Auto Fine Tune Confusion

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Swiftler

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So I went through the process of fine tuning 2 of my lenses on my new D850 today and am a bit perplexed. I decided to do it because I was shooting some moths on some leaves yesterday in the backyard using my 24-120mm @120 and was noticing that the focus was clearly off. I figured that since I would have the system setup for calibration I might as well test both of my lenses. The 2 lenses I calibrated are the Nikon 200-500mm and of course the Nikon 24-120mm. The 200-500mm did not have any noticeable misses so I didn't expect much from the fine tuning. Below are the averages for the extremes of the zoom ranges after completing 10 Auto Tunes for each range.

Nikon 200-500mm @200 = -2.1
Nikon 200-500mm @500 = +0.3

Nikon 24-120mm @24 = +5.6
Nikon 24-120mm @120 = -15.8

I know Steve's video says that if working with a zoom lens to just average the 2 values and manually set it, or favor one end more if you tend to shoot at that focal length more often. Clearly the 200-500mm doesn't need much adjustment if any at all since I shoot mainly at 500mm. I am really confused on the 24-120mm. There is such a huge difference between the 2 focal lengths and I use that lens throughout the entire range. Any advice from those who have experienced similar results?
 
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Swiftler said:
So I went through the process of fine tuning 2 of my lenses on my new D850 today and am a bit perplexed. I decided to do it because I was shooting some moths on some leaves yesterday in the backyard using my 24-120mm @120 and was noticing that the focus was clearly off. I figured that since I would have the system setup for calibration I might as well test both of my lenses. The 2 lenses I calibrated are the Nikon 200-500mm and of course the Nikon 24-120mm. The 200-500mm did not have any noticeable misses so I didn't expect much from the fine tuning. Below are the averages for the extremes of the zoom ranges after completing 10 Auto Tunes for each range.

Nikon 200-500mm @200 = -2.1
Nikon 200-500mm @500 = +0.3

Nikon 24-120mm @24 = +5.6
Nikon 24-120mm @120 = -15.8

I know Steve's video says that if working with a zoom lens to just average the 2 values and manually set it, or favor one end more if you tend to shoot at that focal length more often. Clearly the 200-500mm doesn't need much adjustment if any at all since I shoot mainly at 500mm. I am really confused on the 24-120mm. There is such a huge difference between the 2 focal lengths and I use that lens throughout the entire range. Any advice from those who have experienced similar results?
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Those results are pretty typical for wide range zooms and there's not much you can do about it. Some third party lenses like Tamron's allow you to adjust focus at multiple zoom points and even multiple focusing distances for each zoom setting which helps a lot but it's tough to completely correct AF accuracy issues across a wide range of zoom focal lengths and focusing distances.

I'd probably reset the AF Fine Tuning to zero for the 200-500mm if you mostly shoot at the long end of the range.

The 24-120mm is a tougher call if you use it across a wide range of focal lengths and don't prefer one end of the zoom range to the other. Personally I'd probably see it to be most accurate at the wide end of its focal range but that's just how I tend to shoot with wide angle zooms.

One tip for the 24-120 that might help is to shoot more static subjects like macro, landscapes and the like in Live View mode using the rear display which uses on-sensor contrast detect auto focus and does not rely on the separate PDAF sensor used in through the viewfinder focusing which means it doesn't really need focus fine tuning. Yeah, that doesn't really work for active subjects where you need fast auto focus and doesn't work for through the view finder shooting but it's one way to get more out a lens that has a wide range of focus tuning variation.

For situations where Live View shooting isn't a viable option I'd probably get in the habit of stopping down the 24-120mm to buy a bit of depth of field margin so slight front or back focusing errors are less obvious. The good news about wide angle shooting is there's more DoF compared to shooting long lenses and that already gives a bit more margin for slight AF focusing distance errors but stopping down a stop or more increases that DoF margin which can help.
 
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Were you using the Dot Tune method? Were the results for the 24-120 fairly consistent for each test at the 24 and 120 settings?
 
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MartyD said:
Were you using the Dot Tune method? Were the results for the 24-120 fairly consistent for each test at the 24 and 120 settings?
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Here are all of the results I got. Focused using magnified live view.
 
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The AF fine tune value is an offset for the difference between apparent focus with the mirror in place and with it out of the way and the light being focused on the sensor. It is not going to be a linear value with a zoom lens and Nikon advises to set the zoom at the most often used camera to subject distance when making the adjustment. With a 80-400mm lens I would have it at 400mm and focus on a target subject at a distance of 100 feet from the camera.

For what it is worth my copy of the 24-120mm was much softer than the 24-70mm f/2.8 lens with both at a 70mm focal length. The difference was too great for me and I sold the 24-120mm lens. I bought the Sigma 24-105mm but that lens, while being very sharp, was impossible to focus manually.
 
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