Donald Miller
Allowing Ads
- Joined
- Dec 21, 2002
- Messages
- 6,230
- Format
- Large Format
Several have indicated an interest into additional information on a point light source for use in enlarging. There is a relative lack of information available online and since I recently converted my Durst 138S condenser enlarger to a point light source, I will write up a brief theory of light, it's application, and it's effects insofar as photographic enlarging.
Before I begin, let me say that what I am about to state is based upon my education and my experience. It is not designed to be a point of contention for differing viewpoints. It will be factual and factually supported.
To begin, let's recognize a couple of things. The first is that the smaller the source of light in relation to it's illuminated subject, the greater the inherent contrast. When we apply this to the world we observe, we need look no further then the effects of the sun on a clear cloudless day. We observe that inherent brightness ratios are at their highest then. Shadows are sharply defined. Not only are shadows sharply defined but also contrast between different reflective values are at their highest.
However when we compare this to day when we have thin cloud cover , then we observe that the shadows are less well defined. Not only are they less well defined but also the contrast between different reflective value objects less well defined. The reason is that we have converted the relatively small source of light, the sun, into a much larger and diffuse source of light by converting the cloud cover into the source of light. The thin clouds effectively scatter the light rays emanating from the small source, the sun. This could be compared to firing stobes in a bank in studio versus bare bulb lighting.
When we take this still a step further and, rather then a thin cloud cover, we have a heavy cloud cover then the shadows will be totally absent and the contrast of differing reflective values will be at their lowest. The heavy backlit clouds have effectively converted the small light source of the sun into a much, much larger source of light. Again we find that the light rays are very diffuse in this case.
This effect is what we encounter when comparing differing light sources in photographic enlarging. The cold light head, color and variable contrast diffusion light sources all serve to create a large light source in relation to the illuminated object. Which in this case are the individual density regions of the camera negative. Not only are these light rays in these types of light sources diffuse (scattered) but they are also not collimated (directed along a defined axis). Thus we experience a loss of sharpness between different density regions of the camera negative, we also experience a lack of information on the print. We need look no further then the absence of spotting when comparing a diffusion light source enlarger to a condenser enlarger. The same dust and defects may exist on the negative in both cases. However because the light is not collimated and diffuse we have effectively "washed" away the defect by the scattering of light. We must recognize, it would seem, that we can not "wash" away the effects of dust and defects without suffering an accompanying loss of local contrast and sharp detail.
The next step is to look at a condenser enlarger. Condenser enlargers collimate (direct) and focus the light from a frosted bulb at a given point. In a well designed condenser system, this light is focused at the nodal point of the enlarging lens. Not all condenser systems have well designed optical systems. Additionally, however, I think that it is important to recognize that we still have a relatively large light source in a condenser enlarger. The frosted Thorn lamp in my Durst 138S was approximately 4 inches in diameter. Also, because it is encapsulated in a frosted envelope, it is diffuse at it's origin. Thus while some of the loss of sharpness and local contrast experienced in a diffuse light source are solved by the optical system of the condensers they are not totally solved because the light is still diffuse at it's source. It may be focused but not well collimated.
When we look further at this matter, and create a much smaller light source. We reach the point where we have light that is sharp, collimated, and focused. The effects are, so much are possible, the sharpest detail and greatest local contrast available within a conventionally available photographic enlarging system.
At this point I want to interject that I disagree with those who contend that the same prints can be gained from all differing light sources. That simply is not true. In this context, it is important to note that while one can utilize diffusion as an addition of a point light source, should that be desired for whatever reason, it is impossible to transform a diffuse light source into a sharply collimated and focused light source. To contend that would be to say that we would suddenly bring the sun beneath the clouds. I think that we can agree that just will not happen.
With this in mind, we begin to seek out lamps that have small filaments that are capable of incredibly bright light housed in a relatively small clear quartz envelope. These lamps typically operate at or near 500 C. Thus cooling of the lamp enclosure by induction cooling is normally warranted.
The change from a large diffuse light source to a small well defined light source does carry certain design requirements in addition to the heat encountered in lamp operation. The other problem is that the light must be even across the projected area. This calls for designs that are not encountered in the use of diffuse light sources.
