Nikon DP-20 finder for the F4

Recently, as part of a beautiful, practically unused, late model Nikon F4s which (ahem) came my way, I immediately noticed a problem with the DP-20 finder. There was quite a heavy infestation of fungus on at least one of the internal eyepiece surfaces which was so bad it made viewing the lower meter readout difficult, blurring the LCD. I’d already knocked the seller down quite a bit (without knowing about this issue, of course) and so my initial reaction was not to demand a refund. I’ve fixed this kind of thing before – in lenses and more simple finder pieces – so how difficult would it be to eliminate the fungus from this otherwise pristine DP-20 and restore it to minty condition?

Well, as it turns out, quite difficult. But feasible, given care and patience.

This project required some careful planning, given the condition and obvious complexity of the finder. I got hold of the service manual for this finder which, unfortunately, is an x-generation photocopy scanned by an ancient scanner (probably) and so the image quality gives rise to a fair amount of guesswork. I was so determined to get this right that I bought a well-used finder for a cheap price, so that I could practice and note any caveats before working on the “real thing”. At worst, I’d spam it up completely and learn something in the process; at best I’d have a working spare finder that I could use for parts if need be. The practice run went very well in the end; a very grubby, fungus-infested DP-20 (though not nearly as badly affected as the minty one) with decomposing foam seals is now very clean, fungus-free, re-sealed and functioning perfectly, with just a few external scratch marks. I’m keeping it as a spare.

Now much more confident that I could do this, I set to work on the minty one. I’ll describe the steps I took in case this is of use to anyone else who wants to have a go at restoring their own finder. Note that this doesn’t require a complete disassembly but it’s worth mentioning that the finder optics and areas around are quite complex and fragile. With care there should be no need to de-solder any wires, but some patience is required.

The finder optics comprise three separate lenses in front of the prism (not including the screw-in eyepiece on the outside of the casing). The first lens is held in place by two small screws, whereas the second, which is able to travel forwards and backwards, is held in only at the top by the dioptre adjustment mechanism. The third and final lens, before we reach the viewing edge of the prism, is attached to the meter cell unit and is the most difficult to work on. The worst possible area for an infestation of fungus, if we were to try to remove it, would be the back-side of the third lens. Guess where it was in my finder.

Step one, a challenge in itself, is to remove the centre parts of the dioptre dial and meter selector dial without causing too much damage. These plastic discs cover the screws that need to be removed in order to remove the dials. They are held in with glue and are, frankly, a pig to remove because it’s likely that there will be some degree of marking to the outer parts of the dials during removal. To minimise this I covered the outer dials with insulation tape and then gently levered the covers out using a very small screwdriver. Once the screws are exposed both dials can be removed. Take care with the two washers for the meter selector dial: the white plastic one goes against the inside of the dial and the black foam one goes against the case.

After removing the four black screws of the outer case, the top can be lifted away from the body. A little bit of wiggling may be required here. The wires attaching the flash shoe to the main body are quite long and I didn’t need to detach them at all. With the main cover off the first lens can be seen directly, with the two screws holding it in place. 

Take the first lens off and inspect both surfaces; with luck any fungus will be here (this is where it was, mostly, in the “scruffy” finder with a tiny bit also on the front surface of the second lens). If there’s some on the back surface of the second lens then this can be taken out by first loosening the dioptre adjustment unit which sits on top. Notice the two small screws holding this part down (not the larger one; that can stay in place) and loosen them a a bit. Be careful here as the two mercury switches are held by this unit and they are glued to the unit underneath, yet there’s some play to take advantage of. The top adjustment unit may be lifted just enough to wiggle out the second lens, which is held there by the pin of the swinging arm. Again, inspect both surfaces.

If there’s fungus on the front surface of the third lens, it can be treated in-situ. If it’s on the back side, though, then the meter cell housing needs to be taken out. There are two screws holding this in place – a small one and a larger one, but this unit is the most difficult to get out so take extra care here. Remove both screws. Facing the lens, first lift the left side and then lift the right side a little. It may not appear to want to move much but persevere here, gently lifting either side and twisting slightly, firstly at the top, outwards, and then push up, swinging out at the base. Watch for wires snagging and gently lift them out of the way so that the lens/cell unit can swing outwards. Don’t pull it too far out. To clean the back side of this lens it will be best to turn the whole lot upside-down and move the unit out far enough to get at the surface, but no more. 

DP-20 insides
Lens no. 3 of the eyepiece group – clean and clear. Nearly half of the inside surface was covered in fungus.
DP-20 eyepiece lens group holder- side view
Side view. Notice one of the mercury switches and wires going into one of the meter cells in the lower left of the picture.

Once clean, re-assemble everything in reverse and look for any wires that may have become detached. I had just one such casualty – the black wire of one of the mercury switches – which was a simple fix with a soldering iron.

Neither the fungus in scruffy nor that in minty had etched the glass coatings at all and so I was able to remove it completely, using my usual method. There’s no secret to this, although people tend to have their preferences as to which tools and compounds to use. I have cleaned fungus from photographic optics for a few years and I’ve never had any return, using simple, easily obtainable isopropanol, cotton buds, cocktail sticks and certain brands of optical cleaning wipes which are, specifically, impregnated with alcohol. It’s the cocktail stick wrapped in an optical wipe which really does the trick, with gentle rubbing over the affected area. If necessary, the area can be finished off with an optical-grade microfibre cloth. Perhaps stronger chemicals, such as hydrogen peroxide, may be required for very well-established fungus but I’ve yet to encounter any.