The Nikon FTn finder in common with a lot of cameras from the golden age of film cameras contains a fair amount of foam. Like the Olympus OM-1, the Canon FTb and many others the foam around the prism slowly degrades. When it does the foam turns into a toxic goop that will de-silver the prism and ruin it. While other cameras have the same issue the Nikon FtN finder is complicated by the fact that the prism itself is quite a complex piece and spares are hard to find.
In this easy, step by step guide, I’ll show you how to take the FTn finder to pieces and avert disaster with your Nikon – left to itself the internal foam WILL eventually destroy your prism.
Notes on working on the FTn Finder
As ever Nikon have a love affair with thread-lock. You may come across screws that wont shift. As ever a dose of Isoprop will usually get them moving but you also need correctly sized screwdrivers. Be careful of using Acetone during any disassembly work, remember there are optical parts inside that can be damaged easily.
The work is not hard but as ever with any finely engineered optical system you need extreme care. Take your time and exercise patience and care. The parts inside the FTn finder are almost irreplaceable so its as well not to rush in and wreck things.
Nikon FTn Finder – Stripping Down
Make sure when the finder is off the camera the prism face on its base is kept safe. I normally use a soft lint free cloth underneath.
Apply some Isoprop to the edges of the leatherette to weaken the glue, you may need several doses. The leatherette can then be lifted at an edge using a fine scalpel and then slowly pulled back. Be careful not to tear it.
Below the leatherette you will find 4x screws that retain the FTn finder top cover. Remove these and slip the top cover away.
With the top cover removed you will see the variable resistors which trim the battery check and the meter functions.
Note the round cut-out to the side with the wires.
Note: this pic was taken during rebuild and tuning but it will look the same.
Slacken the screws that retain the wires, do not remove the screws just loosen them enough to release the wires. Note that the left most screw retains a copper strip, slacken this screw as well and very carefully free the copper strip.
It’s not possible to separate the two halves of the FTn finder until these wires and the copper strip are released.
Remove the face plate screws from the finder.
Be careful as you remove the front plate from the finder not to damage the aperture tab. You may need a little bit of wriggle to free it.
With the front plate removed you can examine the aperture sensing mechanism of the FTn finder. Be careful not to disturb the small spring.
Often these are caked in grease and dirt. Use a soft brush lightly moistened with Isoprop to clean any debris away.
Turn the finder over and remove the four screws indicated.
Note that the screws at the rear of the finder are a different size to those at the front. The rear screws are slightly smaller in width.
With the screws removed the finder can be separated. Take care that the wires do not snag as you separate the two halves.
The top half contains the mechanics while the lower half contains the prism and electronics.
With the two halves of the finder separated watch out for 4x small washers located at the corners of the lower half. These are usually stuck down and will stay secure but you should be aware of them.
Note: This picture was taken during a later stage of the strip down as the original picture was not good.
Remove the prism retainer tab screw indicated and remove the retainer.
De-solder the brown wire running from the battery case to the circuit board.
Be very careful handling this circuit board. It’s an old school mica board and will crack very easily under any stress.
Remove the screw indicated that retains the sprung metal part which retains the meter cells and extract the retainer.
Remove the circuit board retaining screw. This is the screw that was holding the copper strip when the wires were released earlier.
It was not removed earlier in case it slipped down inside the finder. Its far safer to remove now.
The circuit board can now be removed, exercise extreme care.
It is easiest to remove it by gently lifting the meter cells upwards out of their holders.
Remove the second prism retainer screw and extract the retainer
Remove the two screws indicated. These retain the meter cell platform. Once removed pull the platform free and remove it.
You will now be getting your first look at the state of the foam beneath.
The prism itself can now be carefully lifted out of the finder base.
Note that the prism contains various reflectors for the meter and shutter speed read-outs and also has two small prisms glued to its back for the meter cells. Be careful using chemicals around these as the balsam used to cement them may fail if exposed to chemicals like Isoprop.
