Zeiss Planar 50mm f/1.7 haze on rear element: disassemble and clean? Recalibration issues?

Question

I recently purchased the notorious Planar 50mm 1.7 at a bargain price, due to its faintly fogged rear element (see attached photos), which my trusted local photographer diagnosed to be an early spread fungus.

Now, I know this will lightly affect image quality - I bet even not at all. The things I worry about are fungus possibly spreading to the inner elements, since it now appears to be living only on the last one, as well as avoiding possible contamination with other lenses.

My question is: is that rear element easily accessible?

It appears to be so, provided one owns a lens spanner: just by looking at the lens it seems very easy. The two notches just around the glass (and the screws as well) suggest it's possible to unscrew just that element.

However, I couldn't any video and/or step-by-step article and/or documentation that explains how to begin from the rear side of the lens, which makes me wonder whether it's possible at all or not. Moreover, from the available videos (e.g. like this one) I noticed that in order to access the blades, for example, one needs to start from the front side, actually...

Even if it eventually turns out to be possible, I'm worried about possible recalibration issues: can dismantle this lens lead to this kind of problem, whether I'm starting from the rear or front side?

Thank you in advance.

P. S. How should I clean it in the meanwhile? I have brushes, blowers and a lenspen. Should I use them without any worry?

Answer 1

I looked into this and it does appear the only want to know for sure is to try unscrewing the rear lens ring to see if it gives you access to the rear element. My belief is that it will. However, no matter how careful you are, you would likely need a collimator in order to reassemble the element and ensure that it is in proper alignment. My estimation is that the rear ring is set to ensure proper distance from the film plane for the rear element, and that is a likely reason that repair videos recommend disassembling from the front.

In the meantime, you should have no issue using a brush to remove dust and a microfiber cloth to keep the external surfaces of the front and rear element clean. I prefer to use a bit of hot breath with the microfiber unless there is more stubborn dirt or oil, which may require a small bit of lens cleaner applied to the cloth (not directly to the lens).