Reflectors are used in the design that I developed and installed. The reflector design must be clearly defined. It's dimensions must be carefully calculated and adhered to. The fact that light does have defined and predictable characteristics does help in this regard. In building a point light source as in all aspects of lighting the law that the angle of reflection equals the angle of incidence holds true.
I hope that this has answered those who have had questions.
Before I begin, let me say that what I am about to state is based upon my education and my experience. It is not designed to be a point of contention for differing viewpoints. It will be factual and factually supported.
To begin, let's recognize a couple of things. The first is that the smaller the source of light in relation to it's illuminated subject, the greater the inherent contrast. When we apply this to the world we observe, we need look no further then the effects of the sun on a clear cloudless day. We observe that inherent brightness ratios are at their highest then. Shadows are sharply defined. Not only are shadows sharply defined but also contrast between different reflective values are at their highest.
However when we compare this to day when we have thin cloud cover , then we observe that the shadows are less well defined. Not only are they less well defined but also the contrast between different reflective value objects less well defined. The reason is that we have converted the relatively small source of light, the sun, into a much larger and diffuse source of light by converting the cloud cover into the source of light. The thin clouds effectively scatter the light rays emanating from the small source, the sun. This could be compared to firing stobes in a bank in studio versus bare bulb lighting.
When we take this still a step further and, rather then a thin cloud cover, we have a heavy cloud cover then the shadows will be totally absent and the contrast of differing reflective values will be at their lowest. The heavy backlit clouds have effectively converted the small light source of the sun into a much, much larger source of light. Again we find that the light rays are very diffuse in this case.
This effect is what we encounter when comparing differing light sources in photographic enlarging. The cold light head, color and variable contrast diffusion light sources all serve to create a large light source in relation to the illuminated object. Which in this case are the individual density regions of the camera negative. Not only are these light rays in these types of light sources diffuse (scattered) but they are also not collimated (directed along a defined axis). Thus we experience a loss of sharpness between different density regions of the camera negative, we also experience a lack of information on the print. We need look no further then the absence of spotting when comparing a diffusion light source enlarger to a condenser enlarger. The same dust and defects may exist on the negative in both cases. However because the light is not collimated and diffuse we have effectively "washed" away the defect by the scattering of light. We must recognize, it would seem, that we can not "wash" away the effects of dust and defects without suffering an accompanying loss of local contrast and sharp detail.
The next step is to look at a condenser enlarger. Condenser enlargers collimate (direct) and focus the light from a frosted bulb at a given point. In a well designed condenser system, this light is focused at the nodal point of the enlarging lens. Not all condenser systems have well designed optical systems. Additionally, however, I think that it is important to recognize that we still have a relatively large light source in a condenser enlarger. The frosted Thorn lamp in my Durst 138S was approximately 4 inches in diameter. Also, because it is encapsulated in a frosted envelope, it is diffuse at it's origin. Thus while some of the loss of sharpness and local contrast experienced in a diffuse light source are solved by the optical system of the condensers they are not totally solved because the light is still diffuse at it's source. It may be focused but not well collimated.
When we look further at this matter, and create a much smaller light source. We reach the point where we have light that is sharp, collimated, and focused. The effects are, so much are possible, the sharpest detail and greatest local contrast available within a conventionally available photographic enlarging system.
At this point I want to interject that I disagree with those who contend that the same prints can be gained from all differing light sources. That simply is not true. In this context, it is important to note that while one can utilize diffusion as an addition of a point light source, should that be desired for whatever reason, it is impossible to transform a diffuse light source into a sharply collimated and focused light source. To contend that would be to say that we would suddenly bring the sun beneath the clouds. I think that we can agree that just will not happen.
With this in mind, we begin to seek out lamps that have small filaments that are capable of incredibly bright light housed in a relatively small clear quartz envelope. These lamps typically operate at or near 500 C. Thus cooling of the lamp enclosure by induction cooling is normally warranted.
The change from a large diffuse light source to a small well defined light source does carry certain design requirements in addition to the heat encountered in lamp operation. The other problem is that the light must be even across the projected area. This calls for designs that are not encountered in the use of diffuse light sources.
Reflectors are used in the design that I developed and installed. The reflector design must be clearly defined. It's dimensions must be carefully calculated and adhered to. The fact that light does have defined and predictable characteristics does help in this regard. In building a point light source as in all aspects of lighting the law that the angle of reflection equals the angle of incidence holds true.
I hope that this has answered those who have had questions.
