Showing our age

<p>I've been stalking a 24mm f/2.8 "chrome nose" lately. I've bought two excellent examples only to find that they each have a separation in the cemented doublet behind the diaphragm. I've removed the doublet from the first lens and pictured it below. It's quite bad. The other lens is going back to the dealer tomorrow. Sure enough, I inquired today about a third lens, and the seller believes it is also separating.</p>

<p>These are the first I've seen in an FD lens. Does anybody have any other unfortunate FD separations? I hope this isn't the beginning of a wave of them. At least the few other early breech lenses I have are fine. Just curious whether we can isolate this to the one lens type, or if we're looking at something more.</p><div></div>

<p>I don't have an answer to your question, Alan, but I have noticed a less severe separation in an older (LTM) 50 F1.8 Canon lens. The service guy told me it was deteriorating Canadian Balsam cement which most manufacturers used in the earlier years which caused the issue. Was that still used as a cement when the chrome nose lenses were being produced? Like you, I'm concerned about the integrity of the cement in my older lenses.</p>

<p>FWIW, I was shopping for a nFD 135/F2.0 this weekend and came upon one sample which the store clerk and I both agreed was probably early separation. Those lenses weren't produced until 1980! (Of course, the lens in question could've had a different issue but "something" was happening on the edge of the outermost lens group which was causing some discoloration. It didn't seem to have the visual fingerprint of fungus.)</p>

<p>It may also have to do with mechanical shock to the lens somewhere along the line.</p>

<p>Thanks for your responses. I saw pictures of the third lens this morning, and it too is separating for sure.</p>

<p>I'm trying to find out whether these early FD's used Canada balsam or a synthetic cement. While I'm sure that the LTM lens is old enough to have used balsam, I suspect the FD is synthetic, which makes it a far more difficult repair. </p>

<p>There's also the issue of centering when recementing, though I was surprised to see that the ground edges of my doublet are not concentric. If they were ground accurately around the optical center, then the assembly was off by a few thousandths of an inch.</p>

<p>I hadn't thought about shock, but it makes sense. How much of an issue is temperature? Even balsam takes a lot of heat to soften the cement, so perhaps ordinary careless storage isn't much of a factor.</p>

<p>I've not seen a 135/2.0 with a problem, but both the 28/2.0 and 35/2.0 can have very cloudy internal elements, almost opaque. I think they have been affected by outgassing of the anti-reflective paint Canon used on the edges of the large elements. The paint is also what causes the apparent "bubbles in the coating" you sometimes see in the 28/2.0.</p>

<p>I've seen quite a few of the Zeiss auxiliary lenses for the Contaflex that are separating, yet not all of them are. It was always comforting to think that the FD's are so much younger and that they'd outlive me. Hope that proves true!</p>

<p>There were two versions of the chrome front 24/2.8. The first one is marked FD and has an unsprung mounting collar. The second version is marked FD SSC and has a sprung mounting collar. This is the one I have. It came with separation in the rear element group but I did not send it back t the seller. I sent it to Ken Ruth at Photography On Bald Mountain in Davenport, CA. He repaired the lens expertly and I enjoy using it. On Sunday it was warm and sunny here so I went to the Botanical Gardens Of NJ in Ringwood and brought an F-1 with some lenses, including the 24/2.8 chrome front FD SSC. This was one of Canon's early floating element designs. While I don't have any other 24mm Canon lenses to compare it to I have both versions of the 24/2.8 Konica Hexanon and a number of other 24s. I was actually watching a black front 24/2.8 FD SSC on eBay recently with the idea of sending it to Ken.</p>

<p>I've wondered about the long-term effects of daily swings in household temperature and the resulting thermal expansion / contraction cycles. If there's a differential in expansion rates between two bonded surfaces there will be mechanical stress, and if that bonding agent loses strength with time...damn you, entropy!</p>

<p>Can someone explain why elements are bonded? Are the two pieces of different diffractive indices? Is it too difficult to grind the shape? This is something I've been wondering ever since I learned (here on photo.net) about bonded elements. When I looked at the lens construcions, I just assumed the two lenses were just positioned next to each other... Thanks.</p>

