Lens AF Fine Tune ?

Started 2 weeks ago | Questions thread
Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 12,955
Re: Lens AF Fine Tune ?

steveperry wrote:

Hi Len -

I'm actually the one who created that target and your comments caused me some concern - and to do some testing.

I created a Nikon-style split target and taped it up next to mine and ran the Auto AF Fine Tune operation on numerous times to see what happened. The results are interesting:

I tested five combos - my 600 F4, 600 F4 + 1.4TC, 300 PF, 300 PF+TC, and 200-500 on my D850.

Consistency was roughly the same from target to target. The 600 F4 (with and without TC) only was swapping between two values , the 200-500 had a three point gap with each target.

The 300 PF is the least consistent of my lenses and was slightly point more consistent with the spilt target over my target both with and without the TC (although only by a point ot two - it averaged out the same in the end).

However, the 200-500 was interesting. There was a three point variance with both targets, but a two point difference in the final tuning value. So, I checked it with Lens Align and the value with the split target was showing slight back focus with the value from my target being on the money.

So, I don't know what to think. My initial thought was to simply dump my target and replace it with the Nikon Style one with a little note of explanation, but I'm not sure there's any advantage to doing that. In fact, the OP and I both had an instance where my target seemed to give more accurate results. However, it also seems like Nikon would know the right thing to recommend / show in the book!F

An ideal Phase Detect AF target can be a topic for debate part because it can be extensively contrast dependent. There are some subjects such as fields of flowers where AF can work in cross sunlight and yet with the same equipment can fail in overcast lighting.

Digressing Nikon has a few "gotcha" AF issues such auto area AF (or 3D-tracking) is switched is automatically switched to single point AF with a TC and a combined aperture smaller than f5.6. This is at the top of page 104 in the D850 manual.

Back to AF targets I photograph wild flower meadows in summer and find this type of subject definitely can fool AF. Your target looks very similar to the Nikon field of flowers illustration of a difficult phase detect subject.

Nikon fail to mention in advice on using phase detect AF that it works best with either horizontal or vertical detail relative to the long or short dimension of the frame when using a cross type sensor but only works well with detail parallel to the long edge of the frame when using a single direction "line" sensor.

Nikon mentions in contrast detect (Liveview) AF guidance that detail parallel to the short direction of the frame is best. This is in the opposite direction to single line phase detect AF.

Good horizontal and vertical direction detail can help when comparing PD and CD AF. There can be an issue with phase detect AF with geometric patterns - Thom Hogan has a parallel line target with lines of varying thickness to avoid this.

Below is a deliberately low contrast small segment of an old target I have used extensively. It has limitations with D500/850 AF systems but can work good with 51 point lower MP bodies

Part of old test target

The basic principle was to photograph at 27x focal length with AF lined up on the thicker white lines parallel to the top of the image in the black box to obtain detail of varying thickness using PDAF. Then the resolution reading at nil fine tune could be compared to + 5 and -5 to see which was best. Higher MP cameras can resolve finer detail than on this target.

Using targets at lower contrast than the common 1000:1 ratio and more in line with common UK outdoor lighting combined with a resolution measure can provide useful additional information as resolution usually falls a lot when subject contrast is low.

You and I may never get round the issue of some not changing focus distance each time they change focal length, not using a light source with variable colour output, or pinning a sheet of paper in a post not parallel to the sensor This aside it seems from this thread there is a wider appreciation AF does not always focus accurate with all subjects.

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Leonard Shepherd
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