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04-29-2019, 02:16 AM - 1 Like   #1
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Firmware magic going on?

This is a post about something wierd I noticed about auto-focus with the Pentax K1 and D-FA24-70 (haven't tried with other lenses).
Using live view, zooming in to 4x (or more), it is possible to evaluate how good was the auto-focus (wide open), and how good is the focus when closing the diaphragm (using optical DoF simulation).

Here is an interesting observation:
-> Auto-focus is done with the lens aperture wide open (f2.8 for the D-FA 24-70)
-> Zoom in x 4, lens aperture f2.8 (wide open), manual tweak of the lens focus ring to check if it is possible to increase sharpness => yes, a tweak of the lens focus ring allow to get slightly better focus then AF.
-> Activate DoF simulation (now the diaphragm is set to f8), manual tweak of the lens focus ring to check if it is possible to increase sharpness => yes, a tweak of the lens focus ring allow to get slightly better focus then AF.
-> Capture images:
--> 1) Perform AF on target => capture image (Image #1)
--> 2) Perform AF on target => manual focus adjust for best sharpness (aperture = f2.8) => capture image (image #2)
--> 3) Perform AF on target => simulate DoF (aperture = f8), manual focus adjust => capture image (image #3)

Compare sharpness (focus) between images #1, #2, #3:
- Image #2 isn't focused as well as image #1 or #3
- Image #1 and #3 show similar sharpness.

I conclude that:
- the lens focus isn't constant with changing aperture = focus shift
- if the f8 focus is best when f2.8 is performed automatically by camera AF, it means that the firmware accounts for focus shift when focusing the lens for an f8 shot.

Apparently, camera firmware perform some magic (camera correct focus shift based on lens model) for best focus when the lens is stopped down. Not bad.

04-29-2019, 08:50 AM   #2
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QuoteOriginally posted by biz-engineer Quote
(using optical DoF simulation).
?????

Simulation? What is seen on the rear LCD is not simulated, it is real, just a little on the small side and down-sampled to the rear display pixel pitch.

That cleared up, I am curious as to how you are doing the test, specifically:
  • Focus peaking?
  • Flat target?
  • Repetitions?
  • Why the apparent assumption of focus shift?
Thanks in advance! Also, perhaps a few examples with annotations indicating intended point of focus.


Steve

(...does a fair amount of manual focus in live view...hence my interest...)
04-29-2019, 09:28 AM   #3
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QuoteOriginally posted by stevebrot Quote
(using optical DoF simulation).
I mean, pulling the shutter button fully to right end side [ Off => On => DoF preview].


QuoteOriginally posted by stevebrot Quote
Focus peaking?
I wasn't using focus peaking, instead zooming in LV mode

QuoteOriginally posted by stevebrot Quote
Flat target?
Yes, a newspaper on the wall (black text on white paper)

QuoteOriginally posted by stevebrot Quote
Repetitions?
I haven't done repetitions as design of experiment but I tried 2 or 3 times and I saw the same effect.

QuoteOriginally posted by stevebrot Quote
Why the apparent assumption of focus shift?
Because, when I achieved the best focus wide open (f2.8) manually adjusted, the DoF preview (f8) wasn't as sharp as if I didn't achieve the best focus at f2.8.
Now, if I pulled the DoF preview (f8) and refine the focus with LV at the same time, I did get the sharpest image ( exposed with the lens aperture set at f8).
04-29-2019, 10:14 AM - 1 Like   #4
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QuoteOriginally posted by biz-engineer Quote
Because, when I achieved the best focus wide open (f2.8) manually adjusted, the DoF preview (f8) wasn't as sharp as if I didn't achieve the best focus at f2.8.
Now, if I pulled the DoF preview (f8) and refine the focus with LV at the same time, I did get the sharpest image ( exposed with the lens aperture set at f8).
Thanks! In theory, precision should be lower stopped down,* but I have often wondered if magnification might cancel that out, allowing consistent focus in an aperture range having better contrast than most lenses have wide open. As for focus shift...that should be demonstrable by doing manual focus in magnified live view wide open and activating the DOF preview.

To test whether there is "firmware magic" it should be easy enough to see if your test shows similar results with non-supported AF glass (no inside knowledge passed from the lens).


Steve

* Hence my question about repetitions. Repeatable fine focus is pretty difficult even with lenses designed for manual focus, but doubly hard with the short focus throw and funky focus rings found on many AF lenses.

04-29-2019, 12:10 PM   #5
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QuoteOriginally posted by stevebrot Quote
To test whether there is "firmware magic" it should be easy enough to see if your test shows similar results with non-supported AF glass (no inside knowledge passed from the lens).
Unfortunately, I do not have any third party AF lens (sold all of them when I bought the K1). I do have a Samyang 85 1.4, and intially wasn't impressed with the sharpness even at f8, to later find out that the focus shift is rather significant with that lens.
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