<p>It could possibly be as Rick suggested - temperature changes. Many years ago I had a Canon 50mm f/1.4 lens that I left in my car trunk while I was away on business. This was in the winter, and the temperature was below -20C overnight. I later noticed the same separation between lens elements. Can't recall what I did with the lens probably scrapped it and replaced it with another. And this lens was relatively new at the time.</p>

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<p>So, would this separation be easy to spot without disassembling the lens?</p>

<p>I just checked my FD 24/2.8 SSC. I held it up to a bright light and looked through both ends while tilting the lens back and forth and it looked OK to me. Both of my chrome nose 50s (a 1.4 and a 1.8) were fine as well.</p>

<p>Cheers! Jay</p>

<p>I have a 50/1.4 chrome front as well as several 50/1.8 chrome fronts. It is uncommon to see separation in these lenses. The 24/2.8 is a floating element design and the rear element group of this lens, chrome front or black front, seems to have the separation condition fairly often. A solvent is used to completely separate the elements and they are then cleaned and recemented properly with a modern material. According to what I have been told, the solvents which work most quickly are not an environmentally friendly but slower working and more environmentally friendly solvents are available. This is why it can take some time to get this lens serviced. If you need a 24 in a hurry then you can look at the 24/2.8 New FD, the 24/2 New FD or either of the 24/1.4 lenses. The older 24/2.8 FD and SSC lenses require you to be patient to get them serviced but in my experience still provide a good level of IQ. </p>

<p>From my reading, I've learned that cemented elements are under a certain amount of stress simply from shrinkage of the cement when it cures. It is said that the particular curvature of the cemented surfaces affects the degree of stress. I have no doubt that temperature cycles would be a factor.</p>

<p>Jay, the separation isn't too hard to see with the lens assembled. My copy shows its rainbow just about as clearly either way. Earlier stages of the separation don't seem to show so much color, but tend to look like a slight discoloration with a defined border, as if a film of honey had found its way onto the lens. In either case, it is more visible at an angle rather than looking straight down the lens axis. My lens shows some faint color by transmitted light, on-axis, but not as strong as the reflected light result you see above. My other early lenses are also all fine.</p>

<p>In this 24mm, there are two single elements to the rear of this doublet, so the separation is a little distance down into the lens. But the sellers I've had check have been able to see it. Look into the rear element at about a 45 degree angle.</p>

<p>Steven, I'm no expert and I hope someone will correct this as needed, but as I understand it, the two (or more) cemented elements still refract light independently as if they were not bonded. In other words, the two surfaces that are joined still function as separate surfaces. There is a difference in refraction because the medium between them is not air, though; the optical cement makes the pair behave differently than if they were air-spaced at the same close distance. Hence, the cemented pair can solve optical problems that an air-spaced pair cannot. It's not the material--air, cement, or glass--that actually refracts. It happens at the interface between materials, all a "surface thing," if you will. I think that statement is actually not 100% true, or there would be no benefit to glass with high refractive index, or fluorite (though that gets into dispersion).</p>

<p>Jeff, I didn't know there was an SSC with a chrome front. I wonder if that combination arose while Canon was depleting the stock of original filter rings? I have a lot to learn about the historical intricacies of the breechlock lenses.</p>

<p>Thanks Alan!</p>

<p>Cheers! Jay</p>

<p>It's very encouraging (and equally amazing...) that these separations can be fixed- just one more reason to support the top camera technicians! Surely they're a dwindling breed. Given the choice between repairing a defective lens or outright replacement, even if it costs somewhat more I prefer the former- it keeps a piece of FD gear "on the road" AND rewards an individual for their skill. That's a win-win in my book.</p>

<p>I think the explanation that Canon was using up the remaining chrome rings is probably right. I wonder whether there were any other chrome front FD lenses with the SSC marking. It's funny because there were aeven few FL lenses, like the 300/2.8, which were marked SSC.</p>

<p>Might there also be some repair-bench "specials" out there, damaged lenses being returned to service using a mix of part vintages?<br>

SSC-marked FL series lenses: 300/2.8 and 1200/11, any others?</p>