Bonjour le forum,
Frédéric a écrit :Que signifie "délamination" ? (je connais ça pour les bateau en fibre de verre... mais pour des objectifs ?)
Une réponse assez complète en anglais :
Ultraphot Shop a écrit :So-called 'delamination' is the separation of cemented elements, mostly in objectives, but not unknown, as far as Zeiss equipment is concerned, among wide-angle eyepieces, the telan lenses of widefield nosepieces, and Nomarski prisms. It seems to be more prevalent in lenses of more complex construction and, particularly, where a cement other than Canada Balsam has been used. The low power Neofluars, Epiplan HD 100/1.25 oil and all the planapos appear to be prone to it, the 4/0.16 (rear doublet), 40/1.00 oil, 63/1.40 oil and 100/1.30 oil being commonly affected. Unless the condition in an objective is very advanced, it will be necessary to use a Bertrand lens or phase telescope, together with careful focusing and adjustment of the illumination in order to detect it. It will then be seen as a faint (usually)grey film, spreading across one or more elements. Where it is restricted to the outer margins of lenses, it appears to have no effect on the image but, as the area of separation increases, there is progressive loss of contrast and the worst affected objectives are useless.
In the case of the separation occurring within objectives, telan lenses and Nomarski prisms, the most likely culprit is the heating effect of the illumination system. Both filament bulbs and arcs radiate large amounts of infra red and much of this is probably focused in the objective or somewhere close behind it. In busy research and medical screening laboratories (the probable source of many second-hand planapochromats and Neofluars, anyway), microscope lamps may be switched on at 9.00 AM and left on all day, so be particularly careful when buying instruments from them. Since the coefficients of thermal expansion of the metal mounts, glass, and fluorite differ, long-term damage is probably inevitable. The presence of a C-mount adapter among the bits and pieces of an Ultraphot outfit should also set off alarm bells because it usually means that the instrument has been used for cinémicrography - probably with a 12v 100w lamp blazing at full voltage for long periods. I once had a 40/0.75 Neofluar phase contrast objective from such an outfit in which the balsam in the area around the margin of the phase ring had obviously been boiled. In my experience, Ultraphots with a well-used CSI 250W lamp should also be viewed with suspicion. These lamps, together with HBO 200's, and, worse still, XBO 150's, are particularly potent objective cookers since they cannot be dimmed, and are frequently left burning because switching them on and off shortens their life. It is worth noting that neutral density and Polaroid filters are usually almost completely transparent to infra red and so offer no protection to lenses - or eyes.
As a result of the current 'delamination phobia', even slightly affected lenses are virtually unsaleable, providing excellent opportunities, for those prepared to take a risk, to acquire high quality objective at bargain prices.
What is the risk, anyway? The common assumption, of course, is that separation is a progressive condition which will continue independently of how the lens is treated, but there seems to be no evidence for this. My guess is that, treated prudently, many an apparently doomed planapo will enjoy a longer useful life than its owner!
Les bons vendeurs précisent
toujours l'état de leur objectif par rapport à une éventuelle délamination (pas toujours visible, mais quelquefois très nette). On trouve quand même des objectifs semi-apo et apo Zeiss d'excellente qualité sur Ebay, et pour du
fond clair au moins une légère délamination n'affecte guère la qualité de l'image restituée. Après tests, je ne regrette pas d'avoir acheté un Zeiss Planapo 100/1.3... il y avait certes un "risque", mais quand même.
Cordialement,
Eddy
