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Thread started 13 Mar 2015 (Friday) 11:46
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Autofocus in Live View mode - Bad idea?

 
WaltA
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Mar 13, 2015 18:36 |  #16

AJSJones wrote in post #17473851external link
This is based on, wait a minute, it's actual dataexternal link, not opinions or guesses:D:D:D (There's a follow-up hereexternal link.) Roger Cicala is one of the few people who is in a position to test these issues on multipleexternal link lenses and is extremely methodical/systematic about his testing.

Wait wait wait - an opinion based on facts?

That's just crazy talk. Opinions need to be based on speculation.

The only difference between an opinion and a fact is the number of people that believe it.


Walt
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BigAl007
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Mar 13, 2015 21:06 |  #17

Well I guess the OP is is full of you know what and just looking for an argument! I guess he just wanted to ask a question to then contradict the answer he knew he would get. I did actually suggest in my first post that MF in LV using 10× magnified view was the most reliable way of getting what YOU want in focus.

The question though implied that CDAF in LV was bad because you could not calibrate it. The fact is that CDAF is perfectly accurate without calibration, as you are using the sensor to do the measurement of the actual state of the focus. It also does this without a minimum aperture requirement, unlike PDAF. It is certainly sub optimum for speed though, as it requires many small drives of the lens AF motor. To be perfectly frank it doesn't really matter what type of electric motor/gearbox you use to drive the actual focus mechanism. Ring USM drive is probably somewhat over specified if it were to be used solely with CDAF, but is not a disadvantage when used in CDAF situations using LV for still images. Using a stepper motor to drive the STM lenses is actually a drive type that is optimised for use with CDAF and even video with onboard sound; rather than PDAF, where Ring USM is still the best option for speed, and the ability to offer real Full Time Manual override. When it comes to speed PDAF always has the advantage that it knows how far and in what direction you need to drive the lens to reach correct focus, at least as far as the AF sensor is concerned. Also it seems from the reports on Lens Rentals blog pages that using the latest Canon bodies and MK II L series lenses that the PDAF systems are now very often using a Look, Drive, Look methodology to improve consistency/accuracy in PDAF, and if necessary adding in an additional drive step. Previous bodies and older lenses seem to be limited to Look, Drive, Shoot.

I do not find it surprising that the quote the OP provided suggested that CDAF in LV was bad. The author has a vested interest that anyone using DSLR AF systems continues to rely on the use of PDAF, so that he can continue to sell his products for performing PDAF AFMA. If those who shoot non moving subjects were to start to use the much slower but more consistently accurate CDAF in LV far more, he would lose sales to them.

Alan


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texshooter
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Mar 16, 2017 16:59 as a reply to BigAl007's post |  #18

I want to resurrect this thread to ask a related question.

I bought a split prism focusing screen to help me manually focus better through the eyepiece. Using Live view all the time runs down the battery, but manually focusing with the eyepiece does not.

If my specific lens/camera combination needs an Autofocus manual adjustment in order to improve phase detect auto focusing, then how can I be sure the split prism focusing screen alternative will improve my efforts to manually focus through the eyepiece?
If the lens is miscalibrated from the factory, wouldn't that necessarily mean that manually focusing would be off as well?
How could the PDAF mechanics be off mark whilst manually focusing be spot on? I would think that a messed up PDAF would also mean a messed up manual focus screen. Yes?

Moreover, could the manual focusing screen be miscalibrated (ie, positioned incorrectly inside the eyepiece) even whilst the PDAF system operates perfectly? And if so, how does one calibrate or fix the manual focusing screen/mechanism so that manually focusing is as reliable as PDAF? I've never heard anybody ever complain about manual focusing problems (other than perhaps their prespyopia), but it only makes sense there could be reason to mistrust your camera when focusing manually due to lens-to-prism-to-sensor alignment imperfections.




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Bassat
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Mar 16, 2017 21:16 |  #19

texshooter wrote in post #18302730 (external link)
I want to resurrect this thread to ask a related question.

I bought a split prism focusing screen to help me manually focus better through the eyepiece. Using Live view all the time runs down the battery, but manually focusing with the eyepiece does not.

