I agree! I would love to. The prices are insane for the F prisms.As I understand from what technicians have told me, the same little part wears out, Therefore, no replacement parts. A plain meterless prism makes a well-balanced F that is a delight to shoot with. Try it.
As I understand from what technicians have told me, the same little part wears out, Therefore, no replacement parts. .
Probably the ring resistor...
Sover Wong has the ring resisters he indeed has them made but he only uses them to service Nikon F's and F2s he doesn't sell them.Sover Wong has that part. He has had new ones made.
Sover Wong has the ring resisters he indeed has them made but he only uses them to service Nikon F's and F2s he doesn't sell them.
With all due respect, aren't we getting a little ahead of ourselves here?As I understand from what technicians have told me, the same little part wears out, Therefore, no replacement parts. A plain meterless prism makes a well-balanced F that is a delight to shoot with. Try it.
If it is called a Photomic it is the DP1. It has a needle for exposure indication, doesn't it? And takes pre-Ai lenses? At any rate it should say on the bottom of the finder housing towards the front, I think.I have a photomic meter for the F2 that works, but is inconsistent. Sometimes it seems accurate, other times way off. I think it is the DP1 head.
Plenty of Minolta srT meter perfectly, and they date to roughly the same era as the photomic T or TN finders. 60's CdS cells are often as not in fine shape if the rest of the camera is.My Nikon F metered prism works okay and seems to be accurate (tweaking the ISO less than a stop to adjust for modern batteries). I have had it for over 30 years and used it only sporadically recently, so it has been kept clean and the ring resistor hasn't been abused.
I don't think CdS photoresistors "wear out" with age. It is claimed that CdS photo-resistors could die from corrosion that causes the contacts to go bad, but this would be externally induced damage, while some of the age effects on selenium cells are claimed to be damage from the internally generated current. (CdS photoresistors, of course, don't generate their own current.)
....
I don't think CdS photoresistors "wear out" with age. It is claimed that CdS photo-resistors could die from corrosion that causes the contacts to go bad, but this would be externally induced damage, while some of the age effects on selenium cells are claimed to be damage from the internally generated current. (CdS photoresistors, of course, don't generate their own current.)
Plenty of Minolta srT meter perfectly, and they date to roughly the same era as the photomic T or TN finders. 60's CdS cells are often as not in fine shape if the rest of the camera is.
What makes the meter in the DP-1 and DP-11 or Spotmatic for example go non-linear (eg off by 2 stops in bright light and of by 4 or 5 stops in dim light )? I've always been under the impression that it was the CdS cells...it does not happen to light meters with SBC cells.
First, to keep this really on-topic, I found this schematic of the FTn meter, which might be useful to the OP:
http://www.peterrussellphotography.com/nbr/Nikon_F_Photomic_FTn_meter_repair_instructions.pdf
A couple of things to note:
- The circuit is extremely simple. It's a voltage divider with the CdS photoresistors in series with a fixed 41 ohm resistor and then the meter+trimpot in parallel with the main pot (the infamous variable resistor). The circuit must be designed and trimmed for whatever the CdS resistance vs illumination curve is, over a wide range. It's impressive that such a simple circuit works.
- The currents involved are tiny. CdS cells have a resistance up to over a megohm in a relatively dark scene. The author of that schematic says the meter is centered with a current of 16 micro-amps.
What this means is that any extra resistance could really mess things up. For example, corrosion at the CdS cell terminals, or extra resistance at the wiper of the potentiometer (which seems more likely). That extra resistance could easily make the circuit non-linear.
SBC cells are silicon photo-diodes, not photoresistors, so a circuit with SBCs would be completely different. I have no doubt that SBCs are superior, but they're also a lot newer. A major reason for going to SBCs (as I understand it) was that they are a little more sensitive and a lot faster to respond to changes in light.
I thought that Sover only works on the F2 body and finders?
....ah, wait...l think I get it now...
The CdS cell's response is non-linear with a region that is linear enough to be useful. So, adding some small resistance to the circuit could push the CdS cell up into the highly non-linear area of it's response curve !!!
Thanks for helping me think this through. It has bugged me for a very long time.
I do not see how adding resistance can possibly make the circuit respond such that the meter is off by 2 stops in bright light and off 5 stops in dim light - a situation I have seen in at least four different DP-1 meters and several Spotmatic meters.
EDIT:....ah, wait...l think I get it now...
The CdS cell's response is non-linear with a region that is linear enough to be useful. So, adding some small resistance to the circuit could push the CdS cell up into the highly non-linear area of it's response curve !!!
Thanks for helping me think this through. It has bugged me for a very long time.
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