DIY: TTL metered prism for Pentacon Six – Part 2

TTL metered prism calibration

The Pentacon Six TTL metered prism measures the amount of light entering through the lens using a CdS photoresistor. First I’m going to talk a little bit about how it works. If you don’t want to know how it works, or getting more confused as you read on, you can still get the job done by just skipping the next three paragraphs and jump to the one after those.

Cadmium sulfide, or CdS, is a semiconductor that is often used for making light detectors. They are called photoresistor because of its variable resistive property to light: the resistance decreases as more light (more photons) hitting its surface. The reading is realized by a galvanometer – an ammeter that measures current. If you understand Kirchhoff’s circuit laws, having only one resistor (even though it’s a variable one) in a circuit loop is not enough to establish a current measurement because no electrons are branched off throughout the entire circuit. We need at least a simple current divider circuit to achieve that.

In our TTL meter prism, we have a set of two adjustable resistors which are serial to each other, but parallel to the photoresistor. They are labelled 3 and 4 in the picture above. R3 has a large resistance for quick ball-park adjustments while R4 has a small resistance for fine tuning. This set of resistors is often called a “shunt” in our electronics  jargon. The problem is that the current measurement (Io) is not exactly linear vs. the resistance of the photoresistor (Rx), as shown in our current divider formula:
Io = I_input * Rx/(Rx + R3 + R4)
We can see that linearity occurs only when R3 + R4 ≈ C – Rx, where C is a constant larger than Rx. Practically, it means that the shunt needs to be readjusted accordingly if the photoresistance is very high (dark) or low (bright), in order to maintain (approximate) linearity. The purpose of having an adjustable shunt is exactly for this: to facilitate measurement in very low or high-light situations. Unfortunately, if you screw up the linearity by turning these resistors carelessly as I did, you’ll have a hard time readjusting it.

The next comes to offset adjustments. Offset adjustments are sometimes necessary to compensate variations such as temperature and aging of the coil inside the galvanometer. This is realized by the use of adjustable resistors serial to the photoresistor. They are R1 (large) and R2 (small). The offset is not exactly linear either as we now have this:
Io = I_input * (Rx + R1 +R2)/(Rx +R1 + R2 + R3 + R4)
It’s OK as long as R1 and R2 are relatively small.

In order to calibrate your prism, first you need a reliable reference – a good light meter, a digital camera, or even a sunny day using the Sunny f16 Rule. You’ll need a gray card as target as well. If you don’t have one handy, asphalt concrete or healthy green grass are good approximation. This is how I did:

1. Peel off the leatherette on the top of the prism as pictured above.
2. On a sunny day, use a reliable meter (I use my Canon 40D )to target the gray card at f5.6, ISO125 and Av mode, and get the shutter speed reading. It should give you around 1/1000s shutter speed using Sunny f16 Rule.
3. Then use your Pentacon Six with the TTL metered prism and your favourite lens (I use my 80mm f2.8 Biometar) and follow the same setting – set the lens to f5.6 and set the prism meter dial to ASA125. Turn the power of the prism on. Press and hold the stop-down lever of the lens. Adjust the shutter speed ring of the meter dial until the needle points to the centre of the scale. Check if the recommended shutter speed aligns with the reading from the reference. If not, adjust knob 1 or 2 pictured above using a small screwdriver. Attention: Make sure the screwdriver is not touching any knobs when you take a reading, as the conductance of the screwdriver intervenes with the meter.
4. Then repeat Steps 2 and 3, but this time with f16 (if your lens can go to f22, go for it). You should get something around 1/125s (or 1/60s for f22) this time. If the TTL meter is linear, it should align with your reference this time around and no adjustment is needed. If so, congratulations and you’re done with the calibration. If not, read on –  we have to play around with knobs 3 and 4 as well as 1 and 2.
5. First check how much knob 3 can trun. Mine can turn about 240°, which is from 12 o’clock to 8 o’clock.
6. Turn knob 3 all the way counterclockwise. Reapt Steps 3 and 4. If the reading does not closely align to the reference at the end of Step 4, turn knob 3 by 30° clockwise and do again. Repeat until you get something close to the reference in Step 4. My “sweetspot” for daylight use is happened to be in the middle, which is 4 o’clock position. Yours can be different.
7. Use knob 4 for fine-tuning if necessary, again by repeating Steps 5 and 6 – where states turning knob 3, turn knob 4 instead.