If my specific lens/camera combination needs an Autofocus manual adjustment in order to improve phase detect auto focusing, then how can I be sure the split prism focusing screen alternative will improve my efforts to manually focus through the eyepiece?
If the lens is miscalibrated from the factory, wouldn't that necessarily mean that manually focusing would be off as well?
How could the PDAF mechanics be off mark whilst manually focusing be spot on? I would think that a messed up PDAF would also mean a messed up manual focus screen. Yes?

Moreover, could the manual focusing screen be miscalibrated (ie, positioned incorrectly inside the eyepiece) even whilst the PDAF system operates perfectly? And if so, how does one calibrate or fix the manual focusing screen/mechanism so that manually focusing is as reliable as PDAF? I've never heard anybody ever complain about manual focusing problems (other than perhaps their prespyopia), but it only makes sense there could be reason to mistrust your camera when focusing manually due to lens-to-prism-to-sensor alignment imperfections.

The $15 solution here is to just buy another battery.


Tom

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RDKirk
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Mar 17, 2017 08:30 |  #20

texshooter wrote in post #18302730 (external link)
I want to resurrect this thread to ask a related question.

I bought a split prism focusing screen to help me manually focus better through the eyepiece. Using Live view all the time runs down the battery, but manually focusing with the eyepiece does not.

If my specific lens/camera combination needs an Autofocus manual adjustment in order to improve phase detect auto focusing, then how can I be sure the split prism focusing screen alternative will improve my efforts to manually focus through the eyepiece?
If the lens is miscalibrated from the factory, wouldn't that necessarily mean that manually focusing would be off as well?
How could the PDAF mechanics be off mark whilst manually focusing be spot on? I would think that a messed up PDAF would also mean a messed up manual focus screen. Yes?

Moreover, could the manual focusing screen be miscalibrated (ie, positioned incorrectly inside the eyepiece) even whilst the PDAF system operates perfectly? And if so, how does one calibrate or fix the manual focusing screen/mechanism so that manually focusing is as reliable as PDAF? I've never heard anybody ever complain about manual focusing problems (other than perhaps their prespyopia), but it only makes sense there could be reason to mistrust your camera when focusing manually due to lens-to-prism-to-sensor alignment imperfections.

Phase detection autofocus is a very complex relationship between sensor position precision, mirror position precision, detector position precision, lens motor precision and accuracy, and the calibration of all of them as a system. Lots of things to be done properly and inspected carefully, lots of things to go out of whack.

Manual focusing on the screen (which the split prism enhances at the center) depends on a fixed and quite stable relationship between the mirror box focusing screen frame and the mirror box sensor bed. Grind those two surfaces of the solid mirror box precisely, drop the components in correctly, and you're done. Precision machining is old, old tech that SLR manufacturers perfected more than half a century ago. That's the last thing that might be wrong.

A modern problem is that the accommodation of autofocus actually requires a manual focusing screen that is less accurate for manual focusing than the old-school screens that were designed for it. The problem is not the precision of the focusing screen position, it's the loss of accuracy in the way the focusing screen groundglass is ground: It's ground for screen brightness rather than for focusing accuracy. But your split prism makes up for that.




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texshooter
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Mar 17, 2017 14:23 |  #21

RDKirk wrote in post #18303282 (external link)
Manual focusing on the screen (which the split prism enhances at the center) depends on a fixed and quite stable relationship between the mirror box focusing screen frame and the mirror box sensor bed. Grind those two surfaces of the solid mirror box precisely, drop the components in correctly, and you're done. Precision machining is old, old tech that SLR manufacturers perfected more than half a century ago. That's the last thing that might be wrong.
.

If the manual focus mechanism is precisely machined as you suggest, the lens may still not be. (ergo the often need for autofocus microadjustments). If the AFMA is compensating for a bad lens, how does the "perfect" from factory manual focus assembly correct for the bad lens without some sort of do-it-yourself manual focus calibration test kit? Are you telling me that the manual focus apperatus need never be adjusted regardless of how poorly calibrated the lens may be?
I suppose the manual focus system cannot be adjusted at home anymore than the lens itself can be adjusted at home-- the factory must do that. But I could (and seemingly should) conduct my own manual focusing chart tests before blindly trusting the camera's manual focus accuracy.

Wait a minute!

Even if I did that, i would need a separate camera for each lens because the factory can't adjust a single manual focus assembly to correct for any and every lens. There is no such thing as a fine tuning knob on the camera to adjust the manual focus parts +\- up or down in a similar way the AFMA is designed for. Perhaps I'm over thinking it.




