You have to think how light behaves. Light goes line of site and bounces off of objects at 90 degree angles. Think about how a pool ball acts when you hit it with the stick and bounces off the bumper. Light bounces the same way. This means that it's predictable and controllable. A good way to tell if you are going to see reflections come back at you is put a mirror where your glass is and shine lights on it. Then look through your camera view finder. If you can see the light, you will see reflections. You don't want your lights at 45 degree angles from you. that means the light hits your glass and bounces at 45 degrees. 45 degrees plus 45 degrees = 90 degrees and the light bounces right back at your camera. So you want to make the angle of your lights less or greater than 45 degrees. When you use 2 or 3 lights, you have to consider all those angles that the light is bouncing off the glass at. One or more of them is going to be bouncing at a 90 degree angle from your camera position unless you are careful where you place them. Don't forget that this is 3D also. Any light placed to the right or left as well as above and below that will create a 90 degree angle back to your camera is going to show up as a reflection of some kind. Of course this is assuming a flat reflective surface. Once you introduce curved, round, convex, concave, odd shapes, etc., it gets even more interesting.
Light placement is something that is learned... some learn faster than others. If you practice enough and understand the properties of light direction, pretty soon you can do all the calculations in your head as to where the best angles are to place lights.
You also want the largest light source you can create. This is where light panels and diffusers come in. You put a smaller light source behind it, the light hits the diffuser and gets spread out into a bigger light source. Of course you lose light in the process, both from absorption and from light reflecting back towards the light and at angles away from the diffuser. Only a certain percent of light goes through the diffuser and falls on your object. You light tent is turning the lights that fall on it into a bigger light source, which softens and diffuses it. Think of the sun shining. You have a very dark shadow when there are no clouds. This is because the sun is a "small' light source. Now bring in a layer of clouds. Your shadow gets lighter and softer. This is because the sun has become a bigger light source. Now bring in another layer of clouds and your shadow gets even fainter and less distinct. It also gets less bright out. The sun is still a big light source, but now the thicker clouds are diffusing the light even more. If the clouds get thick enough, you will have hardly any shadow at all and the light will be nice and soft and smooth... also it will be low contrast, which makes thinks look dull. So you have to find the balance between light and diffusion to go with what you are trying to do.
If you don't like using your light tent, make some light panels. These work great and can really soften the light. I made a 6' x 4' light panel out of PVC and sport nylon for under $35 (and the $35 bought almost enough to do 2 of them, but one was plenty).
Here is a PVC frame and panel I made out of a window screen repair kit that I hung from the frame:
Here is the larger panel I made:
The PVC panels had a C clamp (what is used to connect two chain link fence panels together) connecting them so the two sides could be adjusted making it free standing. That way you can move them around with your lights to get the best angles.
One other trick that most people think is opposite to what you should do when getting reflections and highlights... most people move their light away from the object. But again, this makes your light smaller. The trick is to move your light closer, in fact as close as you can get it without it being in the picture. This makes your light source a bigger light source as compared to your object and also reduces the amount of bounce you get from the light since it doesn't have as far to travel before it hits your object.
Ok, I've probably rambled enough to just make it more confusing, but if you learn how light behaves, you will be well ahead of the game.
Mike
Question on really reflective items
