Unexpected AF Issue - Nikon

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Steve
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#1
Hello folks,

I have a Nikon D750 with Nikon AF-S 70-300 VR lens and recently I purchased a Kenko 1.4 TC to stretch the reach a little. This 70-300+TC is a combination I have owned in the past, albeit on a D300 body, so thought I knew what to expect. Anyway, I have been putting off adding the TC until I was satisfied with the focus of the D750 and 70-300 alone, which I think I have now aced.

Yesterday I thought it's time to give the TC a go so set up the camera to do an AF fine tune and was somewhat surprised to find the camera struggle to get a focus on the (static) focus target. Despite trying maybe 50 times to focus through the viewfinder it never really got it right. So I thought I would check it out in live view and hey presto it hits focus straight away every time - yay!

I appreciate that the AF principle and light path is different between LV and viewfinder but none of my other lenses has this problem with the TC mounted.

There's obviously something I am missing here so was wondering if anyone can put me right and help me out or I'm gonna have to annoy the wife and spring for a 150-600 - which is what I wanted in the first place :)
 
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Name
Geoff
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#2
Go for a 150-600mm if you can. Since buying mine about 4 years ago I've used it for so many things, and got some fantastic shots that I could never have got before.

Anyway, to your issue. It could be that the TC is interfering with the AF in ways I could only guess at, but I wonder if it might be heat haze? I have found many times with my 150-600mm lens that if I'm focusing on something a bit further away and zooming up high, the heat haze effect is surprising, and it not only makes images look blurred but it plays havoc with the phase-detect AF. It's not something I would have thought of if I hadn't experienced it myself, but it's worth considering as the cause if you're finding this problem with the TC and approaching the long end of your zoom range. One way to see the heat haze and judge whether that's actually the problem would be to zoom the screen up to 200% (I think you can take it to that on your camera) in live view, and look for the shimmering. If you see it, that's the problem.

That doesn't answer why you can't manually get the focus right with your eye, but personally I've never found I'm able to get accurate focus using my eye in the viewfinder.
 
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#4
I might have guessed the D750 would do better than the D300 with the TC mounted, since that reduces the maximum aperture at the long end to f/8. The D750 spec says this is the limit for AF, and the D300 supposedly needs at least f/5.6. Nikon would probably say that with a third party TC, all bets are off, but it sounds like fine tune would be worth trying as Mark suggests.
 
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Geoff
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#5
I think the OP is trying to setup the lens with TC in order to fine-tune focus of both together, but the camera won't lock focus correctly. Hence my suggestion of heat haze being a possible reason.

That's how I read it anyway, but I may have misunderstood.
 

Nod

Krispy and Kremey
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35,036
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Nod (NOT Ethel!!!)
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#6
I have the same body and lens but my Kenko TC is a 1.5x. While AF is slower with the TC, in decent light, it does achieve AF through the VF, even at 300mm. Having said that, I don't use the TC now - if I want/need any extra reach, I'll have taken the Fuji with the 100-400 and 1.4x telecon which works very well or, if I only have the Nikon kit, I'll crop into an unconverted image and print smaller!
 
OP
OP
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Steve
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#7
Did you fine tune the 70-300 to your body?
Adding a tc will require fine tuning that combo again.
Hi Brazo, thanks for answering. Yes, I have fine tuned the 70-300 to the D750 and I was attempting to fine tune the 70-300/TC/D750 combo that led to this problem coming to light. It was a beautiful, clear day with visibility >50km(1) and I had set up the focus target and camera tripod outside in the garden to do the fine tuning.
(1): I'm the UK sales manager for a company that makes meteorological sensors and have a portable visibility unit - www.biral.com
 
OP
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Steve
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#8
I think the OP is trying to setup the lens with TC in order to fine-tune focus of both together, but the camera won't lock focus correctly. Hence my suggestion of heat haze being a possible reason.

That's how I read it anyway, but I may have misunderstood.
Quite correct, Geoff :)
 
OP
OP
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Steve
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#9
Go for a 150-600mm if you can. Since buying mine about 4 years ago I've used it for so many things, and got some fantastic shots that I could never have got before.

Anyway, to your issue. It could be that the TC is interfering with the AF in ways I could only guess at, but I wonder if it might be heat haze? I have found many times with my 150-600mm lens that if I'm focusing on something a bit further away and zooming up high, the heat haze effect is surprising, and it not only makes images look blurred but it plays havoc with the phase-detect AF. It's not something I would have thought of if I hadn't experienced it myself, but it's worth considering as the cause if you're finding this problem with the TC and approaching the long end of your zoom range. One way to see the heat haze and judge whether that's actually the problem would be to zoom the screen up to 200% (I think you can take it to that on your camera) in live view, and look for the shimmering. If you see it, that's the problem.

