Schneider convertable lens: why the short focal distance when converted?greenspun.com : LUSENET : Large format photography : One Thread
I just bought a Schneider Symmar 150/265 convertable lens. You remove the rear element and get a so-so 265 mm lens. I haven't made any exposures yet, but noticed that infinity focus with the 265 mm is way shorter than infinity focus on my 210 lens. Seems like a big difference given that one lens is two inches shorter in focal length and yet focuses over an inch further down the rail.
I'm just curious. I know that lenses of different design have different focusing distances, but I thought they were a lot more similar. Does removing the rear element make some kind of semi-telephoto?
-- Erik Ryberg (email@example.com), October 04, 2000
I believe you should be using the rear element alone instead of the front element alone.
-- C. W. Dean (firstname.lastname@example.org), October 04, 2000.
I have a 210/370 convertible. The previous post is correct. You get the converted length by removing the front element i.e., using the rear element alone. Be prepared for the bellows extension to be a bit more than the converted focal length since the nodal point is well inside the bellows. The performance is actually surprisingly good. Using the front element alone gives very so-so results. Extremely soft with lots of diffusion and what looks like halation - works sometimes as a soft focus look but the performance is pretty bad. And yes, it acts like a telephoto although I don't know whether that is because the nodal point now lies in front of the front standard, somewhere inside the front element - I never made any accurate measurements to check but I wouldn't be surprised if it actually functioneed like a tel with the nodal point way out in front of the element itself. The converted length (i.e., the rear element alone) actually works pretty nicely. Have fun. Cheers, DJ.
-- N Dhananjay (email@example.com), October 04, 2000.
Yes, you should remove the front element to convert the lens. You don't need a convertible lens to do this, though. A symmetrical lens, like the G-claron, works as well. Just remove the front element.
-- William Marderness (firstname.lastname@example.org), October 04, 2000.
Regarding William Marderness's point, having heard previously that most symetrical lenses can be "converted" merely by removing the front element, I tried it with my 250mm Fujinon, and sure enough was able to focus a good-looking image on the ground glass. This leads me to wonder whether lenses like the Convertible Symmars are really no different than other similarly designed lenses (plasmats, I think), or whether they are called "convertible" because they are designed to do especially well with the front element removed. Does anyone know?
-- Chris Patti (email@example.com), October 04, 2000.
Just one other comment, when converted and using black and white you should use a yellow or orange filter behind the lens and focus at the taking aperture with the filter in place for best results. Removing the element changes the formula of the lens. The filter reduces fringing from the colors of light being focused at different distances from the film plane. With the filter in place and the lens converted there is a focus shift that occours as the lens is stopped down hence the need to focus at the taking apeture.
-- Marv (firstname.lastname@example.org), October 04, 2000.
To respond to Chris: Convertibility goes back 100 years or so in lens design and very generally, the best results come with symmetrical lenses where the single element is used behind the iris diaphragm. Many manufacturers did not advertise or promote convertibility and photographers more or less had to experiment to discover which brands would work that way. In most cases the single element is much slower than the combined doublet sometimes up to three stops slower--two stops is about normal. This means that unless there are separate engraved iris markings for each configuration, you must assume that a wide open single element correctly mounted in the rear is about two stops slower than the whole set. They usually need to be stopped down two to three more stops to eliminate certain aberrations that are normally corrected in the complete set. In black & white many use a yellow filter with the single element especially on the antiques. The Schneider convertible Symmars are probably the most well-known of the relatively modern convertibles dating from the late 1950's or so and Schneider engraved separate iris scales for the rear element alone which were painted the same color as the serial number on the rear element. Schneider discontinued marketing the feature but the Symmars remained convertible. The story I got was that they stopped marketing the convertibility feature when people kept dropping the removed element in rivers and gorges and calling Schneider for replacements. I suspect the truth may be that they wanted to sell more lenses. They were considered state-of-the-art and were expensive. The famous Dagor was an early and popular convertible but I don't think the manufacturers promoted that feature very much. You see a lot of mention of using the long focal length as a portrait lens and the implication is that it's somewhat softer but all of the Symmars and Dagors I've used that way were plenty sharp if stopped down two or three stops.
