monitor a CRT or LCD?

might be something to do with the sweep frequency of the CRT if it is one.
04-12-2012, 02:07 PM   #3
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Thanks - it's lcd =(
04-12-2012, 03:07 PM   #4
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The problem is the shutter speed. The flash is lighting the subject. The subject is masking the background. When you let the background burn in over the course of a second, any slight movement means the "masked" part of the background will show. This presents itself as a "shadow". The strobist talks about it a bit here. If you zoom in on the image from the article (here) you can see the same shadow around the subject.

Basically, you need a brighter background so that you can reduce your shutter speed a lot. You should be close to sync speed (1/180) to reduce any motion blurring.

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