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apersson850
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Mar 17, 2017 14:40 |  #22

Yes, you are. Automatic focusing may need calibration because there can be a discrepancy between what the camera want's the lens to do, and what the lens actually does.
This doesn't matter when using contrast focusing in live view, since the camera will order the lens to move the focusing mechanism until the camera measures the contrast to be the highest.
When using manual focusing, provided the camera has the focusing screen positioned properly, i.e. at the same efficient distance from the bayonet mount as the sensor is, then the image will be in focus if it looks sharp on the focusing screen. The calibration of the lens is in this case irrelevant, since it's you, the photographer, who has to adjust the lens until the subject is in focus.

Cameras with automatic focusing frequently have focusing screens that can't resolve precise focus. What you can see in the viewfinder is frequently equivalent to the depth of field at around f/4. Thus with a lens with a large max aperture, you may not be able to see if you've focused it precisely. Your special focusing screen is hopefully better in resolving sharpness, but will then be darker instead. The split image center works fine for some targets, but not for all of them.
You still have the issue that lenses designed for automatic focusing usually have a pretty coarse manual focus setting ring. Precise adjustment may be difficult.


Anders

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RDKirk
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Mar 17, 2017 15:35 |  #23

texshooter wrote in post #18303525 (external link)
If the manual focus mechanism is precisely machined as you suggest, the lens may still not be. (ergo the often need for autofocus microadjustments).

It doesn't matter. Lens calibration is a PDAF issue. If you're focusing manually on either the LCD screen or the groundglass, you can use any kind of adapted lens--not even autofocus, not even Canon, not even communicating with the camera--and the focus will be correct if the basic machining of the mirror box is correct. No lens calibration needed.




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texshooter
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Mar 17, 2017 17:20 as a reply to apersson850's post |  #24

It makes sense now. Good to know I can trust the camera's manual focusing.




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Azathoth
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Post has been edited 1 month ago by Azathoth.
Mar 20, 2017 07:21 |  #25

Some time ago i made some tests and I concluded that contrast detection on my 70D and on my 1100D was more acurate than phase detection. I would get photos as sharp as using manual focus with max digital zoom on Liveview.
Using dual pixel's 70D liveview AF was also as sharp as MF and sharper than phase AF.


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AJSJones
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Mar 20, 2017 18:43 |  #26

Azathoth wrote in post #18305780 (external link)
Some time ago i made some tests and I concluded that contrast detection on my 70D and on my 1100D was more acurate than phase detection. I would get photos as sharp as using manual focus with max digital zoom on Liveview.
Using dual pixel's 70D liveview AF was also as sharp as MF and sharper than phase AF.

Any system that uses information coming directly from the sensor will ultimately be better than one that relies on an off-sensor AF system (even a well-calibrated one will have variability within its specs) - the main issue is speed - or it used to be. The performance of EVFs has only recently approached the advantages of the OVF, where real-time view is instantaneous, while in the EVF the quality and lag have been problematic (in many cases they still are), so "Live"View and EVFs which allow sensor based AF are compromises, especially for action. On-sensor DPAF or PDAF with LiveView reduces the compromises somewhat in terms of AF speed but needs a good EVF. The additional information displayed and low-light image visibility of EVFs have always been a plus...


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Colin ­ Glover
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Post has been edited 1 month ago by Colin Glover.
Mar 20, 2017 19:36 |  #27

I noticed that Mr. Tapes designed "Lens Align." Wouldn't that give him a vested interest in saying what he said? AJSJones, I didn't know there was a digital zoom on my 70D. Where is it, and how do I use it please?


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AJSJones
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Post has been edited 1 month ago by AJSJones.
Mar 20, 2017 23:57 |  #28

Colin Glover wrote in post #18306358 (external link)
I noticed that Mr. Tapes designed "Lens Align." Wouldn't that give him a vested interest in saying what he said? AJSJones, I didn't know there was a digital zoom on my 70D. Where is it, and how do I use it please?

P. 247 of the manual shows how to do manual focusing using Live View and using the magnify icon to zoom in (I don't have a 70D)

This is a pdf of the manual http://media.the-digital-picture.com ...Manuals/Canon-EOS-70D.pdf (external link)


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Autofocus in Live View mode - Bad idea?
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