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Nikkor-AM ED 120mm f5.6s Question

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Michael Erlewine:
The Nikkor-AM ED 120mm f5.6s goes from f/5.6 to f/45. If anyone knows, at which f/stops is it sharpest? Wide open? If so, how far can we narrow the aperture and still get sharp results?

Bjørn Rørslett:
For my usage of this lens with 4x5", I routinely closed the aperture down to f/22 and still got very sharp pictures. Do note this was for close-ups 1:2 or higher. By f/32 the onset of degradation became visible.

As you are using only a small portion of its projected image circle, due to a much smaller sensor format, I guess one should be stricter and not go beyond f/16. Perhaps f/11 is a good compromise between depth of field and attainable sharpness.

As this lens is perfectly symmetric in its optical design, I have a feeling it is more tolerant of stopping down. Experiment to see where you find the sweet spot for your own applications.

Michael Erlewine:
Thanks for that experience. I have been trying it at f/16 and am still fiddling with it. A while back, you detailed how to make an F-mount for this lens out of odds-and-ends. I did that, but for some reason when I put it all together it puts pressure on the aperture slide so that it locks up and does not move. If I start to unscrew the mount, then it will move, but is loose. It looks like I must need a slight amount more of room, a shim or something, but have been unable to figure what will work. Any ideas?

Bjørn Rørslett:
Here are the parts I used for my makeshift F-mount adapter for the 120 AM.

The bits and pieces are combined as follows,

First, remove the rear lens group, but keep the retaining ring (the one used to mount the lens on a lens board) in its position. Then, put a step ring 37-52 mm over the retaining ring with the wide part towards the shutter. This ensures a minimum spacer so as to avoid jamming the aperture ring.

Next step, put a T2-M39 step ring inside a BR2. This ring can go in in either orientation as it just acts as a secondary spacer. Place this package on top of the 37-52 ring.

The last step is placing the rear lens group into the opening of the BR2 and screw it home. This locks all the parts together.

Voila.

Frank Fremerey:
I use it between 11 and 22 depending on the subject and the camera. I am a bit reluctant to go lower than 16 with the
D600. For most of my purposes f=16 is more than enough to get all parts I want in focus Scheimpflug focussed

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