Hi, I have a third party 60-300mm ƒ/4 - 5.6 zoom in Nikon F AI-s mount. I have it mounted on a pre-AI body (Nikkormat FTn), with which it is compatible thanks to the "rabbit ears" on the aperture ring.
But it seems this would be a problem with the internal meter :
I can set it to ƒ/4 and zoom to 300mm, but then it means the max aperture is ƒ/5.6, however the aperture ring still tells the meter it's ƒ/4. Will my images be underexposed ? Must I correct manually by adding a stop of exposure ? And would it be different on a AI or AI-s body ?
Thanks in advance
It probably depends on the zoom but I would say, based on looking at my two pre-AI bodies, including an FTn like yours, you would definitely need to compensate manually if you want a guaranteed precisely correct exposure. I would also make sure that there is nothing about the lens that can damage your camera. Almost every Nikon body can mount a large percentage of Nikon lenses, but every single one has a few that can be useless or harmful if mounted. (The most compatible is the Nikkorex F which can mount any Nikon lens barring the ones without manual focusing and aperture rings, and those that require MLU)
I would say that If you're at the narrow end of the lens's zoom range, you might not be far off if you centered the needle then decreased shutter speed by one stop or opened up the lens, assuming it's not wide open as in your example.
Now, the interesting thing is that the lens is actually compensating a little. In the example you mention, the camera is metering through the lens, and at 60mm it's seeing through an f/4 entrance pupil and at 300mm it's seeing through a f/5.6 entrance pupil. Now, it doesn't necessarily matter exactly what aperture the camera thinks it is, as long as it's pretty close, because the amount of light it's receiving is determined by the entrance pupil, which is a function of the size of the diaphragm and the magnification of the elements in front of it. It may meter almost accurately without compensation in most cases.
Now, there will be some problems. Some camera do want to know the max and min apertures of your lens, because of some limitations of the metering system I don't quite understand... Minolta cameras from this same period just sense how many stops down from wide open the lens is set at, and calculate exposure from that information. The Nikkormat FTn on the other hand does "want" to know the actual value of the aperture. I think there's a linearity problem, since the Minoltas calculate exposure mechanically using strings and pulses and the Nikons calculate it electronically with variable resistors built in to the aperture ring, the film speed ring and the shutter speed ring. So there are likely to be small miscalculations if the camera is wrong about the functional aperture of the lens. I would compensate by +1 stop at 300mm and maybe a half stop if possible in the middle, but you may find that the actual error is smaller than that and wish to not bother adjusting.
I don't know how these lenses behave on AI bodies but honestly a lot of zoom lenses don't compensate in a mathematically perfect way and just rely on the exposure latitude of the film.