Dots on Rear Element of Rokkor Lens

[FilmOnly]

I have owned many lenses, but I have never seen anything like this on any of my glass. I received a Minolta MC 58/1.4 PF today, and, while the front element is immaculate, the rear element has these very small dots or specs. I first thought they were "cleaning marks," but after looking at the lens carefully, and with a flashlight, I see that they are not marks or swirly lines, but dots. I tired some Formula MC and a microfiber cloth, and it had no effect.

Overall, they are somewhat difficult to notice, but they are there. They also seem to be grouped in the center of the element, while the outer portion of the element seems clear. The dots themselves look to be clear, not having any particular color or tone. Could there be something sitting on the inside surface of the rear element? Is it perhaps early stages of fungus? The "cleaning marks" theory just does not seem to describe this well. I have a seven-day return privilege, but I would like to see if I can keep this lens, as it is fine otherwise. I welcome your thoughts.

 

[tkamiya]

With unknowns like that, I'd use the "return privilege." I don't know what it is but it shouldn't be there. If this was a used item purchase and it wasn't disclosed, I'd be unhappy.

 

[tomalophicon]

Could they be bubbles?

Stole this from another site. not my information: Pay attention to the last paragraph

Why the Bubbles in the Lens Glass?
Modern glass making has changed a lot in modern times. It wasn't so long ago that the bubbles in the glass were a sign the lens was of the highest possible quality. How quickly we have gotten used to photographic lenses that do not have bubbles in them. When optical glass is made all of the ingredients are mixed in a container that can take high temperatures called a crucible. The ingredients are melted at very high temperature. Even when it is melted, glass is very viscous and thick. When the melted ingredients are stirred to mix them together, the mix tends to pick up air which forms bubbles and the ingredients used in the older optical glasses tended to generate gasses as they melted. The bubbles are removed from the glass by allowing the crucible to sit in the high temperature furnace while the bubbles rise to the surface of the melted glass. Up until about 25 years ago the materials of which crucibles for optical glass making were made could not withstand the effects of the extremely high temperature at which glass is made for very long. There was a time limit on how long the crucible could stay in the furnace. In order that the crucible not fall apart it was necessary to remove the crucible from the furnace before all of the bubbles had time to rise through the melted glass and escape from it. Another problem is that the melted glass was corrosive to the inner surface of the crucible. It was necessary to remove the crucible from the furnace before corrosion of the inner surface of the crucible caused impurities to ruin the batch of glass. Another problem is that making the highest quality optical glass required higher temperatures than did lower quality optical glass. So it was possible to make lenses without bubbles, but only if the glass was of a lower quality. Back then, no bubbles in the glass meant it was cheap.

The overall effect of these factors was that for a very long time small bubbles in the glass of a lens were an indicator of the highest quality. It is still such an indicator today for lenses which were made prior to around 1975. Sometime around 1975 crucibles for optical glass making were developed which could withstand the high temperatures required for making the highest quality optical glass long enough so that the molten glass could be left in the furnace until all of the bubbles had risen out. These crucibles were also resistant to the corrosive effects of molten glass on the inner surface of the crucible. Since then high quality optical glass has been virtually bubble free.

So when you are looking deep into a Zeiss lens for your Contax camera, and you see some little bubbles deep inside, rejoice because you have found a rare and special jewel that will take most excellent photographs. The bubbles are photographically insignificant and do not affect the quality of the pictures the lens will take. They are merely an indicator of great quality.

Also keep in mind that when a lens blank contains bubbles it is only natural that during the grinding and polishing process where glass is removed that some of these bubbles will become partially exposed. These exposed bubbles will appear to be small tiny pits on the surface of the glass on both the front and rear elements. These are natural, to be expected and cannot be avoided. They are an artifact of the lens glass being superior.

 

[FilmOnly]

Hmm...

They do have something of a "bubblish" appearance, and they appear to be inside the lens, not on an exterior surface. To me, they look like they are toward the rear, perhaps on the inside surface of the rear element. They do not at all seem to be on the exterior surface of the front element.

