Undoubtedly, the purely scientific aspects of photography are extremely important, especially in the areas of the physics of light and the related optics. Most of us know that the literal meaning of the of the word, as per it's root, is painting or imaging with light. Generally speaking, it is interesting and extremely helpful to understand how things work, mechanically, physically chemically and theoretically in order to better and more appropriately apply certain methodologies to the artistic aspects of photography or any other art-form or craft.
The remarks, this far, that have been made about the workings a Fresnel light source are mostly correct, however, the aesthetics have not been address in terms of practical photographic results especially in the areas of portraiture and commercial photography where the aesthetic aspects of the resulting imagery are vital.
Technically speaking, with any and all photographic light sources it is certainly a basic requirement to understand certain theories and formulas including those of; angle of incidence, reflectivity, color temperature, the components of a beam of light that enable an understanding of “feathering” and the concepts of softness and hardness of light as influenced by size of the light source, the optics of the light source, if any, and the distance of the light source from c the subject. Each and every light source type has its own suggested applications, prescribed usages, properties and limitations. Even units of the same ilk can have some differences and variations due to differences in size, construction, adjustablity and/or engineering concepts.
In the “old school” of portraiture, the diehards believed and professed that spotlights and raw unmodified floodlights were the tools of “real photographers”- the stuff that separated the boys form the men. Soft-boxes and umbrella “contraptions” were looked upon as “idiot-boxes” with which lesser photographer could “cheat and get away with murder” in terms of lighting mistakes. A few of theses old guys may have “condescended” to use a thin sheet of “spun glass” material as a diffuser. “Real” light control was accomplished by very precise lighting placements, feathering, barn-dooring and, snooting. No doubt, some outstanding and absolutely stunning work was and still can be created utilizing theses simple methods and principles but it ain't necessary, especially the first time around.
Raw lighting devices (floods in particular) were the common tools in portrait and commercial studios for time immemorial. Spotlights were mostly found in theatrical stage lighting grids so as to to place light in specific areas of the stage, act as back or background lighting either unfiltered or in conjunction with colored gels. All of this eventually graduated into cinematography where raw floods and spots were the common tools back in the day. It was common practice, again, back in the day, for the stills photographer on movie sets to take temporary command of the lighting crew for photo-calls and make portraits of the stars and images for lobby cards and publicity photographs right on the sets. Eventually, spot lights of various configurations and types found their way into the photo studio to facilitate dramatic and theatrical effects. Most were electrical- the originals were oftentimes illuminated by carbon-arc systems that were rather cumbersome and somewhat dangerious.
There are two basic types of spotlights, the optical spot and the Fresnel. The optical spot works like a movie or slide projector or an analog condenser photo-enlarger. It incorporates lenses that can be focused and thereby condenses the light beam in a very precise and definite manner. Some of theses units have variable apertures that further narrow the beam. Some of them also have internal and external features that can introduce colored gels patterned cutouts call “cookies” that can be projected onto a white, gray or colored background. Theses spots are recommended where an extremely defined or even harsh main light is required or for some of the aforementioned special effects are required.
The Fresnel spotlight is more commonly found in the photographic studio. Its effects is quite a bit softer or more diffused that its optical counterpart but it is still, as compared to a flood light, rather a hard and concentrated light source. There, however, are many variation in its aesthetic effect. Much has to do with the internal light source; a conventional incandescent projection lamp, a quartz tube or an electronic flash tube; its size in relation to the Fresnel lens, the size, shape and reflectivity of the reflector behind the bulb, or tube, its adjustable position within the housing, the characteristics of the Fresnel lens itself and the distance of the unit from the subject and the degree of feathering as well! The all over size of the unit, obviously, also factors in too. Both, a mini-spot or “inky-dink” and a keg-light can be of the Fresnel class but have decidedly different effects and applications. We can conclude that all Fresnel spots are not created equally.
To say that a Fresnel spot does not qualify as a light modifier may be scientifically correct but only a half-truth when the aesthetics of photography are considered. The lamp or flash tube, would not have any of the aforementioned usages, advantages or even disadvantages of an entire Fresnel unit with the lens in place. There are also a number of adaptions for popular electronic flash systems that accessorize the standard lamp-heads whereby they can preform as a Fresnel unit.
In most shooting situations, there is not time to make mathematical, trigonometrical or formula based calculations. Theses assessments can be applied perhaps when choosing a lighting unit but the gold standard here is to assess the lighting effects visually by moving the light around latterly, vertically, back and forth and using various degrees of feathering. Feathering means to use the edge of the beam of light rather than the center “hot spot” part of the beam.
No doubt, Fresnel spots are certainly useful additions to any lighting “arsenal”! There can be, however, certain pitfalls in their nature and usage can tend to be frustrating and like everything else in photography, some research, study, experimentation and practice are needed to attain the full potential of any equipment and technique.
To illustrate this outlook in spot light or even raw floodlight usage you need to make a list of just about everything that can go wrong with lighting technicalities and aesthetics: Out of control ratios, results that are way out of the dynamic range of our cameras' sensors, totally washed out highlights and/or blocked up shadows, stray light spilling where we don't want it, washed out highlights due to steep angles of incidence, flare from spilled light, overly harsh and unflattering results with certain subjects, problems such as “raccoon” eyes due to a main light that is placed to high in relations to the subject, misplaced shadows for the nose projecting onto the lips- the list goes on and on.
Many of the aforementioned gremlins can appear even with the softest and most forgiving lighting configurations but with spots and raw lights- just double the bad potential by 10 fold. As one other poser in this thread aptly alluded to; if the subject of the light source is moved so much as an inch away from the planned postilion, the entire effect can radically change. Photographing a impatient subject such as a very active child can become an impossible chore. Even with a totally cooperative subject, the photographer has to really watch is lighting Ps & Qs. A lighting change may be required when the same subjects in the same position simply changes expression for a smile to a more demure look. A spot light as a main light makes the photographer very aware how even minor shifts in musculature of the face can noticeably effect or affect the precision of a light scheme. For some, all of this can be a nasty pain in neck but let me tell y'all; it's a great way to learn and refine your lighting savvy even when using softer light sources. One becomes more observant and aware of facial characteristics, lighting patters and cosmetic lighting techniques. Wanna thin out a round face, broaden a thin face, hide a double chin, straighten out an irregular nose? Practice with a spotlight! Sometimes it's hard to see the subtle changes or exactness of remedial lighting techniques with soft lights. The spotlights more easily and succinctly demonstrate the differences. For the “Old Hollywood” look- the Fresnel Spotlight is the tool of choice! A great combination is the use of spotlight with a soft-focus lens- it called hard light/soft lens. This combo imparts a lovely ethereal look without a “mushy” out of focus impression- great for retro stylists. Inversely, soft light with a tack-sharp monochromatic lens can yield an equally dramatic effect.
Although spotlights have a very controllable beam spread, it is oftentimes necessary to modify them as well. Barn doors and snoots can prefect stray light from becoming a distraction and some experienced portraitists will even employ a diffuser and/or a colored gell for subtle variations in effects.
Hope this helps!