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02-26-2016, 09:48 AM   #1
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Lens fine focus adjustment - interesting result

As per the heading, I thought that Id have a look at the fine focus of my lenses, not particularly out of any concern but because I just wanted to try it and see if there was anything unexpected well there was, but not what I was thinking about.

My equipment is a K-S2 with SMC DA-L f4-5.6 18-50mm DC WR, SMC DA-L f4-5.6 50-200mm ED WR, SMC D-FA 100mm F2.8 Macro WR and a SMC FA 4.5-5.6 80-320. I used test charts from Yvon Bourque, all tests were done at the widest aperture and minimum focal distance (plus 100mm or so) using spot focusing. Tripod and 2sec delay used. I took several shots with each lens.

Day 1. Daylight in a bright room. I used Live View as I thought that would be easiest. On review mode it is set up with a 4x magnification and I thought that would give me a good start to check the focusing.
18-50 test, quick focusing, result marginal back focus, adjusted to plus 1.
50-200 test, quick focusing, result spot on.
80-320 test, a bit of lens hunting before focusing, result large front focus. Adjusted to minus 7 and could not get it to focus. Put back to zero and could not get it to focus even when it lit green (focus indicator) the chart was completely out of focus. I tried increasing the focal length to no avail. Then I tried turning Live View off and viewfinder focusing instant focus success.
At this stage I was getting fed up so stopped testing and reset the adjusted lenses back to zero.

Day 2. Daylight in a bright room. I only used Live View (LV) to set up the lens to test chart alignment. Then switched to viewfinder focusing.
18-50 test, instant focusing, result spot on.
50-200 test, instant focusing, result spot on.
80-320 test, instant focusing, result spot on.
100 macro test, instant focusing, result spot on.

Other than the results suggesting that all my lenses are focusing fine, there are two things of interest to me here, Live View v Viewfinder results and Macro focusing.
1. You can see that there were differences between the LV and Viewfinder results, why?
2. I use Live View for macro photography, its easy to view at all camera positions and using the zoom feature you can see exactly what is in focus. But now Im wondering if LV is not correctly showing what is in focus which is odd as LV has good reviews on the K-S2

Any thoughts?

02-26-2016, 10:05 AM - 2 Likes   #2
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QuoteOriginally posted by whyhaveone Quote
I used Live View as I thought that would be easiest.
Live View uses contrast detection in the image as recorded by the sensor for focusing, as long as the lens can achieve focus, back or front focus shouldn't matter. Viewfinder focusing uses phase detection for focusing based on getting two offset images to line up. If those offset images line up in front of or behind where you are focusing, then AF adjustment corrects this. Live View is slower to focus because it uses trial and error to find the best focus, whereas viewfinder focus can measure how much offset there is between the two split images and tell the lens how much to move.

You should use viewfinder focus to check the AF adjustment of your lenses.
02-26-2016, 10:26 AM   #3
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QuoteOriginally posted by whyhaveone Quote
1. You can see that there were differences between the LV and Viewfinder results, why?

Any thoughts?
Simply, viewfinder focusing uses PDAF whereas LV uses CDAF mechanism, they are different units/components in the camera. AF fine adjustment in the camera applies to PDAF only IMHO.
02-26-2016, 10:34 AM   #4
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QuoteOriginally posted by whyhaveone Quote
Any thoughts?
Thank you for confirming what I have been telling people on this site for several years. Your experience is essentially normal.
  • A need for lens adjustment is generally the exception rather than the rule
  • Manual focus using magnified live view is the standard and usually agrees very closely with CDAF under live view
  • PDAF using the regular viewfinder has fairly poor precision (ability to consistently attain the same plane of focus) with a "focus sensitivity" of about f/5.6.* This is usually not a problem unless you are using lenses f/2.8 and faster.
  • CDAF using live view has much better precision than PDAF. What's more, there are no "front/back focus" issues since the evaluation is made from the actual sensor image.
  • Precision of both systems is limited by the mechanical design of the lens focus mechanism
  • Depth of field applies to auto-focus in the same way as with manual focus. The narrower the lens aperture, the more difficult it is to do fine focus.
  • Attempts at focus adjustment require multiple attempts at the anticipated focus distance. A single attempt is not enough (see point above regarding precision). Some methods such as the Dot-Tune procedure take a statistical approach to finding the middle value from the spread of indicated adjustments.
  • Lens "hunting" is not related to calibration.
You may have noticed that the word "precision" is used a lot in the above list. That is because precision counts more than accuracy and should drive our expectations. A driving analogy might be sloppy steering in an old car. Or if you prefer, hitting the center when playing darts.

