Disclaimer: just because I did it doesn't make it a good idea. In my case it was a gamble of cost of replacement against the extortionate cost of professional cleaning. Having done a lot of research I was confident that best case would be that I fixed the problem and saved myself some cash and worse case the lens would be in the same condition as when I started. This is simply a log of what I did.
The lens
Several years back I bought myself a nice Nikkor 40mm 1:2.8 DX macro lens. Not exactly top of the range Nikon glass but a a good lens never the less. It was relatively cheap (for a genuine Nikkor) off Amazon, very quiet and a good around portrait and macro lens. I know some would disagree but I like it and besides it's not fair to compare such lens to pro-glass costing £1000s.
Anyway, all was well until I tried to use it for widefield astrophotography. I started to notice that when I stacked multiple images (subs) there were dark spots (or streaks depending on the tracking) - a sure sign of dust in the optical train. First stop was the sensor but that was clean and besides it only seem to affect this one lens. After some meticulous inspection with a loupe I tracked it down to a 'foreign body' stuck to the rear element - on the inside!
(sorry about the quality of the images, they were done in a hurry whilst trying not to lose any bits).
I know that most photography experts will say that all lenses will eventually get dust in them and that it really doesn't make much difference unless it's at a focal point such as the sensor. (Some even profess that it's impossible for internal dust to affect the images but that's clearly wrong). The fact is that internal dust and particles will always create small shadows on the image as they are interrupting the light path. The impact the shadow is dependent on how close they are to a focal point - far away and the shadow is spread out and feint, close and it is small and pronounced. The truth is that in normal daylight photography involving single frames you probably won't notice the effect (I certainly didn't) and even if you do you can edit them out easily.
If you are engaged in astrophotography, however, these things tend to show up like sore thumbs in your images. In daylight the small spec may lose you a few photons out of millions but when you're only getting a few photons in the first place it makes quite an impact. When you then stack dozens of images the effect gets magnified causing dark spots (or smears if your tracking is out).
The cause
Having taken the lens to bits I can see exactly how such things get into the lens. The lens effectively consists of a bellows-like assembly with the rear element fixed and the front moving in and out. This relatively small assembly is contained within the considerably larger lens (fixed) housing. The upshot is that when the lens telescopes in and out it vacuums up particles into the outer housing.(but not into the lens assembly at this point). Obviously with any semi-sealed telescopic lens assembly there ultimately has to be a vent for air to get in and out, In the case of my lens this is a small slot right by the bottom edge of the rear element. It's therefore only a matter of time before something is sucked out of the housing, into the internal tube and from there onto one of the lenses.
The image above shows the slot where the particle entered.
The fix
The challenge having identified the culprit was to find a way of getting it out. I watched numerous videos of people doing similar things - mainly aimed at fixing the mounting plate it has to be said - but couldn't find any relating to my specific lens. On the of the older lenses the rear element unscrewed (which would have been ideal) but in my case it seemed to have been glued in. The only simple means of access I could see was through the small slot (as shown above),
Although the picture above makes it look like quite a simple task, there's quite a bit of dismantling needed to get to that stage. On the plus side there wasn't any hidden spring-loaded things to shoot out when that last screw was removed :) I have to say at this point that I could probably have slid off the outer casing and done a more thorough clean but I didn't want to push my luck.
As can be seen above the lens has a very nice metal (not plastic) bayonet mounting ring. This needs to be removed to gain access to the side of the rear element. Before this can be done it has to be detached from the small block of contact pins. This is extremely important as they are connect to the electronics in the housing by a very small and fragile ribbon cable. On the bright side in the case of this lens the electrical contact to the metal ring itself is via a small spring-loaded pin underneath and not (as in come cases) a very thin wire.
As can probably be seen above, the connector pin block is held in place primarily by the large black inner ring. Removing this was straightforward - it's held in place by three very obvious screws. Once the screws are removed it pops out easily.
The ring also acts as a stop for the aperture adjustment lever which pushes against it meaning that there is a small click when the ring is removed - a bit alarming first time. This also means that you have to move the lever out of the way to reinsert the ring later.
Having removed the inner ring the next step is to detach the contacts from the metal outer ring. If you don't do this then you can't remove the ring fully and run a real risk of snapping the connecting cable.
The connector pin strip is held onto the outer metal ring by two obvious by extremely small screws. This is were a very fine magnetic screwdriver comes in handy. The screws are probably only about 1mm long. Once the block is detached it can be pushed slightly inwards (I.e. safely clear of the outer ring). Once this is done it's plain sailing to remove the outer ring. This is held in place by four relatively large screws.
As can be seen above, the outer ring has a long lever attached, This is what operates the aperture mechanism. It is therefore necessary (and obvious) that the ring be lifted off vertically. Also when re-attaching the ring it is important that the lever is carefully inserted back into the small slot from which it was removed (to be honest it won't fit back otherwise),
The outer ring has three small metal gasket rings underneath it. These are a bit delicate and it is also a good idea to remember there orientation for when you come to re-assemble things later.
Four of the round holes are where the screws go through. A fifth round hole fits over an alignment pin. The large hole is where the electrical contact from the lens body sticks through to make contact with the bayonet ring.
So, at this point I'd gained access to the small slot in the side of the rear lens element. Now came the challenge of attacking the particle adhered to the lens. The first weapon of choice was my trusty Giottos rocket blower however it made no impact at all, I did consider using a small wire to poke at the speck but decided against this in case I scratched the lens coating. Finally, I opted to use a plastic coated wire tie to poke a lens cloth (a fresh FroKnowsPhoto cloth - sorry Mr Polin) into the slot.
The initial problem was that although the slot was reasonably wide on the outside it seemed to narrow considerably on the inside. This it turned out was due to the fact that the front element was fully retracted and hence almost touching the rear element. Once I'd figured this out a quick turn of the focus ring made life much simpler. The next problem was how to navigate the cloth to the offending particle without scraping across the lens element. The solution was simple - retract the font element slowly compress the cloth (gently) against the particle then reverse the process. It worked! The particle was now safely attached to the cloth and easily withdrawn through the slot.
Putting it back together was amazingly easy - essentially the exact reverse of taking it apart, I was a bit apprehensive as to whether it would still work but thankfully it's all a good as new. No more annoying dark spots on images and it still auto-focuses - always a bonus :)
Overall a useful learning experience :-)
