There are several techniques used to create the isolation shot, but on white there are a few keys that need to be adhered to in order to make it work. The background needs to be pure white, the subject must not be over-exposed and there must not be any hard shadows.
Before you can understand the three points above, you will need to know a couple of things about the lighting to use. First, direct light, like from a lamp, will create a shadow on the opposite side of the model. If the light is pointed directly over head of the camera, then you will have a shadow on your background.(Fig 1)
So in this case, the easiest way to fix this is to either use a very large light source, or move the lights far enough to the side, or top, that the shadow does not show in the picture, by either moving the shadow to the side, or directly under the model. Now it is significantly easier to sidelight than to suspend lights above the model, so for this example we will use side lights.(Fig 2)
Great, now you've created a side light, however now there are shadows from detailed parts of the model, so there are 2 ways to overcome this - a reflector or another light. A reflector could be as simple as a piece of paper, or cardstock, or crumpled tinfoil. In this case I used another light. (Fig 3)
Okay, now we've got the basic lighting problem overcome with the direction of light, the background and shadows, but now there is a new problem. Hard light produces dark shadows from the limbs or other details from a model, so what we need to do is "soften" the light. To understand this concept I like to think of light as being similar to water. If you turn on a hose, and pinch the end of the hose, the water comes out as a concentrated, piercing force. But if you spread the water out through a large hose, it flows smoothly. Light behaves in a similar fashion, so making the light bigger by bouncing it, or through a "diffusor" like wax paper or white fabric will soften the impact of the light. In turn the shadows transition from hard light, to soft shadows in more stages. So to do this, I have rigged up a simple white fabric over top of the model, to form a lightbox. This can also be done with wax paper on a frame, over the head of the lamp, in a cardboard box... There are many versions of this, and it is all much easier than buying professional lighting kits. (Fig 4)
Now that we have our basic lighting figured out, and the shadows are softened, the last thing to do is add accent or effect lighting. The most simple and in my opinion needed light is a "fill" light. This is to bring more light on the most important part of the model - in this case, the face. This light is above the camera to keep the shadow below the model, but to give the front of the model enough detail to be seen clearly. This light is just an accent, so it cannot over-power the sidelights which are in fact the "key" lights. So back it off, or bring it closer if it does not have a dimmer.
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Other effect lights can be rigged up above or behind the model to create different effects like a "hair light" above, or a coloured light on the background to interesting effect.
The final consideration and toughest part of an isolation shot is balancing the proper exposure on the model, and a pure white background. There are 2 common ways of doing this, one of them requires benchmarking your exposure off of the background with a lightmeter. Or in photoshop(or free editing software like gimp) to clean up your whites, you might need to move the white point from 255, usually to about 240 in a levels adjustment layer. Ideally you shouldn't need to do this if you've gotten it right in the camera.
Some general tips to improve your images: If at all possible, use manual settings and a tripod. Common settings are iso 100, 1/30th shutter and f-11 or more. If you're going to change a setting, change your shutter speed.
My pictures have only a little bit of the model in focus!? To get more of the model in focus, just back up the camera and zoom in, pictures from farther away obscure mistakes and give the model a smoother finish. Many jewellery shots are taken at 180mm, so get as far away as possible. Also use as small an aperture setting as reasonably possible, f11-f22 is usually good.
My background isn't white, it's grey, and when I increase my exposure the model is over exposed!? This is a balance between your fill light on the model and the sidelights. Move your model closer to your background, or move the lights back so they are slightly behind the model, but not pointed toward the model. This will give more light to your background. Then increase your exposure.
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