Nikon AI question
I just got my first Nikon camera - an FM2. I have a question about the AI system's precision for meter readings.
I realize that the system calculates exposure based on the position of the AI lever/ring on the camera body. ie if the ring is in a particular position the camera knows how many stops away that is from the maximum aperture, and adjusts the exposure reading accordingly.
However, I am wondering how finely the ring positions are graded. Does the ring position only convey information to the meter in half stop increments? In other words, if I put the camera a third of the way to F8 from F5.6 will the meter read this as a half stop between the two (F6.7) or is it able to detect an aperture of F6.3? I ask because I am used to using 1/3 stop increments on my RB67, but not sure if I need to be quite so precise with the FM2.
There aren't intermediate half-stop detents on Nikon MF lenses, though the meter will measure movement between full stops. It's just a bit difficult to do more than guesstimate fractional f-stops when the distance between full stop detents on the aperture ring are so small. Needle displays help, I suppose. Early Mamiya RB lenses have half-stop detents and markings on the aperture ring; later KL lenses have half-stop markings but no detents. Not sure how you're dialing in precise 1/3 stop settings.
The FM2 does not have any electrical contacts that communicate in any way with any lens. The camera doesn't know how many stops away the lens is set from the maximum aperture. It simply 'knows' how much light will enter at a given setting, combines the data from that setting with the given shutter speed and arrives at a meter reading. The aperture settings on the lens are referred to as being 'stepless', which is to say you can set the aperture ring to any setting whatsoever and the meter will respond accordingly. At the moment of exposure, the aperture will be closed down to the precise setting you made on the ring, whether it's f6.3 f6.7, etc. Shutter speeds, however, are not stepless.
Originally Posted by nwilkins
The aperture settings for the Nikon metering system are 'stepless', which means that you can set the aperture ring to any marked setting, or in between, and the meter will respond accordingly.
Agreed. The FM2 and FM2n use an analog circuit to read the amount of stopdown you're doing from maximum aperture, so they're functionally stepless. I'm sure there's a resolution value that the circuitry has, but for working purposes it can be ignored. It's much finer than 1/3 stop.
... even 1/3 stop is more resolution than is really needed in most situations! :)
okay great thanks everybody. I realize that for example F6.3 vs F6.7 is not usually too crucial but good to know how it works. It is much easier to alter the aperture in thirds on the big RB67 lenses than on the small Nikon ones, so I will probably be rounding to the nearest half stop on the Nikon most of the time anyway. I just got into the habit of thinking in 1/3 increments because that is what is marked on my sekonic studio meter.
Aperture setting information is coupled to the metering circuit through a variable resistor.
Originally Posted by PhotoJim
It's continuously adjustable, like the volume control on a classic radio or television (some modern ones are digital).
The 'resolution' would be 1/10000 of an f-stop or better.
The concept of 'resolution' is only meaningful in digital systems.
Nikon pre-digital cameras are completely analog.
The amount of stop down from maximum aperture is stepless. There is no steps whatsoever. I am sure that up to the F3 the aperture is stepless. I am not sure if the newer camera like the F5 would divide the resolution in step being heavily microprocessor controlled camera.