G'day Dallas & M. Carter,
In short I would have to answer "yes" to your question. If you hold your arm out with your thumb extended towards the sun on a cloudless day the size of the sun would not be larger than the size of your thumb (depending on latitude and season and time of day). N.B. don't look at the sun directly as you can burn your retina by focussing the U.V. Glancing down you would see a very crisp shadow on the ground. If you did the same thing on an overcast day you would be lucky to see any discernible shadow. In the first case the sun is the source of light and in the second the entire cloudy sky becomes the source.
The same holds true for fresnels. If you look at the lens you will see concentric rings of angled glass held together by troughs of glass. Each ring and trough becomes the source. Look at a turn indicator or warning flasher on an automobile and you will see ripples and bumps that refract the light and the whole of the plastic cover becomes the source and glows evenly. If they did not have these refractive elements built into the lens you would simply see a red or yellow filament flashing on the car.The fresnel lens in this case becomes a larger source of light than simply the filament in the globe.
Thinking about the sun/cloud analogy above, the larger the source of light the softer the light becomes, and hence somewhat more attractive.Going back to the sun analogy; the sun is way further away from the earth than clouds. Another given in lighting is that the further away the source is the apparent size of the source is diminished creating a harder light. Hence the idea of placing a model closer to the fresnel lens for cosmetic reasons, as cited in your book. The closer one is to a relatively larger source increases the wrap-around factor with light spreading around a face, for instance, and into otherwise un-cosmetic shadows.
Your mention of umbra and penumbra comes from the nature of the lamp body itself. There are in fact two sources of light hitting the back of the lens - one from the filament and one from the reflector. In an ideal world both of these would line up exactly but we don't live in an ideal world. Many profile spots in theatre have a reflector adjustment to try and align these two sources and mechanically they appear to be moderately successful; as my learned friend M. Carter suggests in his post in this thread. This misalignment is also partly to blame for chromatic aberration as M. Carter discusses in his post. In my humble opinion, though, the use of the umbra or penumbra would not make a useful difference in the outcome aesthetics of the setup - but this is certainly my own opinion and do not mean to second guess your book.
On a further note, M. Carter discusses the usefulness of fabrics which I can highly recommend. I have spent hours in fabric shops overcoming my initial hesitancy at plowing thru women's dress fabrics for that certain type of embroidery or fabric to hang in front of a source to make it look more interesting (see my previous post in this thread about camouflage nets). I also am adept at discussing nylon stockings and denier counts for use in front or behind the elements of my lens as well as using them mounted in a hole on a black card when burning in blacks under my enlarger to "spread" the blacks a bit. But that's another topic I suppose. At any rate I hope that I have not confused this issue but maybe have shed a bit of light on it.
Cheers for now,
Sam
