I bought a used Pentax DA 50-200mm WR lens, but it was dirty and had fungus growing inside the front element.
Disassembling the front element
The ring with the lens name (nameplate) on this lens in particular is not screwed-on but simply glued. With the aid of a thin stilleto / exacto knife, carefully lift the nameplate at one point and slowly turn around the barrel to cut the glue and separate the nameplate. Do it from the inside pointing the knife outside, avoiding slippage and damaging the glass.
After cutting the glue on the underside all around, carefully lift the nameplate.
The underside of the nameplate should retain some of the glue for putting it back together later.
Use a 1.5mm Phillips driver to take apart the four screws fixing the front element to the outer barrel.
Take note of the alignment of the front element - you can use a drop of liquid paper or soft marker pen for this - as the underside assembly has more than four screw holes. I guess this is so the lens can be rotated for perfect centering during factory assembly, therefore it's best to maintain the same alignment.
You can use the nameplate as a guide for the four screw positions, since the underside glue will keep the screws in place.
After removing the four screws, carefully turn the lens barrel on the palm of your hand and the front element should fall. It's a good idea to keep the barrel upside-down or covered to avoid dust falling into it after removing the front element.
Since this is a WR (weather-resistant) lens, on the underside of the front element there's a rubber O-ring that seals the front element against the barrel assembly. This O-ring is very thin and delicate, be careful removing it and keep it apart until you need to reassembly the lens.
Here is the underside (inside) of the front element where the fungii is growing.
For cleaning it, I used my own solution of distilled water and 99% isopropyl alchool mixed in equal parts, but you can use commercially available lens cleaning solution.
First, I remove any possible hard particles that could scratch the glass with a soft brush. Then, I moisturize a sheet of lens cleaning paper (lint-free paper), rub the fungii out of the lens element and let it dry naturally. After that, I inspect against the light if another wet rub is necessary. I finish by rubbing with another dry sheet of lens cleaning paper to remove residue.
Here's what to expect after it's perfectly clean.
To reassembly, first carefully position the O-ring inside the barrel assembly. It should sit flush, sealing the bottom and sides.
Put the four screws back, taking note of the alignment. Glue the nameplate back, applying light pressure with the fingers all around it.
Finally, clean the outside of the front element. By now you should have a brand new lens.
- Perform this procedure on a clean, closed room, to avoid air currents that could carry dust inside the lens while you put it back together.
- Wearing a cloth mask and hair cap will help maintaining a clean environment.
- A rubber or silicon mat for keeping the lens in place in the table while you work is a good idea also.