AF Fine Tuning wide aperture lenses

Started Apr 7, 2018 | Discussions
Maeil New Member • Posts: 9
AF Fine Tuning wide aperture lenses
1

I decided to AF fine tune my lenses again, since I have been having some small focus issues lately.

Some time ago I got a long tethering cable and I decided to try AF tuning while tethered to make it faster / more precise.

As it was so fast to rack shot after shot, I started to take photos also with smaller apertures to check how severe the focus shift is between apertures with wide aperture lenses.

I have been aware of the focus shift phenomena, but never really checked how much it affects focusing. When AF tuning Sigma 85mm F1.4 Art at around 1.35m / 4.5ft distance, the diffrence is very noticeable, so much that I ended up offsetting my AF fine tune from being perfect at F1.4 to being slightly off.

The Sigma dock allows focus tuning at 4 distances for this lenses, this is the reason that AF tuning distance is so small in my example.

For long my shots have been just "slightly" out of focus when using this lens, this "fix" looks to correct this problem mostly, when using apertures smaller than F1.4. At short distances I rarely use F1.4 due to the minuscule DOF.

My lens went back to 0 with all AF tune values, the F1.4 optimized values were +5 / +6 at 1.35m / 2.6m, this makes me wonder; maybe Sigma calibrates this particular lens this way as an compromise to combat focus shift?

The focus on this 85mm Art had very good repeating accuracy, almost no difference between photos when I took many photos per aperture changing focus after each shot.

Here are before / after photos of the AF tuning (pictures cropped without downscaling, so 100% size from D850 RAW)

F1.4 with F1.4 AF-tune -> exactly on the center

F1.8 with F1.4 AF-tune -> Sharpest focus creeping away from intended focus point. Very noticeable back focus.

F2.8 with F1.4 AF-tune -> Sharpest focus moving more away, focus area moderately sharp due to increased DOF

AF Tune settings changed with Sigma Dock -->

F1.4 with Offset AF-tune -> Slightly front focused

F1.8 with Offset AF-tune -> Sharpest focus almost perfectly at middle, very very slight front focus

F2.8 with Offset AF-tune -> Sharpest focus just slightly over middle, no problem.

 Maeil's gear list:Maeil's gear list
Nikon D850 Samyang 14mm F2.8 ED AS IF UMC Tamron SP 24-70mm F2.8 Di VC USD Tamron SP 150-600mm F5-6.3 Di VC USD Sigma 85mm F1.4 Art +3 more
Nikon D850 Sigma 85mm F1.4 Art
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Pepege Regular Member • Posts: 387
Re: AF Fine Tuning wide aperture lenses

Maeil wrote:

As it was so fast to rack shot after shot, I started to take photos also with smaller apertures to check how severe the focus shift is between apertures with wide aperture lenses.

I have been aware of the focus shift phenomena, but never really checked how much it affects focusing. When AF tuning Sigma 85mm F1.4 Art at around 1.35m / 4.5ft distance, the diffrence is very noticeable, so much that I ended up offsetting my AF fine tune from being perfect at F1.4 to being slightly off.

Thanks for your detailed report. It is quite interesting.

Does anyboy have an explaination why this focus shift phenomena exists at all? The camera acquires focus wide open, and closing the aperture should not change the position of the optical groups. How is this possible at all???

 Pepege's gear list:Pepege's gear list
Canon EOS 80D Canon EF 70-200mm F2.8L IS II USM Canon EF 35mm F2 IS USM Sigma 50-100mm F1.8 DC HSM Art Canon EOS M5 +11 more
OP Maeil New Member • Posts: 9
Re: AF Fine Tuning wide aperture lenses

Nasim Mansurov has a done good article about focus shift at his site here:

https://photographylife.com/what-is-focus-shift

 Maeil's gear list:Maeil's gear list
Nikon D850 Samyang 14mm F2.8 ED AS IF UMC Tamron SP 24-70mm F2.8 Di VC USD Tamron SP 150-600mm F5-6.3 Di VC USD Sigma 85mm F1.4 Art +3 more
Pepege Regular Member • Posts: 387
Re: AF Fine Tuning wide aperture lenses

Maeil wrote:

Nasim Mansurov has a done good article about focus shift at his site here:

https://photographylife.com/what-is-focus-shift

Thanks for this link, this explains it very well.

At first, I thought that the "0" symbol in both of your images (first image with 1.4 and second with 1.8) is equaly sharp. But this seems to be only an optical illusion.

And now I can also see it in your second image: Although the sharpest focus is creeping away from intended focus point, the "0" has still the same "sharpness" as in the first image.

 Pepege's gear list:Pepege's gear list
Canon EOS 80D Canon EF 70-200mm F2.8L IS II USM Canon EF 35mm F2 IS USM Sigma 50-100mm F1.8 DC HSM Art Canon EOS M5 +11 more
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