Nikon FTn Finder – Cleaning and Foam Replacement
With the FTn finder stripped is now time to get to work cleaning and removing old foam. All of the external metal work is cleaned up and the interior blown clean of any old foam fragments which, in this one, had migrated everywhere. Check all parts carefully.
You can see in this finder the foam was in a very poor state. The prism needed serious attention as it was an ace away from the foam eating the silvering. Extreme care is needed as foam breakdown can affect the stability of the external paint on the prism.
Clean the prism carefully with a small amount of Isoprop just dabbed on to the old foam. Give the Isoprop 10-15 minutes and then gently wipe. Be very careful – on this prism the foam had started to degrade the external black paint. Go slowly and carefully.
The meter cell platform can be cleaned with Isoprop but keep chemicals away from the small lenses for the meter cells.
These foam pads around the viewfinder area had also badly disintegrated so needed to be cleaned off. Some Isoprop removed them easily.
To get the right width of the replacement pads I have replaced the meter cell platform and am adding a slight score line so I know where the pads are supposed to end.
Fresh 1.5mm thick foam pads have now been added. These are cut from Japan Hobby Tools stock open cell 1.5mm foam.
The meter cell platform is given a strip of 1.5mm open cell foam. Getting it to run correctly with the twists and turns was a challenge.
I wanted to use felt but the felt stock I had was just not thick enough or compress-able enough when layered. This foam will last another 15-20 years!
Clean the small reflector mirror if needed. DO NOT USE CHEMICALS – like the main mirror this is a front silvered mirror. Use just breath and a gentle wipe with a cotton bud.
The top half of the finder contains the mechanics but also the carbon track which provides the variable resistance for the meter. These are a known source of trouble for the Nikon FtN meter head as the tracks can be worn away and there are no replacements.
Take note of the small brushes with the red dab of paint on them and avoid cleaning anywhere near these. They are very fragile.
The carbon track can be cleaned gently with a cotton bud and dilute Isoprop to remove dirt from the track. Go carefully here, the track can easily be damaged.
To avoid the very fine brushes turning the shutter speed control on the finder will cause the track to move backwards and forwards to allow access to each part of it.
Nikon FTn Finder – Assembly
During disassembly the eyepiece surround can fall out. Replace this in position and make sure it stays in place.
Clean all prism surfaces and replace the prism back into the lower half of the finder. Avoid chemicals especially around the joins for the meter and speed reflectors and the rear mirror for the meter cells.
Make sure it is seated correctly as it has a tendency to get caught and jam on its front against the finder lower part.
Replace the left side prism retainer and its screw. This needs to be held firmly against the prism but not so much pressure you risk damaging the prism.
Replace the meter cell platform using the 2x screws.
Replace the circuit board into position, make sure the meter cells locate correctly into their fixing points and then apply the retaining screw – dont tighten the screw – you will need to release it later.
Replace the sprung meter cell retaining part and refit the screw.
Solder the brown wire from the battery case to the circuit board.
Refit the right side prism retainer.
Replace the base of the finder onto the top half.
Replace the 4x screws – remember the smaller screws go to the rear, larger screws to the front.
Check and replace if necessary the foam pads either side of the viewfinder on the base of the finder. These are often ok as they are easy to change and a sloppy repair tech may change these when they wontget inside to check the inner foam. These can be removed using a dab of Isoprop and replaced if necessary with 1.5mm open cell foam.
Fresh pads added here. Check the raised part as well for old foam contamination, the camera body has a foam pad which butts up here. Check that this is clean and has no glue residue. If it does make sure to replace the pad on the camera body.
Check the mechanics at the front of the finder are clean and apply a very thin smear of lithium grease to the slide slots as shown. This needs to be no more than a very thin film of grease.
Replace the front plate making sure that the slot for the aperture lever goes over the lever and does not foul on the lever.
Reconnect the wires and the copper strip to their terminals and tighten down. These dont have to be much more than hand tight. REMEMBER the circuit board is very fragile.
Replace the finder on the camera for setting up the meter.
NOTE: You can be easily thrown here and think the finder is not mounted properly – this is because you are used to seeing the side of the finder fit flat to the camera body. With the top cover removed there is a sizeable gap between the sides of the finder and the camera body.
