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Author Topic: Any Front/Back Focus Problems on Mirrorless Cameras?  (Read 2364 times)
JimAscher
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« on: September 15, 2013, 07:22:44 AM »
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This may be a "duh" query, but I'll risk it.  As there is currently much discussion elsewhere in this forum on this subject of auto-focusing in DSLR's, presumably because of possible misalignment of the mirror, can I assume this problem does not/cannot exist in cameras where there is not only no mirror, but where the photographer (me) focuses manually (with legacy lenses)?  Because what is seen in the viewfinder is actually the same sensor image as ends up on the final "shot?"
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ErikKaffehr
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« Reply #1 on: September 15, 2013, 09:50:22 AM »
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Hi,

There are still some problems:

- There is a phenomena called 'focus shift', correct focus depends on aperture. So correct focus at f/2.8 is incorrect focus at f/5.6. 
- Focus throw on lenses may be less than optimal
- Contrast sensing AF is great in theory but your mileage may vary
- Personally, I am quite sceptical about the accuracy of focus peakig

Still, I much prefer live view MF at actual pixels to phase detecting AF...

Best regards
Erik

This may be a "duh" query, but I'll risk it.  As there is currently much discussion elsewhere in this forum on this subject of auto-focusing in DSLR's, presumably because of possible misalignment of the mirror, can I assume this problem does not/cannot exist in cameras where there is not only no mirror, but where the photographer (me) focuses manually (with legacy lenses)?  Because what is seen in the viewfinder is actually the same sensor image as ends up on the final "shot?"
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JimAscher
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« Reply #2 on: September 15, 2013, 10:08:40 AM »
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Erik:  I don't believe I will encounter the problem of focus-shift for the specific purpose I had in mind when I posted my query.  I am experimenting with the use of a Rokinon 85mm at its maximum aperture setting, i.e., f. 1.4.  As at this aperture there is, of course, almost no depth-of-field, thus little or no margin for focusing error.  The focusing has to be "right-on."    I am trying this lens on both my Sigma SD15 DSLR and my Sony NEX 5n.  On the Sigma, there's a mirror, and on the Sony, no mirror.  The sensors of both cameras are the same size.  I am trying to determine not only which is easier for me to focus, but which, if either, gives me the sharpest focus.  Thanks for your quick, and informative response.  Regards, Jim
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Vladimirovich
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« Reply #3 on: September 15, 2013, 10:12:21 AM »
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can I assume this problem does not/cannot exist in cameras where there is not only no mirror, but where the photographer (me) focuses manually (with legacy lenses)? 

focus shift is not an issue when you focus @ actual aperture
legacy MF lenses that you might want to use tend to have a good helicoid w/ properly dampened long rotation.
decent mirrorless cameras will supply you w/ focus peaking or magnified focus point view, and sensor stabilization


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stever
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« Reply #4 on: September 15, 2013, 10:45:30 AM »
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in theory, autofocus at the taking aperture should eliminate focus shift issues, on the other hand, focusing at small apertures, particularly in low light places heavy demands on the AF and you may have small errors that are more or less random
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scooby70
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« Reply #5 on: September 15, 2013, 12:52:40 PM »
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If it helps at all...

My keeper rate with my Panasonic G1 + manual lenses is very high. Focusing manual lenses with these cameras should be easy with the magnified view and you should be able to focus on any point along an eye lash  Grin

I haven't used my AF lenses for quite a while but when I did the keeper rate was equally impressive. CSC's seem to be fast to focus and very accurate.
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wildlightphoto
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« Reply #6 on: September 16, 2013, 08:57:04 AM »
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The responses so far seem to be blurring the distinction between focus shift and AF front/back focus.

Focus shift is a property of the lens independant of the camera's focussing system, caused by spherical aberation.  It's been with us since the first lens was ground.  As others have mentioned, focus shift isn't a problem when using live view or other TTL focussing system such as a properly-calibrated ground glass and focussing at working aperture.

AF front/back focus is an entirely different beast and is typically found in DSLR cameras using phase-detect AF.  Its causes are many but include misalignment & manufacturing tolerances that result in the 'equivalent' image plane that the AF system uses not really equivalent, and lens focal lengths & apertures not exactly the same as the AF system expects.

A mirrorless camera using data from the sensor for its AF input doesn't have nearly as many alignment issues and contrast-detect AF or focus peaking (which is a contrast-detect system) doesn't have the lens variance issues.
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ned
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« Reply #7 on: September 16, 2013, 05:10:44 PM »
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With the EM-1 having phase detect and the ability to AF adjust your lenses I think the answer is yes.
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ErikKaffehr
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« Reply #8 on: September 18, 2013, 03:15:35 PM »
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+1

Just to make clear, focus shift is a problem also on mirrorless if focusing is as maximum aperture and not working aperture. Another point is that focus shift is mostly a problem of large aperture lenses. Moderate aperture lenses like f/2.8 are probably less problematic.

Best regards
Erik


The responses so far seem to be blurring the distinction between focus shift and AF front/back focus.

Focus shift is a property of the lens independant of the camera's focussing system, caused by spherical aberation.  It's been with us since the first lens was ground.  As others have mentioned, focus shift isn't a problem when using live view or other TTL focussing system such as a properly-calibrated ground glass and focussing at working aperture.

AF front/back focus is an entirely different beast and is typically found in DSLR cameras using phase-detect AF.  Its causes are many but include misalignment & manufacturing tolerances that result in the 'equivalent' image plane that the AF system uses not really equivalent, and lens focal lengths & apertures not exactly the same as the AF system expects.

A mirrorless camera using data from the sensor for its AF input doesn't have nearly as many alignment issues and contrast-detect AF or focus peaking (which is a contrast-detect system) doesn't have the lens variance issues.
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fike
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« Reply #9 on: September 18, 2013, 03:22:18 PM »
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I have had a lot of trouble getting autofocus to accurately focus on small objects like bird heads.  It always will try to focus on the background. You can, of course, manually focus with the zoom-in feature, but I am not fast enough to consistently get that to work.
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wildlightphoto
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« Reply #10 on: September 19, 2013, 12:20:33 AM »
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I have had a lot of trouble getting autofocus to accurately focus on small objects like bird heads.  It always will try to focus on the background. You can, of course, manually focus with the zoom-in feature, but I am not fast enough to consistently get that to work.

Birds are my "thing" and I focus manually with the camera's viewfinder.  It's not an AF or mirrorless camera so it's actually possible to focus manually with the viewfinder.



Backgrounds are not a problem.
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Isaac
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« Reply #11 on: September 19, 2013, 12:38:23 PM »
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Birds are my "thing" and I focus manually with the camera's viewfinder. ...

And importantly, you are "fast enough to consistently get that to work" ;-)

While not "my thing" I've just been photographing egrets, because while they're feeding I can stay close enough for my medium telephoto lens. I focus manually, with an EVF magnified view, because on a good day that will work really well for me 1 out of 10 times.

That doesn't work at all when the egret jumps toward me (too close, not enough time) and it doesn't work at all when the red tailed hawk flies from its perch at me. If this was "my thing" I suppose I'd set the camera just for those sudden events and be patient.

What I might be able to manage is to re-focus from the eye down the beak when the egret takes a crayfish, and somehow place the crayfish onto the white background of the egret's body. Still working on that ;-)
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