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focus accuracy vs precision SLR vs mirrorless
Just think how contrast-detection AF works - it tries to detect the maximum edge contrast pixel level.
It literally reads out the main sensor.

RSAs are focus shifts when stopping down. They do ALWAYS occur - the level can range from negligible to very substantial.

If the camera stops down prior to engaging the CD-AF, RSAs are simply irrelevant. There is no bracketing necessary (I suppose you meant bracketing and not stacking? Stacking is really more meant to increase the DoF in an image).

In theory, a DSLR could use bracketing around the PD-AF point - and do a contrast-detect post-analysis to select the best-focused image.
However, bracketing requires time - time you may not have. Bracketing would also introduce a wobbly viewfinder image.

Funnily, the latter has been criticized in Panasonic mirrorless cameras which use CD-AF only rather than a combo of PD & CD-AF (like Sony/Canon/Olympus/Fuji/Nikon) where this wobbling is not detectable.

That all being said - it's all a question of how holy you want to have it. An occasional misfocus is usually not the end of the world.
I do, however, appreciate "animal AF" for instance.

Klaus, I really appreciating you explaining the concept. I am aware I am not likely to totally get it. But the mirroless finds the best focus, but having found it reverts to the original aperture for the shot, I guess. I guess if the contrast is high and the aberrations are low you have the best focus plus the best aperture. It almost seems that the best case is not the sharpest focus. I don't know.

I did look up linear focus, and got a lot of words that were not helpful to me, but then I saw a video of several Sony lenses where one could see the elements focusing at amazing speed with multiple dual motor linear motors. Man, do these huge heavy lens groups move fast! I still don't quite know why they are called linear but silent fast, and accurate focus- plus minimal hunting is one of the top features that makes a lens fun to use for me. I still have to Canon lens that use what I think they call Micro Motors. The original 50/1.8 mark I, and my old Tamron SP 180/3.5 macro. I have never actually seen the way those motors work because there was just no need. My first USM lens pretty much told me there was no going back!

One of the things that some expert said was that linear motors are really micro stepping motors. I don't know if that is true to the point of not being continuous in practice. The Sony groups slide on smooth guides and are controlled by magnetic pulses. I guess there may be a limit to how small a distance they can move, but I was convinced that it had no problem getting close enough to focus for all practical purposes.

Sorry for the long posts. I am really trying to find what camera will be best for me moving forward and I have largely ignored these systems because the DSLR shooting experience is so simple and enjoyable to me. But as an enthusiast, my biggest desire would be to have prosumer level mirrorless camera. And so I have been trying to catch up on the last 5 or 10 years.


(08-15-2021, 11:00 AM)toni-a Wrote: To make it clearer,  if you are using a standard 85f1.8 not even f1.4, at 1.5 meters depth of field is 2cm before or after subject, focus differs whether camera focuses on eyes or nose...
So if you have mirrorless without eye autofocus or an SLR without AFMA done perfectly well and autofocus point perfectly chosen you have very high chances of having out of focus portraits.

(08-17-2021, 09:57 PM)toni-a Wrote:
(08-17-2021, 02:20 PM)Klaus Wrote: That all being said - it's all a question of how holy you want to have it. An occasional misfocus is usually not the end of the world.

All depends on your  priorities, and what you are shooting it's true in 99.99% of cases though

the problem is sometimes you take a lot of shots but you have to choose one or two, it's frustrating when you enlarge thumbnail to discover it's the misfocused one .
Experience taught me that in  portraits photoshoots I take many photos there will be always a close replacement for A misfocused shot.
When shooting any social  event I absolutely avoid wide apertures for important photos using DSLR, increased DOF will almost  always compensate for any tiny focus error, on APS-C USING 17-55 AT f4 I don't have any issues, better have some noise than a misfocused shot.

I should have addressed that quote too.  I've said it before, and I'll say it again:  All modern interchangeable cameras are modern miracles to me.  I've been using my low budget Olympus E-M10 III a lot lately, just for fun.  In most ways the 90D works better, but the little camera is fun!  And plus I can mount any lens I have on it.  Nah, I just am trying to understand the strengths of the systems.  Even my EOS Rebel XT can take very good pictures.  But life is much easier with the 90D.  I always try to move up in cameras.  Since the 90D is my top camera, and I shoot nature, the next camera will have to outperform it in my eyes.  That is the only time I get critical.  When I am trying to find something better than what I had - usually I replace my top camera, not supplement it.  None of my extra cameras were selected and bought by me.  They are mostly castoffs.  The Olympus was a gift, that was given right back!  Oh, well! 


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RE: focus accuracy vs precision SLR vs mirrorless - by Arthur Macmillan - 08-17-2021, 10:17 PM

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