Nikon have a typically awkward set-up procedure in their service manual for adjustment and few people will have access to a calibrated light source. Generally you will need to compare the meter with a known good camera.
Unless these have been messed about with they will most likely be correct and are best left alone.
See the section at the end for calibration settings.
Replace the top cover and secure with the 4x screws.
Replace the leatherette using a good quality gum rubber or contact cement. Pliobond is the best for this kind of work.
Nikon FTn Finder – Calibration
Nikon advise a rather complicated method for calibrating the meter. In part this is driven by having to set the resistance for each of the CdS cells using an external resistor to balance the cells. While this procedure is useful should you change cells it’s of little value in simply determining the meters readings and adjusting for known light values.
A full technical description is given the Nikon FTn service guide which is available at learncamerarepair.com a simplified approach is given below based on light value.
The examples given are from a known good FtN finder using a 50mm f1.4 lens set at its mid point between f5.6 and f8.
Set the finders film speed at 100 ASA .
Using a calibrated light source the meter needle should centre at the following settings.
BATTERY CHECK – Assuming fresh batteries should be as shown in the picture with the needle to the left of the centre point. When the battery check moves the needle to the right of the centre the battery is exhausted.
Calibrated Source at EV14 – Shutter Speed 500th
Calibrated Source at EV11 – Shutter Speed 1/60th
Calibrated Source at EV8 – Shutter Speed 1/8th
The settings above were done using 2x SR44 1.5v batteries as I wanted the meter calibrated for 1.5v. I suspect the meter when new would have centred the needle at f5.6 exactly which would be the correct speed and aperture for the EV and would accord with other manufacturers specifications. However, I believe the meter cells have weakened somewhat and a small error was introduced by the use of 1.5v batteries rather than Zinc Air batteries of 1.35 volt which would be closer to the original battery specified using mercury cells.
The setting above have been used with live film and the meter was found to give perfect exposures.
If you do not have access to a calibrated light source your only option is to check the meter against a a known good camera over a range of lighting conditions. Most guides will tell you to use a grey card that is 18% percent off-white. I find that sort of advise not useful to most people who lack the facilities for a reliable off-white card and stable light conditions.
The best assessment is to compare with a known good camera using a similar lens and assess the camera over a range of lighting conditions and scenes. Dont forget different manufacturers used different metering patterns but, so long as the scenes you assess are identical for each camera then you should be able to get close enough. If you find the meter is causing you to underexpose or overexpose then you can adjust later if need be.
Brief History of the Nikon FTn Finder
The Photomic FTN finder was the last finder developed for the Nikon F and launched in 1968. The FTn is considered the best finder for the iconic Nikon F body for it’s better handling and its ability to ‘automatically’ manage adjusting for the lens aperture range, albeit this was a bit of a fuss compared to Minolta. Minolta lenses automatically couple to the meter as soon as the lens is mounted where Nikon required the Nikon ‘Twist’ – Mount the lens then rotate the aperture ring from maximum to minimum to couple the meter.
The metering pattern was the same as the previous TN finder with a 60/40 centre weighted pattern which Nikon continued to use throughout most of their film cameras.
The FTn started shipping at F body serial number 690000 and continued until the end of F production in 1974.
The FTn was the first and last F Photomic to have shutter speeds visible in finder. More securely attached to the camera body than its predecessors, the FTN used a new front latch over the nameplate. It was released by a black plastic tipped lever on the finder’s left side, this was requested by users at the time as they felt the single latch at the rear was insufficient.
Bodies shipped after serial 690000 had a Nikon nameplate with square edges, the later FTn finder required a rounded off edge to allow it to fit. The nameplate is only secured by 2 screws so is easy to change if you can find one.
Mel is one of the driving forces behind High 5 Cameras and writes all our articles.
Starting serious photography back in 1972. Over the years she got to shoot film with most of the major brands in 35mm and large format as both a studio photographer and content provider for websites in the early life of the web. These days she is rediscovering photography and has become the GOTO person for knowledge on camera repair advice.