Front & Rear Elements of Convertiblesgreenspun.com : LUSENET : Large format photography : One Thread
Hello Everyone.... I was just reading a late 1980s article by Ron Wisner about classic convertibles. He started off w/ a few rules for getting the best from these lenses. The main rule was to stop down. Another was to use such lenses as somewhat long FL. A third that caught me off-guard was to ALWAYS have the diaphragm in front of the element being used (if using just one of the two elements). Well, classic symmetrical triple convertibles have the shortest FL with both elements in place, a longer FL w/ the one of the elements working alone, and a longest FL with the other element working alone. My Wollensak Ia is about 13" with both; 20" with rear only; 25" with front only -- in front of the diaphragm ('course)! Winsner made no further comment about the diaphragm/lens arrangement matter. I wonder if I'm misunderstanding what he wrote and/or what's up w/ this. Any comments??
jeff buckels (albuquerque)
-- Jeff Buckels (email@example.com), June 18, 2001
That is correct - you want the stop in front of the glass. Most convertibles are designed so that the threads on the barrels are the same on both sides. Typically, the shortest length is got by using both elements (typically with the longer length cell in front). Removing the front element (i.e., with the rear cell behind the stop) should give you about 1.5 times the combined focal length. If you wanted to use the front element alone (typically about 2 times the combined focal length), you need to remove the rear element (put it in your pocket) and screw the front element in at the back of the barrel (where the rear element was) and use it that. The reason is that the position of the stop is important for correcting some of the aberrations. Cheers, DJ.
-- N Dhananjay (firstname.lastname@example.org), June 18, 2001.
Hi Jeff, I used the Series Ia. And with this triple, you can use the elements on the front of the shutter. The difference between back and front of shutter is not that great, and if you put it on the front you require way less extension which will probably reduce camera shake and bellows vibration and improve the photo even more. Use of the element on the rear of the shutter with an orange filter is done with some triples where the individual elements aren't corrected for comma, but the Ia is well enough corrected so it's not that big of deal. I've never noticed any comma problems with my Ia. Most of the time I don't use a filter, and when I do, I use a Y2, and that I use for contrast control and not chromatic correction. Have you shot any portraits with the 20" element? Best, David
-- david clark (email@example.com), June 18, 2001.
Hi Jeff, Here is what the Wollensak manual says about the 1a: These lenses are noted for exceptionally large coverage, extreme adaptability, and the versatility of three focal lengths in one. They are of the basic double Protar type design which has found universal acceptance throughout the years. The front and rear components are individually corrected lenses and therefore can be used separately as well as in combination. The shortest and normal focal length is obtained when both halves (components) are used together. The focal lengths of the individual halves are each different and much longer than the normal focal length...(The front one is the longest.) Triple Convertible lenses are recommended for taking landscapes, industrial shots, architectural studies, illustrative and commercial photography. For better perspective, larger images, or telephoto effects, remove the front component and then use either the front or rear component alone IN THE REAR BEHIND THE DIAPHRAGM. (Emphasis added by me.) There were 4 focal lengths - 165 (4x5), 210 (5x8), 254 (6.5x8.5), and 330 (8x10). That said, I have also used the front element on the front by itself to keep the bellows at a reasonable length. Perhaps it's not as good as being behind the aperture, but it does work. Chauncey
-- Chauncey Walden (CLWalden@att.net), June 18, 2001.
As mentioned above, this is Wollensak's version of a Protar and the individual cells are probably corrected for coma. However, the stop position is important for correcting aberrations other than coma (astigmatism, distortion and field curvature, for e.g.). Typically, the stop should be located in front of the lens to help in this regard. However, with many convertibles, it may not matter immensely since the stop position (which is probably designed to be optimum for the combination) may not be optimum for the single elements (even though they are used at the back). However, performance with the cell in the rear should be better than with the cell in front. But again, bellows restrictions may demand the cell be used in front. When using single elements, the principle point lie outside the lens (so it actually does act as a tele when it is used in front - and as a retro focus when used in the back). Combine the bellows restrictions with the small stops often used and one may not notice the difference. Cheers, DJ.
-- N Dhananjay (firstname.lastname@example.org), June 18, 2001.
One of the reasons the elements are used at the rear of the diaphram is that straight lines at the edge of the image using single elements are often curved. The lines usually curve inward if the single element is in front and outward behind the diaphram. I use a triple convertable Wollansak for scenics for special effects and find little or no differece in image quality. They are fun ol' lenses to use, and are quite sharp stopped way down.
-- Doug Paramore (email@example.com), June 18, 2001.