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Glass is a transparent material, therefore the shape of anything made of glass can only be clearly defined by lighting it so that it stands out as a dark shape against a light background or as a brightly lit shape against a dark background. With some subjects it is effective to combine both methods.
With the first type of lighting some of the sparkle of the glass may be lost but every detail of the shape will be perfectly clear. This is a great advantage if the photograph is to be reproduced. Glass most suitable for this treatment should be tall rather than squat, i.e: vases, decanters, jugs, etc, and it should not have too much decoration.
For this type of photograph the subject is placed, preferably on a plain surface or on a sheet of glass, near the edge farthest from the camera. This arrangement prevents the background line from cutting through the subject. A vertical white background, several feet behind the subject, is lit by spot or floodlight. No light is allowed to fall on the subject, therefore it stands out in silhouette. Variations in the background are sometimes made by casting shadows across it. This calls for care to avoid confusing the outline of the subject. For the same reason, textured backgrounds are avoided as they tend to give a false texture to the subject itself.
The best viewpoint for this type of photograph is nearly level with the base of the subject.
It is more difficult to photograph glass so that it appears light against a dark background and the technique varies to some extent with the type of glass. Cut or engraved glass usually looks best against a dark background, more especially a plain surface that does not fight with the pattern on the subject. And it should curve upwards behind the subject so that there is no horizontal edge to cut the photograph in two. Tracing paper laid over a sheet of plate glass provides one suitable type of background as it enables the subject to be lit from below- a form of lighting which is often necessary.
The main modelling light is kept low and almost behind the subject. It is usually provided by a spotlight to ensure that light falls only on the subject, leaving the background in shadow. Where this light does not illuminate the subject sufficiently a second spotlight underneath the background is directed vertically upwards on to the subject; this light helps to outline such parts as the rim of a glass or vase or it may be used to pick up highlights in the base and stem. The viewpoint for this type of photograph can be anything up to 60° above the horizontal, but experienced workers tend to keep it lower.
Combined Silhouette and Highlight Treatment
A combination of light and dark background tones is usual with flatter subjects like ash-trays, shallow bowls and dishes. The subject is placed on a sheet of clear plate glass which is supported 3 or 4 feet above a plain white background. This white background is then lit by a spotlight adjusted until the areas of highlight and shadow look right when seen from the camera viewpoint. The final effect is that part of the subject is silhouetted against a light area, while parts like the rims and upper edges stand out as highlights against the shadow area of the background. A second light source above the plate glass is occasionally directed on to the subject itself to add extra highlights and sparkle.
Really high class work for catalogue and "glossy" magazine reproduction is almost always done with a large format plate camera using a long focus lens stopped right down to give the necessary depth of field.
As with most commercial photography, a lens with a long focal length is generally chosen to avoid close-up distortion.
Normally, when photographing glass, best results are obtained from panchromatic material which helps to prevent excessive contrast in the negative.