Luminous Landscape Forum
Equipment & Techniques => Medium Format / Film / Digital Backs – and Large Sensor Photography => Topic started by: jonathan.lipkin on February 24, 2013, 11:42:34 am
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Posting here because I don't see a distinct lighting forum and hope you all can help with a problem I'm having.
I want to light a sheet of glass with black velvet behind it so that only the fingerprints and smudges show. I've experimented with a variety of light (spot, soft boxes) and angles for both light and camera (using a hasselblad with hts tilt shift adapter so I can maneuver the camera to different positions). Of course, I need to move the camera off to the side so it won't show in the photo. It's also a challenge to position the light so that its reflection does not show either.
I've read the Light Science and Magic book and am familiar with some of the terms in the book, but there's nothing that precisely addresses what I want to do.
I think that skimming the light across the glass seems to work best, but can't quite work out how to do this well.
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I'd try:
Black velvet about 50 -100 mm ( 2 - 4" ) behind the glass with a light source shining into the space between the two. A fluorescent tube the same width as the glass sheet seems like the simplest option. Try to stop any light leaking onto the front surface of the glass and position the tube above the top of the glass and out of shot. Also try having the fingerprints / smudges on the back surface of the glass to see if they pick up the downlighting more effectively.
This is how we used to display sandblasted & acid - etched glass when I was at college.
If you can't have the black velvet spaced away from the glass you could try edge lighting the glass sheet: I've never done this but I'd try a fluorescent tube against the edges of the glass masked off in such a way that only the edge of the glass is lit. You might need lights on two opposite edges if it's a large sheet of glass.
Good luck.
Graeme
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Depending on what result you are looking for "dark field" illumination (google) might help. It's like putting a ring light behind the subject, and background.
Wollom
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If you want glass to appear invisible, or take the reflections off of anything really, you could try cross-polarization:
1. Buy a roll of polarizing gel, you'll need to frame it, or see if you can get one pre-framed. 17x20" is a nice size.
2. Fasten the gel in front of your light source so it has complete coverage, if you use a diffusor, the gel must be on the outside of the diffusor.
3. Put a circular polarizer on the lens of your camera, take some test shots, and adjust the polarizer on the lens until all reflections are gone.
4. Ideally, the lights should be at a 35-55* angle to the glass, but this method is effective enough to almost remove the reflection of the light itself.
The only downside of this technique is that the double whammy of PL's takes off about 2.5 stops of light, but it can be fairly useful, here's a quick comparison I whipped up:
No CxPL:
(https://dl.dropbox.com/u/41183616/_MG_0158.jpg)
CxPL:
(https://dl.dropbox.com/u/41183616/_MG_0154.jpg)
Note - the white line is a cable on the floor, not a reflection.
P.S. Oh yeah, don't use modeling lamps or have the film too close to the light, it can be damaged by heat.
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KP, will try that. Thanks so much for the advice.