I have certainly been having fun lately. If you have read my stories in the previous issues, you’re well aware that I have been working with some of the best enlarging optics in the world. And there is plenty more fun on the horizon.
I have been making resolution tests using a test negative made on high-resolving Adox Docupan of an Edmund Scientific Company Resolving power Chart. If you want a complete description of the procedure, please read my story, “Schneider Enlarging Lenses” (July/August 1985).
I will get to the El-Nikkor lenses a little later in this story, just now I would like to talk about optical testing from a slightly different point of view. Some people have asked why I chose traditional resolution tests for this series of articles, instead of evaluating each lens’ Modulation Transfer Function (MTF), which is a more popular and possibly more accurate measure of lens quality. First, I think it prudent to explain what MTF is and what it can tell you that resolution in lines per millimeter won’t.
It is quite possible to for two lenses to resolve the same number of lines in a test chart and for one of them to be undeniably sharper than the other. The reason for this divergence is a difference in contrast. The sharper lens will show clearer definition between the dark and light areas of a scene. Anyone who has printed a slightly soft black-and-white negative on a higher contrast paper is aware of this effect; the higher contrast print will appear to have greater sharpness. MTF assumes that sharpness and good contrast are the same thing. In fact, in this context, modulation means contrast.
MTF calculations begin by measuring differences in intensity (modulation) between the lightest and darkest portions of a test chart. The chart is then projected through the tested lens and another modulation measurement is made, this time for the projected image. A perfect lens would show matching modulation for the original chart and its projected image, but there are no perfect lenses. Some modulation is always lost. The amount lost is calculated by using the ratio between the original and projected modulation, and is called the modulation transfer factor. Transfer means to lose.
You can see that many transfer factors can be calculated for a single lens, taking into account different positions within the field of view, different apertures, and other variables. This is precisely what is done to calculate a final transfer factor for a particular test chart/ lens combination. Then, another chart substituted, perhaps with coarser or finer detail, and a transfer factor calculated for it, and so on. The test results are plotted and a curve is drawn that sums up lens and performance.
An MTF curve plots transfer factors against spatial frequencies. These frequencies reflect the modulation characteristics of the original test charts. These charts are unfamiliar to anyone who has not worked in an optical laboratory. Basically, each chart is composed of a series of variously spaced lines. The transition area between each line and the background are intentionally made indistinct. When a test chart is oscillated, and sophisticated equipment scans it, a distinct electronic signal is created. Another chart, with thicker lines or different line spacing, will produce another distinct electronic signal when scanned. These two signals are said to differ in frequency. This is precisely how tests are conducted, with a computer conducting the tests and interpreting the results.
I am a firm believer in personal testing. I think every photographer should have a standard set of testing procedures for every aspect of their work. I do, because consistency demands it. I also believe that tests should be repeatable: If I test a film or paper, you ought to be able to conduct the same tests and put together your own results, which you can compare with mine. Where the results differ, you and I differ in technique. Which means a product that is right for me may not be so for you. The same is true for lens tests. My results are no more than a base for comparison.
I chose to conduct resolution tests in this series of articles for one reason alone: MTF tests cannot be done in a small or home darkroom. Such evaluations must be conducted under laboratory conditions with sophisticated equipment that could easily cost as much as a small studio. I should in no way be interpreted as degrading the value of MTF data, because such information is the best yardstick we have for evaluating sharpness of an individual lens. I think it is essential for any photographer to learn enough about the subject to be able to interpret the MTF curves provided by a lens manufacturer. It is one more tool that you have to help you make the decision to buy one lens over another. After you have bought a lens, run your own tests (or mine) and return it if it doesn’t meet your expectations.
El-Nikkor Enlarging Lenses
Nikon, Inc. sent me eight enlarging lenses, knowing full well that I would find them all to be right up there among the very best. Try evaluating top-notch lenses with an open mind. See if you can find anything bad to say about them, when each one performs like a champion.
I requested lenses that you or I would want for our professional or advanced amateur darkrooms, not ones primarily intended for specialized applications. The lenses I received were the following: 40mm f/4N, 50 mm f/4N, 50mm f/2.8N, 75mm f/4N, 80mm f/5.6N, 105mm f/5.6A. The letter designation on each lens is important because Nikon has lenses of the same focal length and aperture that differ greatly in cost and performance.
All eight lenses have the standard Leica-type thread mount (39X1mm) and all except the two longest focal lengths (135mm and 150mm) have illuminated aperture displays. Mounting them on my Durst 605 enlarger was a cinch. I needed an extension tube to mount the 150mm lens because my bellows would not quite extend far enough to focus the image at the easel. No problem.
If you could see all eight of these lenses lined up in my darkroom, I think you would be impressed just with the looks of these beauties. I don’t know why an enlarging lens has to be pretty, after all, it spends most of the time in the dark, but I must admit that I am always impressed by a handsome enlarging lens over an ugly one. These El-Nikkor lenses are a shoe-in for design award, should such a competition ever be held. They have a simple and well thought-out design: black satin finish barrels with easy to read f/stop calibrations. All have click stops for each f/stop and control rings that are very easy to turn. Adjustments between click stops can be made with ease.
Nikon offers two accessory rings for mounting El-Nikkor enlarging lenses onto process cameras. One is for lenses in the range of 40mm to 105mm. The other ring works only with the 135mm and 105mm lenses. These rings allow reverse mounting of the lenses for close-up and macro work.
Like all optical products from Nikon, the El-Nikkor enlarging lenses are constructed from optical glass produced by Nikon. Nikon believes that this is the best way a lens manufacturer can guarantee optimum performance.
Every El-Nikkor lens is corrected for chromatic aberrations beyond the visible spectrum and into the near-ultra-violet wavelengths, which photographic papers are particularly sensitive to. Nikon states that as a result of special optical glass and matched coatings, the El-Nikkor lenses have ultra-violet transmission characteristics (in the 350nm to 450nm range) that are favorable enough to allow them to be used to make color separations, in addition to all other enlarging applications.
Other benefits described by Nikon include: maintenance of focus when apertures are changed; no color fringing; even illumination free from fall-off; computer controlled lens design; chromatic aberration approaching zero; high-dispersion, low-refraction glass construction.
Nikon believes these are the most advanced lenses it has ever produced, and I can’t argue. After having used these lenses, I am happy to recommend them without reservation. Nikon has worked hard over the years to maintain a position of leadership in optical engineering, and this has guaranteed it an equally lofty position in the marketplace for photographic optics. These is no doubt that Nikon, and just a few other companies, are leaders in optics innovation, not followers.