Focusing Unit AU-1 - automatic diaphragm works with Nikon F but not with D300

[Kent Shafer]

<p>I just received a Nikkor 600 mm f/5.6 ED with Focusing Unit AU-1 from KEH. It appears to be in beautiful condition, but there is an initial glitch. When a D300 is mounted on the lens, the diaphragm stays at maximum opening all the time - it doesn't stop down when shooting or when the depth of field button is pressed. With no camera on the lens, moving the lever on the lens mount up and down works the diaphragm as expected. And the lens works fine with a Nikon F.</p>

<p>If you set the lens at f/16, say, and then look through the D300's viewfinder while screwing the body onto the lens, you can see the aperture open, and you can see it close down again as you remove the body. Evidently, the automatic diaphragm lever on the lens is binding somewhere on the D300 mount and can't drop down when the D300's diaphragm actuating finger (or whatever it's called) moves down.</p>

<p>From everything I read, I thought this old lens would work fine on a modern DSLR. Was I expecting too much? Any suggestions? (I really don't want to send it back. It's too pretty, and it does work fine with the old F.)</p>

<p>Thank you.</p>

<p>Kent</p>

[chris_werner]

<p>Kent,</p>

<p>My Magic Lantern guide says that any lens requiring the AU-1, and specifically including the 600mm f5.6, is not compatible with the D300. It cautions that you could damage the camera if you attach it, but that could just be for good measure.</p>

[chris_werner]

<p>FWIW, I also found the list of incompatible lenses and acessories on page 352 of the manual. That was harder to find for some reason.</p>

[Kent Shafer]

<p>Yikes! Thank you Chris. I'm very embarrassed not to have consulted the manual. No harm done to the D300 however.</p>

<p>I guess the logical follow-up question is whether there are any digital bodies that will work with an AU-1. Or whether the lens could be used with the CU focusing unit, which doesn't appear in the D300 incompatible list. It seems that the ED lenses generally come with the AU-1 and the older ones with the CU, but I don't know if it has to be that way.</p>

<p>Bjorn's page says the 800 mm ED works well with a D3 but doesn't say which focusing unit he uses.</p>

[chris_werner]

<p>Kent,</p>

<p>Real men don't read manuals! ;-) Don't let it bother you - we've all done it, me multiple times.</p>

<p>I've read a little more now and it appears the AU-1 was discontinued in 1977, the same year AI lenses were introduced. Since pre-AI lenses are no-no's on the Nikon digital cameras, it would seem to follow that neither focusing unit is kosher.</p>

<p>Look at the bright side - you'll save yourself some back troubles not carrying that big honker around!</p>

[Kent Shafer]

<p>Chris,</p>

<p>You're right about the back issue. That baby is heavy! It puts a scary amount of stress on a humble 1/4-20 tripod screw too. If I were going to use it much, a better mounting arrangement would make sense. Also minions or a pack animal of some kind to carry everything.</p>

<p>Thanks again,<br>

Kent</p>

[chris_werner]

<p>Kent,<br>

 <br>

Your situation got me thinking.  People hook DSLR's to telescopes all the time with connectors called T-rings.  Focusing is manual, of course, but so is the AU-1.  It would seem that there has to be a way to do this, though it might require a custom adaptor.  If you seriously want to use the lens, you might look into that - here's a very fine firm that focuses on the astronomy market, but could almost certainly help here as well - http://www.preciseparts.com/ppmain/index.html.</p>

[bob_sunley]

<p>Odds are the AU-1 could be AI modified just like any pre AI lens.</p>

 

[Kent Shafer]

<p>"Odds are the AU-1 could be AI modified just like any pre AI lens."</p>

<p>That's what I assumed too when I ordered the lens. I figured it would either work as-is or John White, the AI-conversion guru, could convert it. I called him, though, and he said he isn't familiar with the issue and there's nothing he can do. The problem isn't AI vs non-AI, he says. The AU-1 has no provision at all for the lens to tell the body where the aperture is set.</p>

<p>Instead, the problem has to do with the automatic diaphragm lever. Looking a little more closely, I see that on a regular lens, the lever moves in an arc that's concentric with the rest of the lens. On the AU-1, however, it arcs the other way - moving slightly closer to the center of the lens in the middle of its travel and then back to the edge. Also, there's a piece of sheet metal on the AU-1 just next to the lever - a light shield of some kind maybe - that may be interfering.</p>

<p>I'm pretty sure a little judicious grinding or filing would do the trick, but it would have to be done by someone who knows what he's doing - definitely not me.</p>

<p>I did a little looking around in the Nikongear.com forums and found one poster who said neither the AU-1 nor the CU focusing units will work with modern cameras due to the automatic diaphragm problem I'm having. In other posts, though, Bjørn Rørslett says both work fine and in fact he has mounted CPU chips in them. Maybe I will try emailing him.</p>

<p>If no one comes to my rescue, I may resign myself to only using the lens wide open, which is what one would mostly do anyway, or using it only with older film cameras.</p>

[bob_sunley]

<p>I'd just make sure the lens body isn't touching the AI index tab on the camera. You don't want to damage that part.</p>

<p>A while ago I modified an older AI third party lens to fit a newer digital body, it had a large chunk of metal that acted as a rear element protector. That worked fine on non electronic bodies with lots of spare room at the bottom of the mirror box, but it would't fit into a newer body.</p>

 

[Kent Shafer]

<p>Thank you Bob. I will watch out for that.</p>

[Kent Shafer]

<p>Just to complete the story for anyone who's interested: I discovered that the automatic diapragm lever on the AU-1 was binding very slightly on a protrusion in the D300 body just above the finger that moves the diaphragm lever (which appears to be an upper stop for the finger).</p>

<p>I used a jeweler's file to file a tiny, tiny bit off the outer, lower corner of the lever on the AU-1 - just slightly rounded it off - and everything works fine now.</p>

<p>Now if I can just figure out how to lug it around, point it at something interesting, and get it focused before the interesting thing goes away . . . .</p>

[bill_webster2]

<p>Kent,<br>

Thanks for all of the good information on the 600mm lens. I have one of the same located and thinking of buying it. If you don't, mind I would appreciate knowing what the price range of this lenses are.<br>

Bill</p>

 

[Kent Shafer]

<p>I bought mine from KEH for around $800 in "bargain" condition with hood and caps. It's in really great shape except that it's missing the little screw-in handle for the focusing ring. (One could substitute a bolt or something, but I haven't bothered.)</p>

<p>I found that it is important to use a filter in the AU-1's filter drawer, which takes standard 52mm screw-ins. Pictures with the filter are noticeably sharper than without.</p>

<p>It's awkward to focus because the weight of the camera body causes the focusing mechanism to bind - you need to apply a little upward pressure on the camera to relieve the stress while focusing. Nikon made a cradle (sort of an arm) to deal with this problem, but I haven't found one yet. You could never use this lens for birds in flight or anything like that, but that would be tough with any manual focus lens.</p>

<p>So there are drawbacks. But if you can deal with the quirks, it's a very sharp lens, especially stopped down one or two stops, as well as something of a conversation piece.</p>

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