Focus Limiter on Nikkor 300mm f/4 AF ED

[Mike Buckley]

I just bought a Nikkor 300mm f/4 AF ED and can't find any documentation for it because production stopped about 12 years ago. It has a variable focus limiter that can be used to limit the amount of hunting needed for the auto focus to do its job. The only problem is that I can't figure out what the graphics on the lens barrel mean and how to position the limiter. If someone could explain that, I would be grateful.

If a close-up image of the markings would be helpful to refer to, let me know and I'll post one.

Thanks in advance!

[Manfred M]

A picture would be nice. I'm trying to remember; is the design where the focus limiter is a ring that you loosen or tighten with a thumbscrew? I think I may have looked at a used one last year for my wife before our trip to Namibia. We ended up picking up the Sigma 150-500 for her instead.

If it is you loosen it and turn it so that the dot aligns with the maximum focus distance you want it to use and then tighten the screw. If for example, you set it to 20 ft / 6 m then the limter is set from 20' / 6m to infinity.

[Mike Buckley]

Thanks, Manfred. Yes, that's the type of mechanism. I have a feeling that you've got part of it right but perhaps have forgotten the other part of it. Even now that I've seen your explanation, I'm not able to figure out why there are two sets of numbers to the left and right of "full" as shown below.

15ft---------25ft ---------100ft--- FULL ---------10ft----------12ft-----------15ft

Now that you see that, please try running it by me again.

[Manfred M]

Mike - I don't remember that part particularly well. I did mount the lens on my camera in the store and took a couple of test shots. I remember the focus seemed even slower than my 80-400mm (also a screw drive focus). I have enough trouble with missed shots on that lens and felt it would drive my wife crazy, so I didn't look at it any further.

Looking at what you have there, I think it is still essentially what I suggested. Loosen the thumb screw and put the alignment mark to the minimum focus distance marking you want and then tighten up, either to the left or right side of "FULL". I'm not sure why they went with a split design, but suspect it is as simple as it seems. The numbers on the left will allow very short focus travel distance (fast focus), and the ones on the right will let you get a lot closer to the subject, but will result in slower focus speeds and more "seeking" issues.

[Mike Buckley]

The numbers on the left will allow very short focus travel distance (fast focus), and the ones on the right will let you get a lot closer to the subject, but will result in slower focus speeds and more "seeking" issues.

OK, that makes sense.

That's until I see what Ken Rockwell has to say and now I'm confused again: "You may choose any near or far distance as a limit. One of the limits is always either infinity or the close limit of 9' or 2.5m."

I don't understand optics at all, so could it be possible that positioning the dot at a number on the left side of the scale limits the focusing range from that distance to 9 feet? And that positioning it at a number on the right side limits the focusing range from that distance to infinity?

I'm not even sure I would ever position the dot on anything other than "Full." That's because I don't need to be concerned about focusing speed for 95% of the situations that I'll be using the lens; fortunately, mountains and the like don't move very fast.

[Manfred M]

A focus limiter is purely to prevent the lens from going through its entire range to focus. It's all about focus speed because the old screw drive lenses tended to have a long distance to go through from the closest focus point out to infinity. On longer lenses, the subject matter may not be all that distinct and the contrast detect circuit in the autofocus may have issues picking it up and the lens will move through its entire focus range to get it to work. With my 80-400mm, that cycle can take well over a second. The focus limiter on it is a slider and restricts the lens from going all the way back and roughly cuts the focus time in half.

If fast focus is not an issue for most of your shooting, just leave it off and let the lens go through the cycle. That is what I do most of the time on my lens.


It sounds like Rockwell is saying exactly the same thing I am; unless you use the focus limiter, the lens has two built-in hard stops; one at the minimum focus distance of 9 ft and the other at infinity. If you set the focus limiter to another position, it will stop the lens there, instead of going all the way back to the 9 ft focus position.

[Mike Buckley]

I'll take your word for it that you and Rockwell are saying the same thing. I understand everything that you've been writing and I have never fully grasped his sentence that says you're either limiting the long end or the short end of the focus range. You brought my camera back to life and he never did that for me, so I'll pay more attention to you.

[JonathanC]

The stop prevents the focus mechanism from going past where you set it. Set it at 25ft and you won't be able to focus past 25ft from where the lens is focussed when you set it. If you have focussed on a close subject you can focus from close to 25ft. Focus a long way off and then set the stop and you'll get infinity down to 25ft available.
HTH