Austin Harned , Jan 25, 2012; 09:27 p.m.
I recently aquired a well aged Nikon F with the photomic FTN meter/finder.
I did 4 test rolls, and the meter seemed pretty off. All test rolls were done on ISO 100. When I developed the film, every exposure was consistently very underexposed.
So, I broke out my Nikon Digital SLR, and put the 50mm lens from the Nikon F onto my DSLR. I found the correct exposure on the DSLR for ISO 100, 200, 400, and 800.
I then put the Nikon F's 50mm lens back onto its body and tested out the "correct" exposures from my DSLR.
On the Film body, for ISO 100, I had to set the ISO to 20 to get a correct exposure reading.
For ISO 200, I had to set the ISO to 45 to get a correct exposure.
For ISO 400, I had to set the ISO at 75-80 to get a correct exposure.
For ISO 800, I had to set the ISO to 160.
So, does it sound right that I just need to divide the films ISO rating by 5 and then set the Nikon F's ISO to that sum? That is what it seems like to me. I just need a fresh set of eyes to read this and confirm my results. And, does anyone know how I could somehow adjust or calibrate the Photomic FTN meter.
Ah! Sorry for the novel here!
But, thanks for any help!!
Austin Harned , Jan 25, 2012; 09:39 p.m.
Forgot to mention, I'm pretty sure the meter is off because from the factory these Photomic FTN's used 1.35V Mercury batteries, and I have got 1.5V Alkalines.
Craig Dickson 
, Jan 25, 2012; 09:39 p.m.
Are you doing the "Nikon twist" when you attach the lens to your F? (Mount lens, rotate aperture ring all the way to the minimum aperture, then back to whatever f/stop you want.)
It's quite possible that an old FTn won't meter accurately. Whether, or how, it can be adjusted, I do not know.
Austin Harned , Jan 25, 2012; 09:41 p.m.
Yes, I am coupling the lens to the meter the old-fashioned way. (Set the aperture to f/5.6, then mount, then twist to the extremes.)
It's innacurate because of the battery issue. I was wondering if my math/results seemed acurate and could be relied upon.
Craig Dickson , Jan 25, 2012; 09:47 p.m.
I would suggest trying it in a variety of lighting conditions. If you can come up with a formula that works in practice, it would be hard to argue with that. It sounds like your ISO/5 formula works well enough for the conditions you tested it in.
Austin Harned , Jan 25, 2012; 09:50 p.m.
Thanks Craig, good idea! I just tested it at a photo of my vaccum. I will have t get outside when it is sunny, and test my formula out.
Jim Momary , Jan 25, 2012; 10:05 p.m.
ISO 800 halved = 400.
ISO 400 halved = 200.
ISO 200 reduced a bit more ... as you state, really is about 2 - 2 1/2 stops to compensate for under exposure.
That's what I typically see in my older rangefinders that ate mercury batteries and now use SR or LR. It should get worse with more bright lighting conditions. I've modified some with schottky diodes (easier on those simple beasts).
Look up here on p-net options on using hearing aid batteries or getting a battery adapter.
You need to drop the voltage to the 1.35 locale to get the meter closer to 'normal', presuming it's still within factory calibration.
Jim
Matthew Currie , Jan 25, 2012; 10:13 p.m.
If you have found an ISO offset that works at one exposure, it should work pretty much at all exposures, as long as the meter remains in range. Usually when I do this on an old camera I simply check the meter against a known good one with the same focal length lens aimed at the same subject, and determine how many stops off it is. 2 1/2 stops sounds about right for an uncompensated FTn.
It's not terribly hard to recalibrate these meters, and if you go to http://arcticwolfs.net and follow the links through "nikon service manuals" to the Photomic FTn page, you'll find one set of instructions for this. There are only two variable resistors involved. I've done a couple with good results. I recalibrated mine for silver oxides, but I believe the SO cells are becoming nearly impossible to find, so you're probably better off using alkalines, which start at a very slightly lower voltage, unless you plan to make an adapter for the smaller SO cells. O-rings on smaller cells will not work in the FTn battery box, unfortunately, owing to the way the contacts are arranged. Alkalines will slope off as they age, but not so quickly that it's a terrible problem, and since as part of the process you must calibrate your battery check as well, you'll be able to keep track of it.
You do have to be pretty careful to use fresh batteries, and to check your work against a meter you know gives good results consistently. For mine, I used the F with a 50 mm lens, and compared it to a Minolta X-370 with a 50, both aimed at an evenly lit surface from the same tripod. Your mileage may vary, but that's the best meter I've found for this kind of work, and its pattern is similar too.
Remember when doing this that an F with a contemporary lens will work only in one stop increments, and the meter itself is doing well if it gets you within a half or a third of a stop, so don't get too too fussed if it's a little off. You'll still need to exercise a little judgment when shooting, even when it's spot on.
Bob Sunley 
, Jan 26, 2012; 12:24 a.m.
You can pick up a card of thirty of the 675 blue tab hearing aid batteries from Costco for around $15. Add two of the machined brass ring adapters and you are good to go.