At this point, my guess is that they are spots or bubbles of oil, but they could be the "quality" bubbles mentioned above.

 

[FilmOnly]

Your bubble theory intrigued me, and so I got out the flashlight and checked the lens again. After checking and reviewing what you have supplied here, I am pretty well convinced that I have a "special jewel." They do not look like droplets of oil or specks of dirt, but, rather, like tiny bubbles. I thank you very much for supplying this information, tomalophicon.

 

[FilmOnly]

I just found this discussion. None of these folks seem to think that bubbles are a good sign...

http://photo.net/bboard/q-and-a-fetch-msg?msg_id=0008vW

Hmm...I am disappointed again...

 

[FilmOnly]

After reading more about haze and oil, I am thinking that could be the culprit. I say this because the information I read in regard to bubbles described bubbles here and there. Using the flashlight test, my lens shows many, many tiny little dots, and thus perhaps it is haze from droplets of oil.

 

[tomalophicon]

Don't know.
My advice would be to use the lens and see how it goes. maybe set it at minimum aperture and see if you can identify anything in your prints that would suggest that the defect is detrimental to image quality.

 

[tkamiya]

Hello OP....

The point I was making in my original reply to you was that since you can return this lens and the exact nature of these spots are unknown - to seriously consider taking the opportunity to return the lens. An expert can take apart and examine the lens then tell you what they are - problem or nothing at all, cost to do that is not reasonable considering you bought this lens used.

You may have a "gem" or you may have a problem. The problem may not be anything significant or it may grow if that's a fungus. Sometimes it is not easy to determine if the defect is on the lens or in the lens, unless you can see it from both surfaces.

Since you do not have an exact copy of this lens without the issue, making image quality judgment isn't really possible - although if I had to guess, there will be no degradation of quality. I'm purely guessing as I am not looking at your lens and I am not an expert.

It's really up to you. You are not going to be able to make an informed judgment via internet, although we try to help as much as we possibly can.

 

[FilmOnly]

You are right, tkamiya, and I thank you for your kind words of caution. The only reason why I have considered sending this lens out is because it is somewhat difficult to find this version of it. It is the last (latest) version, a version that is non-radioactive (I prefer lenses that do not use thorium and such). The lens is also in very nice shape, otherwise. I have apprised the seller, and she has been very nice. She has already offered a full refund, and even some allowance for a possible repair. Thus, I may send this lens to my repairman. I will keep you posted.

 

[tkamiya]

That sounds very fair. Good luck and enjoy!

 

[Galah]

I just found this discussion. None of these folks seem to think that bubbles are a good sign...

http://photo.net/bboard/q-and-a-fetch-msg?msg_id=0008vW

Hmm...I am disappointed again...

Hmm, I seem to remember (I no longer own this camera) that my old Minolta SR2 with an MC Rokkor 2/50 (or 55) Auto lens had a small bubble or two (big enough to see with the naked eye). I remember being told it was of no importance and, in those pre-pixel peeping days (with smallish prints), I never ever did feel it gave me any problems and people complimented me on the quality of the images.

What did cause trouble, however, was the way the diaphragm blades had a habit of "sticking" (failing to stop down to the required aperture) and needing to be serviced every few years, a thorough nuisance: hence why I no longer own the thing. A characteristic which has made me wary of Minolta/Rokkor lenses ever since (although I still do own some Minolta gear even now).

 

[FilmOnly]

Interesting comments, Galah...my lens has a whole lot more than one or two spots or bubbles.

With regard to the reliability of Minolta lenses, your comments give me concern. I usually have cameras (and sometimes lenses) CLAed before use. It would be annoying, however, to have to do this every few years. I would hope to get at least five years out of an overhaul.

The 58/1.4, and its cousin, the 55/1.7, are now on their way to my repairman. I sent the 55/1.7 because it, too, seemed to have the dots, though not as many. I should have some news within a few days. I checked most of my other lenses, both Minolta and Nikon, and did not notice any dots. After looking at it so many times, I would have to say at this point that it has the appearance of condensation or perhaps oil droplets.