一 Photographed at Braemar Hill around 1340H on 13-AUG-2016
2016年8月13日星期六
2016年8月11日星期四
The Mystery of Centre-weighted Metering: Disassembling Nikon F Photomic TN Finder
The Nikon F Photomic TN finder (1967-1968) is the successor of the Photomic T finder (1965-1967). With 'N' stands for 'new', the TN finder differs in appearance from the T finder only by an extra white button on top of the front plate and a white character 'N' imprinted on the top cover near the meter switch. That extra button serves to allow the user to test the battery. Provided that the meter is switched on, if the battery is working (not used up), the meter needle will deflect once the button is pushed. This is a pretty good improvement.
While the T finder merely provides whole-field-averaged metering, a seeming more advanced feature of the TN finder is the centre-weighted metering, with the central part of the view corresponding to a circle of 12 mm in diameter on the film weighted 60% and the surrounding part 40%.
Centre-weighted metering sounds sophisticated. I have been hard-grasped by the curiosity of how it works. So, I sought to buy a TN finder, one with lots of scratches and a few dents, at a reasonably low price for disassembling in order to find out the answer. Out of my surprise, it is not that hi-tech as I thought. There is no complicated electronics. Instead, centre-weighting is achieved geometric-optically by a small hole of suitable size that limits [and diffracts?] light onto each of the two photo-resistors.
Each of the photo-resistors seems to be covered by a convex lens.
For a brief visual comparison of the TN finder with the T finder, please see also the previous post Disassembling Nikon F Photomic T Finder.
While the T finder merely provides whole-field-averaged metering, a seeming more advanced feature of the TN finder is the centre-weighted metering, with the central part of the view corresponding to a circle of 12 mm in diameter on the film weighted 60% and the surrounding part 40%.
Centre-weighted metering sounds sophisticated. I have been hard-grasped by the curiosity of how it works. So, I sought to buy a TN finder, one with lots of scratches and a few dents, at a reasonably low price for disassembling in order to find out the answer. Out of my surprise, it is not that hi-tech as I thought. There is no complicated electronics. Instead, centre-weighting is achieved geometric-optically by a small hole of suitable size that limits [and diffracts?] light onto each of the two photo-resistors.
Each of the photo-resistors seems to be covered by a convex lens.
For a brief visual comparison of the TN finder with the T finder, please see also the previous post Disassembling Nikon F Photomic T Finder.
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