That doesn't answer why you can't manually get the focus right with your eye, but personally I've never found I'm able to get accurate focus using my eye in the viewfinder.
Hi Geoff, thanks for your answer. Whilst you raise an interesting idea, I don't think there was any heat haze involved as my portable visibility sensor was reading >50km visibility. Heat haze would have dropped that to more like 10-15km.
 
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Steve
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#10
Go for a 150-600mm if you can. Since buying mine about 4 years ago I've used it for so many things, and got some fantastic shots that I could never have got before.
If I can't get this to work satisfactorily then a 150-600 will have to be the answer. I used to have a Sigma before but sold it for some reason I don't even remember ... it's convincing the other half that I need one that's gonna be the problem :)
 
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23,089
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Richard
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#11
Hello folks,

I have a Nikon D750 with Nikon AF-S 70-300 VR lens and recently I purchased a Kenko 1.4 TC to stretch the reach a little. This 70-300+TC is a combination I have owned in the past, albeit on a D300 body, so thought I knew what to expect. Anyway, I have been putting off adding the TC until I was satisfied with the focus of the D750 and 70-300 alone, which I think I have now aced.

Yesterday I thought it's time to give the TC a go so set up the camera to do an AF fine tune and was somewhat surprised to find the camera struggle to get a focus on the (static) focus target. Despite trying maybe 50 times to focus through the viewfinder it never really got it right. So I thought I would check it out in live view and hey presto it hits focus straight away every time - yay!

I appreciate that the AF principle and light path is different between LV and viewfinder but none of my other lenses has this problem with the TC mounted.

There's obviously something I am missing here so was wondering if anyone can put me right and help me out or I'm gonna have to annoy the wife and spring for a 150-600 - which is what I wanted in the first place :)
Most DSLRs won't AF when the f/number rises above f/5.6. It's a limitation of their phase-detect AF systems. Zooms that are f/6.3 at the long end can get away with it by telling porkies about their actual f/number to fool the AF system. Some cameras claim they will AF up to f/8 but that's pushing it and AF will not be as good, and may not work with all lenses. Isn't this a known problem with the Nikon 70-300? I'm sure I've heard something about it before. Have a google.
 
OP
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Bristolian
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3,127
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Steve
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#12
Most DSLRs won't AF when the f/number rises above f/5.6. It's a limitation of their phase-detect AF systems. Zooms that are f/6.3 at the long end can get away with it by telling porkies about their actual f/number to fool the AF system. Some cameras claim they will AF up to f/8 but that's pushing it and AF will not be as good, and may not work with all lenses. Isn't this a known problem with the Nikon 70-300? I'm sure I've heard something about it before. Have a google.
Hello Richard, thanks for commenting. Some additional experimentation is due this weekend.
 
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Steve
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#13
Most DSLRs won't AF when the f/number rises above f/5.6. It's a limitation of their phase-detect AF systems. Zooms that are f/6.3 at the long end can get away with it by telling porkies about their actual f/number to fool the AF system. Some cameras claim they will AF up to f/8 but that's pushing it and AF will not be as good, and may not work with all lenses. Isn't this a known problem with the Nikon 70-300? I'm sure I've heard something about it before. Have a google.
I use AF all the time and shoot stopped down at F11 etc all the time. It never doesn't work.
 
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Geoff
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#14
I use AF all the time and shoot stopped down at F11 etc all the time. It never doesn't work.
That's because the AF works before pressing the shutter, when the aperture is fully open. The aperture only adjusts to whatever you've set it to once you press the shutter. With a long lens and TC fitted, the aperture could easily be down to f6.3 or smaller "wide" open, hence possibly giving unreliable AF on some cameras. I've never had a problem at f6.3 with my Nikon D500 though.
 
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Steve
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#15
That's because the AF works before pressing the shutter, when the aperture is fully open. The aperture only adjusts to whatever you've set it to once you press the shutter. With a long lens and TC fitted, the aperture could easily be down to f6.3 or smaller "wide" open, hence possibly giving unreliable AF on some cameras. I've never had a problem at f6.3 with my Nikon D500 though.
Of course - silly me and of course a TC acts to stop down.
 
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