Condenser vs. diffusion enlarger optics: This is one of the touchiest subjects we encounter; the claimed advantages or shortcomings of one or the other optical system ignite really impressive passions.
To be clear, we will differentiate in this column as follows: Condenser enlargers have one or more large, heavy glass lenses incorporated below the lamp house and above the negative carrier and enlarging lens. These condenser lenses are absent in what we will term here a diffuse enlarger.
The key difference
The singular characteristic of a condenser enlarger is the directional nature of the light arriving at the enlarger lens. It is collimated by the condenser(s), so it is all traveling in the same direction. This means the silver particles in denser portions of a negative can block light from arriving at the easel, both by absorbing it and scattering it. In an enlarger with diffuse optics, the light is already scattered and omnidirectional when it arrives at the same point and further scattering by the emulsion is irrelevant. Net result is, darker areas of a negative “block” more image-forming light when inserted in a condenser enlarger: The negative behaves as though it were higher contrast.
It’s important to note that this only happens with projection printing, not contact printing, and only with traditional black- and-white negatives containing particulate silver, not with chromogenic or color negatives.
The degree of contrast enhancement depends on details of the enlarger’s construction. Typically a negative that prints well on a grade 2 paper in a diffuse enlarger may require a paper grade of 1 or even 1⁄2 to produce a print of the same overall contrast in a condenser enlarger. The alternative, of course, is to shorten the development time for a negative destined to print in a condenser enlarger so that it has less contrast to begin with. And herein lies much of the confusion.
Manufacturers’ time-temperature charts for film development invariably list values intended to produce a Contrast Index (CI) of about 0.58—which is appropriate for printing in a diffuse enlarger, not a condenser. Sometimes they mention this qualification somewhere in the fine print, sometimes not. As a general guide, a CI on the order of 0.42 is more appropriate for film to be printed through condenser optics.
We have encountered countless individuals who claim their film is always way too contrasty despite their care in following development recommendations. “Do you print with a con- denser enlarger?” “Yes,” they say, “but what does that have to do with anything?” And the conversations go from there. Others take it a step further and are adamant that a CI of 0.58 is simply the wrong target value (which, as explained above, would indeed be a universal truth if all the world’s enlargers contained condenser lenses).
When we do meet people with condenser enlargers who develop their film to CI 0.58 (per a quick reading of most developer instruction sheets), we encourage them to shorten their development times by, as a first cut, 25%. The result is consistently a better fit to average paper grades. But the story doesn’t end here. Film speed and CI are tightly linked: Decrease the development time to drop CI and real film speed drops also. Specifics depend on the film and developer, but as a general rule, film speed will decrease nearly a full stop as CI drops from 0.58 to 0.42. Suppose some particular film, developed to CI 0.58 for diffuse printing, provides good shadow detail when it is rated at ISO 200. That same film is then better rated at only ISO 100 or 125 if it is to be developed to CI 0.42 for condenser printing. This may be the biggest single penalty for using a condenser: All the world’s films are nearly a stop slower than for those who print with a diffusion enlarger.
Another major myth that surrounds enlarger optics is concerned with sharpness. Condenser enlargers are widely reputed to offer sharper prints than diffuse units. There are no doubt certain pairs of enlargers for which this is true, but we do not find it to be a general truth. What we do find is that many workers do not adequately compensate for the contrast enhancement built into condenser enlargers and produce prints that are slightly contrastier than what they would create with a diffusion enlarger. Because it is difficult to differentiate between higher contrast and higher sharpness, it is easy to accept the condenser prints as sharper. We have made condenser/diffusion print comparisons wherein contrast is exactly matched, and, like many before us, we can see no sharpness difference whatsoever.
Either type of enlarger can produce fine prints. But to ensure that scenes of average brightness range print well on a middle paper grade, it is essential to adjust both film exposure and development to match the enlarger’s optics. Exact adjustments will vary depending on details of the enlarger’s construction but a useful rule of thumb is to expose two-thirds of a stop more and develop 25% less to projection print silver negatives with condenser rather than diffuse optics.