Cleaning a 50mm f1.8 (type 4) lens diaphragm


I have a sticky lens diaphragm (oil on the blades). How do I clean it?
The lens was bought for £2 from a scrap box. Externally it was in mint condition, with almost no sign of having been used. Glass was clean and un-scratched with no internal dust or fungus. However the iris did not return to full aperture when closed to f/16 with the preview button, or the camera linkage. Serial number was 1735XXX.

The first thing to check was that the sticky iris was not due to gumming up of the back linkage rings just behind the flange (CE104900 & CE105000). I removed the three screws holding the flange and carefully lifted it off, retaining the DOF PV button CE104500 and E button assembly (CE156600, CA923800, PSK1.4x2SN).

I carefully unhooked one end of spring CE102600, and removed ring CE105000. I then removed ring CE104900. Checked both for sticky lubricant.

To see where the problem was I carefully inserted a small screwdriver or tooth pick into the aperture mechanism operated by CE105000 (there is a slot visible with which CE105000 engages), and checked if this moves freely. If it does then the problem is in the rear rings. If it doesn't then the problem is deeper in the mechanism.

In this case I needed to clean the iris blades. I first removed all the lens elements. The rearmost element assembly CE10200,LC069100,CE101300 was finger tight and easily unscrewed. The cemented pair ZC195900 is retained by the rear element assembly, and can then be pulled straight out.

To remove the front frame assembly ZD304100, I first removed the front ring CE168900 with a home made key (see details below). I removed the three screws PUK1.4x4SN, and then removed the front frame assembly. The screws are varnished to prevent them shaking loose so this needs to be removed first with a scalpel or small blade. The front frame assembly carries all three front lens elements.

Next I removed the diaphragm ring CE104100, taking care to find the ball bearing B1/16, and spring CA071500.

The front of the aperture assembly can now be seen. A plastic blade fastener plate CE103400, which is held by three screws PUK16x355SN, covers the iris blades. The screws are also varnished. The six blades (ZC195800) sit in rotating plate CE103300, in the helicoide assembly ZC194800. The blades are a drop fit. I removed the blades with tweezers by gripping the metal post. The blades are steel, and will stick to magnetic tools.

I carefully degreased the iris blades and the diaphragm ring, flooding the linkages ZC183400, CE102900, and CE103200 with methylated spirits, and working the mechanism until it felt free. Ethanol, propanol, or lighter fluid could also be used. The solvent and grease was mopped up with cotton wool tipped sticks (Q tips in the UK).

Before re-assembly, I dusted all parts. A can of compressed gas is most useful for this (Kenro or Jessops in the UK). To reassemble the iris blades, I placed them, one by one, into the diaphragm ring, each overlapping the previous one, and rotated to the full aperture position. The blade post fits into the diaphragm ring, and the blade hole fits over the post on ZC194800. The last blade can not be fitted since the first one hides its slot. To fit, I unhooked the blade hole of the first blade from its post, and rotated it towards its f/16 position. This revealed the posthole for blade 6. Carefully lever up blade 1 to fit blade 6 beneath it. This is the most fiddly bit, but not too difficult. Tweezers are essential for this operation, and ideally non-magnetised ones.

Re-assembly of all the parts was in reverse order to removal. I re-varnished the screws that originally had it. I needed to put a very small amount of light watch oil into parts CE103200,CE102900, ZC183400. No other parts seemed to need lubrication.

I had a set of small cross head screwdrivers. Most of the screws are very small, so jeweller's screwdrivers are essential. I had a no. 4 scalpel, large chrome plated tweezers, and very fine tipped Presista stainless steel tweezers.

For removing the front ring CE168900, I made a key from 1.6mm (1/16") aluminium. I cut a rectangle about 15mm (1/2") wide, and about 48mm long (i.e. just long enough to fit inside the filter thread inner diameter, I trimmed the length to fit ). I then cut a slot about 4mm deep centrally along one edge, to leave 1mm wide tabs at each end. I then trimmed the tabs with a needle file until they fitted neatly into the two slots in the front ring. Both width and thickness need to be trimmed, the final tabs being about 0.75mm square.

Turning gently anticlockwise unscrewed the ring. As it was made of aluminium, the tabs did bend a little. Brass or steel might be a better material, less likely to bend, but also more likely to damage the ring if too much force is applied.

contributed by Chris Barrett