-- C. W. Dean (email@example.com), October 04, 2000.
Thanks for all the very excellent responses. The guy at the store told me to remove the rear element and I never even really thought about it. It would be interesting to make a chart of conversions for the dagor and other lenses that work when converted. Some future project for the web-savvy among us.
-- Erik Ryberg (firstname.lastname@example.org), October 04, 2000.
Jos. Schneider dropped the convertible feature when they re-computed the Symmar (to be renamed the Symmar-S) in the mid-to-late 1960s. By this time convertible lenses were considered to be a bit of an anachronism, and the new computation would have shewn no improvement if it had had to be compromised by making it convertible.
Rodenstock did the same thing with their convertible Sironar (which I believe needs the back element removed).
Incidentally, I'm still trying to find the origin of the term 'Plasmat' as a generic name for double-gauss construction lenses. There appears to be no historical precedent for this. The lineage clearly runs from the Rapid Rectilinear, through the Goerz Dagor to the Planar and on to modern day lenses like the Symmar. There never was a family of lenses called the Plasmat, as far as I can find, until about 1920, by which time symmetrical lenses were well established.
Sorry, I don't mean to hijack this thread.
-- Pete Andrews (email@example.com), October 05, 2000.
Pete is correct that the Plasmat design was introduced in the 1920s, but is was a pretty bad lens. It wasn't until improvements in optical glass came about that the lens really began to show its stuff and became the basis upon which most of the wide coverage modern lenses are designed. I have used a convertable Schnieder lens a lot, as well as an old triple convertable Wollensak. Either works pretty well with a yellow filter. I would not carry a convertable into the field as my only lens, as you are out of business if the shutter fails.
-- Doug Paramore (firstname.lastname@example.org), October 06, 2000.
I also own a Schneider Symmar 150/265 convertible lens. I don't use it much having installed a Rodenstock Apo Sironar N on my camera. I have heard that the convertible lens was good for portraiture and I'd like to experiment with it for that. I always thought that you were supposed to remove the rear element. Can anyone provide a difinitive answer on this?
-- Bill Lester (email@example.com), October 06, 2000.
It's been an age since I used one, but I'm pretty sure, as everyone else says, that it's the front element that has to be removed on the old covertible Symmars. You should be able to tell. Only one set of glasses will give the focal length that's marked in green on the shutter. (Don't go by the bellows extension though. The converted lens has a shorter back focus than its focal length. You'll have to look at the magnification, relative to the normal FL.)
-- Pete Andrews (firstname.lastname@example.org), October 09, 2000.
When you remove one cell of a lens the new 'lens' is made up of only the glass in the remaining cell, and outside of exotic designs the new rear nodal point will probably lie somewhere inside the glass elements. The difference in focussing position reflects that fact: remove the front cell and the new lens is behind the lensboard; remove the rear cell and the new lens is in front of the lens board.
Faced with a choice of in-front or behind, peformance is usually best when the stop is in front of the glass. Crudely, in this position off-axis light never reaches the lens. When the stop is behind the glass it is possible for light passing through the outer portions of the glass to be refracted through the stop opening. So called'zonal' aberrations arise because off-axis light is focussed differently to on-axis light, and blocking the off-axis light stops this happening.
I have heard that the individual cells on convertible lenses are better corrected for coma than on a lens designed only to be used whole. One way to do the correction increases spherical abberration, which produces that glamour glow beloved of portrait lens afficionados. When whole, the slight excess coma of the individual cells in a non-convertible design is corrected by the overall symmetry.
Many problems with the single cells can be corrected by stopping down, but not the colour abberations, which is why yellow or red filters and B+W film are recommended.
The cells on the old convertible Symmars are not identical (the imbalance shifts the optimum focussing distance to infinity) and the second aperture scale is for the rear element. In the shorter focal lengths the convertibility is not such a great bonus, but the 300 and 360 convertible Symmars are attractive options for 11x14 and larger since you get two lenses in one affordable package and the single cells are perfectly good enough for contact prints.
-- Struan Gray (email@example.com), October 10, 2000.