AF operation not working with my 300/4 AF-S on my new D750

<p>All my other AF-S and AF-D lenses correctly auto-focus on my new D750 and when I checked this lens on a D610 or D810, it focused fine. Couldn't find another D750 to try out but will do it in 2 days at the dealer that I purchased the lens a few weeks ago. Any ideas on what's going on? Firmware or software issue with the lens or body?</p>

<p>I have read a few reports where a D750 wasn't focusing with the 300/4 AF-S when a TC was mounted (specifically, TC-14E or EII) - in all cases the 300 by itself did focus though. Does the information in the top LCD blink when you mount the 300 and attempt to focus? How old is your 300 - this should not be an issue with lenses with a serial number starting with 3 (i.e. manufactured in 2006 or later)? With older lenses, there was a grounding issue that was solved then by soldering a grounding wire to the lens' chip; believe the newer solution is to have a new DC/DC converter placed.</p>

<p>It is an earlier version, ser. # US 2075xx. No flashing of the top LCD with the 300/4 mounted. There is no AF operation either with or without a TC-14E II. No problem using my 600/4 AF-S (1st version) or 70-200/4 AF-S with or without the converter. Interestingly, If I slightly dismount the 300/4 AF-S position from it's normal clicked-in position and then turn it back to properly click-in, the lens will autofocus but this is short lived because when I then turn the camera off and then back on, it again fails to autofocus. I would think if it's a lens CPU/contact issue, it wouldn't work on other Nikon AF digital cameras.</p>

<p>How about that top LCD - flicker or no? If it does flicker, then it's a very strong indication for a grounding issue - if it stays normal, then your problem is likely something else. Here's a video of the top LCD behavior: <a href="https://www.youtube.com/watch?v=YsLgz_HxoP4&feature=youtu.be">https://www.youtube.com/watch?v=YsLgz_HxoP4&feature=youtu.be</a></p>

<p>It does flicker with the TC-14E II and 300/4 combination but does not flicker with just the 300/4 mounted on the D750.</p>

<p>Here's a thread on flickr h<a href="https://www.flickr.com/groups/2682619@N24/discuss/72157647884559639/">ttps://www.flickr.com/groups/2682619@N24/discuss/72157647884559639</a>/ that deals with the TC issue - there likely will be an update soon as at least one person will have the fix done. Apparently, a Nikon technical memo has very recently been updated - I just have not been able to locate it. Following that thread and maybe contacting the OP thereof could help you solve the issue?! IIRC, then only one case in that thread is identical to your - i.e. the D750 not focusing with the lens alone - in all other cases, it seems to take the additional electrical resistance of the TC to make the problem surface.</p>

<p>Thanks Dieter, I saw that. I'll see if I can get an answer from Nikon too. Hate to give up my D750 if it's really just a lens issue.</p>

<p>In my experience, the AF-S 300 f/4 has been the most finicky lens I have ever known when it comes to auto-focus. I'm not sure why.</p>

<p>--Lannie</p>

<p>After some delay, I finally received my D750 test sample. I tried my 300mm/f4 AF-S on it; my lens is an early sample, and it happens to be gray market. The plain lens works just fine on the D750. However, once I put the TC-14E on it, the combination wouldn't AF on the D750 any more. I took the lens combo off the D750 and put them on the D810, and AF works just fine.</p>

<p>That D750 AF just fine with the 80-400mm AF-S VR, even with a TC-14E on.</p>

<p>Hey Shun, I tried my D750 with a clean used very late 300/4 AFS (ser. # US 327xxx) and it worked <strong>flawlessly</strong> alone or with either TC-E unit. My lens and converter combinations worked fine with the D610, D800 and D810. I just received the lens back from Nikon service (Melville) but <strong>no improvement</strong> in AF despite repair work on the bayonet, encoder, main FPC and lens FPC. It worked fine by itself on the dealers D750, but not with my extenders attached to their D750. In a few days when I get home, I'll reset all my camera settings to default to see if that helps as suggested by Nikon tech support. The only good news is the warranty is for 6 months so if nothing else works, I'll have to end both camera and lens to Nikon for another look.</p>

<p>I have exactly the same issue as Alan. My 300mm does not af on D750. It works fine on D7000. My other lenses work on both camera. When turned on, the 300mm may try to af and then not action</p>

<p>I have exactly the same problem with the AF-S 300/4 (s/n: 201xxx) on the D750. I have to hold on to the lens locking pin and turn the lens beyond the pin lock, release the pin and turn the lens back slightly for the pin to lock before the AF works which is short-lived. After a while it will fail again. However using a Kenko 2x TC, the lens works flawlessly on the D750. Previously, I've already noticed a very squeaky and slow AF operation when used on a D90. On the D750, when it works, it's fast and silent.</p>

<p>Saxon, your problem seems to be either one of your D750 or 300mm/f4 AF-S or both are a bit off spec. I would clean the electronic contacts around the lens mount, on both sides, to make sure that there is good contact. If that doesn't work, perhaps Nikon repair can make some adjustment.</p>

<p>Well, after sending it back a second time to Nikon, they replaced the silent wave motor and still <strong>no better</strong> on my D750 for AF operation! <strong>But</strong> on their third attempt, they finally got around to replacing the DC/DC converter and it did the trick! Pefect even with a TC-14E II. Good thing it was under repair warranty and they paid for shipping. Looks like the DC/DC converter is the key. Too bad it took so long to fix it. Maybe they should read photo.net!</p>

<p>I just installed firmware 1.07 on 2/7/2016 on my D750 and my 300mm F4 lens focuses properly and I'm pumped.</p>

<p>Doug, thanks for the info. Was your 300mm/f4 AF-S working with the D750 prior to the firmware upgrade?</p>

<p>I have that combo, and I only have AF issues when I add the TC-14E in between the D750 body and 300mm/f4 AF-S lens. Without the TC, the combo works fine. I understand that it is an AF motor issue.</p>