DIY: TTL metered prism for Pentacon Six – Part 1

I have two TTL meter prisms for Pentacon Six. They are nicely built and generally accurate – with the body, it looks simply much nicer than the waist-level finder or the non-metered prism. However, one of my prism has its eyepiece shutter lever stuck at “closed” position. I decide to take it apart…

First, take off the top screw of the meter dial using a tweezer or two mirco screwdrivers. Beware not to scratch the screw.

Then, take off the ASA/DIN plate, and the small washer beneath it. Optional: the eyepiece can be taken off by unscrewing the two tiny screws located at the underside of the eyepiece oriented in V-shape. You can see that the eyepiece shutter (the piece of plastic with diagonal white/blue strips) is at “closed” position. If you have a stuck shutter like I do, fiddle around with it and see if it fixes the problem.

Using a tweezer, take off the big washer.

Note that there’s a little hook on the washer that keeps the ASA/DIN plate rotating in unison with shutter speed ring, unless you disengage it by pulling the ring up – that’s exactly what you do when you change the ASA/ISO/DIN setting!

Next take off the shutter speed ring by unscrewing the three concentric screws. Afterwards, you should see a big dial as pictured above. Rotate it such that two screws at 3 and 9 o’clock positions are exposed. Unscrew them.

Take off the aperture / stop-down metering selection ring, and the cardboard-made washer underneath as well.

There are two film-like washers underneath the big dial that tend to wobble (but shouldn’t come off easily).  Keep them in mind when you put everything back together – I fix their positions by putting needles through the screw mounts.

Next, take off the two shinny screws using a screwdriver. Using a tweezer, take off the little piece of leatherette on the eyepiece shutter dial.

After the tiny leatherette is taken off, a screw is exposed at the centre of the dial. Unscrew it 🙂

Take off the dial and the washer underneath.

At the front of the prism there’s a big piece of leatherette featuring the VEB Pentacon logo. Genently peel it off from one of the lower corners. Expose and unscrew the screw located in the lower middle.

Now you can open the case. It may not the end of the dissembly depending on what you want to do. Below are the most likely reasons that you have read this far to learn how to dissemble your lovely P6 TTL metered prism:

1. The prism has no response even with fresh battery installed – this generally attributes to broken electrical wiring. You can rewire it easily as the electrical contacts are generously big. Use thicker and longer wires if you can.
2. Dusty / dirty prism – in most cases, small amount of dust and dirt are not an issue with meter accuracy or viewing, and not worth the hassle and risk to pop open the case to begin with, let alone going forward and take off the pentaprism. In case you insist, here are the steps:
   • Take off the screws pointed by the blue arrows
   • The pentaprism should be able to come off by pulling it vertically upward
   • Beware not to break anything or scratch the black paint of the pentaprism, which may cause light leaks and result in inaccurate meter reading. The needle-viewfinder periscope (the clear piece of plastic glued onto the needle side of the prism) is particularly easy to break. When it breaks off from the prism, it tends of chip off some glass!!
   • When putting back the pentaprism into the case, there are a few parts that are a PITA hard to position well. Use double-sided tape to help you:
     • the CdS photoresistor and its two connecting wires
     • the gaskets around the prism base used for light-proofing
3. Stuck eyepiece shutter. The opening and closing of the shutter are operated by a spring (a metallic strip) clamped each side by a slit in the shutter on one side and the dail on another. The original design makes it more likely to stuck close than open 😦
   Check if the spring is still inside the slit of the shutter. If so, untighten the screw pointed by the red arrow and pull the whole metal part (dial stopper) against the direction of the arrow by just a little bit. Re-tighten the screw. This way, the dial will spend more of its cycle in open shutter position in each rotation, and thus less likely to stuck close in the future.
   If the spring is dislocated (i.e. out of the shutter slit), you will most likely need to take off the whole pentaprism. Follow #2 above and then do the tweak in previous paragraph.

If your prism has inaccurate meter readings, you don’t need to dissemble it. I will show you how to calibrate the meter in Part 2.

How to glue the leatherette back?

If you peel it off gentally and not too frequently, its adhesiveness should be good enough to stick back to where it used to be without noticeable traces of “tampering”. If it doesn’t stick, you need to glue it back and rubber cement is a no-no. I have tried Weldbond and it worked well. Apply a thin layer of glue on both sides and wait for one minute before bonding. I heard people using Tacky Glue with good results but I haven’t tried – make sure though you’re using the pH-neutral variety because the original one is slightly acidic, which may be harmful to the leatherette in long run.

Good luck!