In regards to "hunting". The AF system is not better than the lens and subject. If the lens is "soft" at the intended focus point, it is more difficult for the system to attain focus. The same is true if light is low or the target has low contrast. Another cause might an ambiguous target. A receding plane or curved surface make things more difficult. If a lens hunts, turn up the light or change the intended point of focus.


Steve

* What this means is that the focus precision with a fast lens (e.g. f/2) is the same as with a f/5.6 lens. Considering the depth of field at f/5.6 at moderate subject distance, that is a lot of slop


Last edited by stevebrot; 02-26-2016 at 10:39 AM.
02-26-2016, 10:42 AM   #5
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QuoteOriginally posted by aleonx3 Quote
AF fine adjustment in the camera applies to PDAF only IMHO.
This is not just your opinion. If is a verified fact Of the four focus systems available, only viewfinder manual focus and PDAF are adjustable.


Steve
02-26-2016, 06:55 PM   #6
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Thanks for the responses all.

I do understand the difference in the focusing methods and had always thought that PD would be more accurate (better?) than CD, but steve you seem to say that is not the case? So now based upon the above comments I'm more confused.

aleonx3 notes that Simply, viewfinder focusing uses PDAF whereas LV uses CDAF mechanism, they are different units/components in the camera. AF fine adjustment in the camera applies to PDAF only IMHO. and steve you confirm that. OK, if that is the case I definitely do not (from my testing) need to adjust the fine focus of any of my lenses - when using the viewfinder.

However, in your (steves) response you comment that CDAF (LV) has more precision than PDAF. So again from my testing at least two of my lenses would need adjustment, but this adjustment cannot be done - sort of confirmed that when I adjusted the lenses on day 1 I could not get any change in the focusing using LV. so if it has more precision, but cannot be adjusted if out what's the point?

I suppose apart from trying to get a general understanding of what is happening, I'm also trying to confirm whether using LV for Macro photography is the best, or whether I should just use LV to compose the shot and then switch to the viewfinder/PD?
02-26-2016, 07:08 PM   #7
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Can any one tell me if adjustment for " apply all" and "specific lens" adjustment add together and if so is there a limit ie. " +10 & +10 = +20 " or is +10 the limit, ( using K30 )

some results..
Sigma 17-70 f2.8-4.5 back focus - adjusted +10, better but still slight back focus, tried to add more using "apply all" and it didn't seem to correct further
Sigma 28-70 f2.8 EX same as above.
Tokina 28-70 f2.8 ATX PRO some back focus, applied +4 to correct. -but noticed that both adjustments now effecting outcome

I think I've answered my own question but has any one had experience with this.

PS. I've always found live view much better for macro focus in fact better all around the problem is the delay when you click the shutter I've had a few shots of tail feathers in the top corner when trying to shoot birds, it's only useful if the subject isn't moving.

Last edited by Cee Cee; 02-26-2016 at 07:13 PM.
02-26-2016, 08:27 PM   #8
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Hi Cee Cee. If you use apply all, every lens you put on the camera will have that adjustment. Apply one is for individual lenses. Your camera will apply that correction each time that lens is used. Apply one and apply all cannot be used together. Hope this helps.

02-27-2016, 10:00 AM   #9
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And to get back to my original post - can anyone assist further with that and my reply to those who originally responded?
02-27-2016, 10:59 AM   #10
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QuoteOriginally posted by whyhaveone Quote
However, in your (steves) response you comment that CDAF (LV) has more precision than PDAF. So again from my testing at least two of my lenses would need adjustment, but this adjustment cannot be done - sort of confirmed that when I adjusted the lenses on day 1 I could not get any change in the focusing using LV. so if it has more precision, but cannot be adjusted if out what's the point?
It is possible that under certain conditions CDAF is less accurate than PDAF, but if the lens is out of adjustment, CDAF will always give better results because it isn't affected by the lens having front or back focus. The LV screen has a lower resolution than your camera sensor, so it is possible that the image on the LV screen looks out of focus, but that doesn't mean you should adjust your lens based on LV focusing.
QuoteOriginally posted by whyhaveone Quote
I'm also trying to confirm whether using LV for Macro photography is the best, or whether I should just use LV to compose the shot and then switch to the viewfinder/PD?
The disadvantages of using LV for focusing is that it is slower than phase detection AF and sometimes the image isn't as clear in LV as looking through the viewfinder (too sunny to see the screen for example). On the other hand, looking through the viewfinder for macro photography isn't always suitable and for manual focus, you can't see as accurately through the viewfinder as on the LV screen. If you have composed and focused your shot in LV, there is no point in switching it off to go to PDAF.

.

Last edited by RGlasel; 02-27-2016 at 11:02 AM. Reason: There is a difference between precise and accurate.
02-28-2016, 04:10 PM   #11
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Thanks all for the responses. I don't think that I'm much the wiser though.
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