the last word

Photography meets digital computer technology. Photography wins -- most of the time.

  • site home
  • blog home
  • galleries
  • contact
  • underwater
  • the bleeding edge
You are here: Home / D850 / Nikon D850 AF fine-tuning with the 58/1.4

Nikon D850 AF fine-tuning with the 58/1.4

November 12, 2017 JimK 3 Comments

This is the 16th post in a series of Nikon D850 tests. The series starts here.

[Horshack asked some questions about this post that caused me to go back and look at the EXIF data for the captures. When I did that, I realized that I’d gotten the sign of the PDAF bias settings wrong. I’ve fixed all that. Thanks,. Horshack, for keeping me honest. The corrections do not change the general sense of the post, however.]

All of the Nikon DSLRs that I’ve used use phase-detection autofocus (PDAF), and they provide a way to tune the behavior of the PDAF for particular lenses. This AF fine-tuning allows systematic biases to be compensated out. Unfortunately, it’s a one-size-fits-all solution, with data stored for each lens, and the same bias used for all distances and apertures for that lens. We’ve seen in this post that there are variations in PDAF bias with aperture (in spite of, or maybe because of, the way that it attempts to correct for focus shift caused by spherical aberrations).

I performed a series of tests with the Nikon 58 mm f/1.4 lens wide open at 3 meters with my three-plane target:

I told the camera to focus on the center crosshatch, using AF-S, single servo, and the small spot centered on the crosshatch:

 

I tested with the AF fine tuning bias set to -5, -2, 0, +2, and +5. 

I used this protocol:

  • Nikon 58 mm f/1.4 on D850.
  • ISO 64
  • AF-S, single servo mode
  • Release priority: focus
  • Aperture exposure mode
  • f/1.4 through f/4 in whole stops
  • 32 exposures at each f-stop
  • Nikon remote release, half-pressed until confirmation observed
  • Lens ring racked between shots
  • Wescott LED panels set to 5500 K.
  • Target distance, 3 meters.

 I measured the MTF50 for the three raw color planes using dcraw in document mode, MTF Mapper, Matlab, and Excel.

 

The vertical axis is MTF50 (a proxy for sharpness) measured in cycles per picture height. The bias that was set in the camera is plotted as the horizontal axis. The average MTF50 of the 32 exposures at each bias setting is the blue line. The best and worst of each series is also shown. It looks like a bias of -2 produces the sharpest results for the red raw channel.

 

With the green channel, which is more important than the red to image sharpness, a bias setting of 5 produces the best results.

 

The blue channel is the least important to sharpness, and also seems to want a positive bias setting.

If we look at the results for each of the three planes of the target for the +5 setting, this is what we see:

 

For the red channel, the far part of the target is very slightly better than the middle plane. Sigma is another word for standard deviation.

 

With the bias set to -5, the green channel peaks in the middle plane, as it should.

 

The blue channel looks a lot like the red one, which is a surprise.

But do we really want to set the bias to +5 if we’re going to stop the lens down a bit?

Here are plots from yesterday’s post with the bias set to zero, for the green channel at f/1.4 (as above), f/2, and f/2.8:

 

This confirms what we’re seeing above. The camera wants a positive bias.

But now zero looks about right.

And zero also looks about right for f/2.8.

 

 

D850

← D850, 58/1.4 PDAF bias at various apertures Do PDAF biases increase variability? →

Comments

  1. Jack Hogan says

    November 13, 2017 at 12:07 am

    Focus shift in action. How did the rest of the f-numbers look? Did you say that Nikon camera/lenses are supposed to compensate for focus shift automatically? By focal length and f-number?

    Reply
    • JimK says

      November 13, 2017 at 7:40 am

      Here’s what Marianne Oelund had to say about the 105/1.4 in a DPR post:

      “If you are planning on using the lens primarily at f/1.4 to f/2 (as I am) then the focus shift shouldn’t be an issue. However, if you need to move back and forth between the widest apertures, and moderate ones, AF will be problematic, especially with camera bodies released prior to 2014 (starting that year, Nikon incorporated focus-shift compensation into the AF system – without telling anyone, of course).”

      That’s all I know about the camera’s compensation for focus shift.

      Here’s the DPR thread:

      https://www.dpreview.com/forums/post/58262673

      Reply
    • JimK says

      November 13, 2017 at 7:45 am

      “How did the rest of the apertures look.”

      Didn’t test with changing the bias in camera. Here are results at f/1.4 through f/4 with the bias set to 0:

      http://blog.kasson.com/the-last-word/d850-581-4-pdaf-bias-at-various-apertures/

      Reply

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

March 2023
S M T W T F S
 1234
567891011
12131415161718
19202122232425
262728293031  
« Jan    

Articles

  • About
    • Patents and papers about color
    • Who am I?
  • Good 35-70 MF lens
  • How to…
    • Backing up photographic images
    • How to change email providers
  • Lens screening testing
    • Equipment and Software
    • Examples
      • Bad and OK 200-600 at 600
      • Excellent 180-400 zoom
      • Fair 14-30mm zoom
      • Good 100-200 mm MF zoom
      • Good 100-400 zoom
      • Good 100mm lens on P1 P45+
      • Good 120mm MF lens
      • Good 18mm FF lens
      • Good 24-105 mm FF lens
      • Good 24-70 FF zoom
      • Good 35 mm FF lens
      • Good 60 mm lens on IQ3-100
      • Good 63 mm MF lens
      • Good 65 mm FF lens
      • Good 85 mm FF lens
      • Good and bad 25mm FF lenses
      • Good zoom at 24 mm
      • Marginal 18mm lens
      • Marginal 35mm FF lens
      • Mildly problematic 55 mm FF lens
      • OK 16-35mm zoom
      • OK 60mm lens on P1 P45+
      • OK Sony 600mm f/4
      • Pretty good 16-35 FF zoom
      • Pretty good 90mm FF lens
      • Problematic 400 mm FF lens
      • Tilted 20 mm f/1.8 FF lens
      • Tilted 30 mm MF lens
      • Tilted 50 mm FF lens
      • Two 15mm FF lenses
    • Found a problem – now what?
    • Goals for this test
    • Minimum target distances
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Printable Siemens Star targets
    • Target size on sensor
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Test instructions — postproduction
    • Test instructions — reading the images
    • Test instructions – capture
    • Theory of the test
    • What’s wrong with conventional lens screening?
  • Previsualization heresy
  • Privacy Policy
  • Recommended photographic web sites
  • Using in-camera histograms for ETTR
    • Acknowledgments
    • Why ETTR?
    • Normal in-camera histograms
    • Image processing for in-camera histograms
    • Making the in-camera histogram closely represent the raw histogram
    • Shortcuts to UniWB
    • Preparing for monitor-based UniWB
    • A one-step UniWB procedure
    • The math behind the one-step method
    • Iteration using Newton’s Method

Category List

Recent Comments

  • Mal Paso on Christmas tree light bokeh with the XCD 38V on the X2D
  • Sebastian on More on tilted adapters
  • JimK on On microlens size in the GFX 100 and GFX 50R/S
  • Kyle Krug on On microlens size in the GFX 100 and GFX 50R/S
  • JimK on Hasselblad X2D electronic shutter scan time
  • Jake on Hasselblad X2D electronic shutter scan time
  • Piotr Chylarecki on Who am I?
  • JimK on Who am I?
  • Piotr Chylarecki on Who am I?
  • Stefan on Swebo TC-1 OOBE

Archives

Copyright © 2023 · Daily Dish Pro On Genesis Framework · WordPress · Log in

Unless otherwise noted, all images copyright Jim Kasson.