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10-20-2017, 04:12 PM   #1
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Front focus and back focus problems with manual focus lenses

I mostly use telephoto (Tamron Adaptall 60-300) and super telephoto (Dollond's S 500mm, Tele-Tokina 600mm) on my K5iis. While with 500mm and 600mm I rarely fail to properly focus , Tamron 60-300mm lets me down quite often with front focusing problem on the long end of the zoom. I experienced the same front focus problem with modern Tamron 17-50. I ignore the focus confirmation sign and sound and rely entirely on my eye believing that what I see sharp on the focusing screen supposed to come out sharp on the sensor as well. I own another two identical manual focus tele zooms and while one of them has no focusing issues, his twin brother consistently focuses ( throughout the zoom range) about 1.5-2 m in front of the object which I can see absolutely sharp through the viewfinder on the focusing screen. I can understand a problem with AF, as it's a separate optical system, but the focusing screen and the sensor are located in the same register distance.
Can any one explain this phenomenon?

10-20-2017, 04:32 PM   #2
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Do you get the same problem in live view with focus peaking/magnification? If not, then it's probably just tolerance that's not discernible via the viewfinder. If so, then the image quality of the lens itself is to blame.

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10-20-2017, 04:59 PM - 1 Like   #3
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It's worth bearing in mind that you see a greater depth-of-field through the viewfinder. Something can appear to be in focus, even when the lens is not focused precisely at that distance. This is a major reason why it's best to use Live View, or a third-party split-prism focusing screen (Live View, however, offers the best accuracy)...
10-20-2017, 08:57 PM - 1 Like   #4
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QuoteOriginally posted by hatsofe Quote
...I ignore the focus confirmation sign and sound and rely entirely on my eye believing that what I see sharp on the focusing screen...I can understand a problem with AF, as it's a separate optical system, but the focusing screen and the sensor are located in the same register distance.
The focus screen registration is supposed to be at the same registration, but whether care has been taken during assembly to properly place and confirm is difficult to assess. If calibration is wrong, one would see consistent front or back focus with ALL lenses.

Front/back focus with PDAF is not related to registration. The topic is complex, but it is probably enough to simply say that it is related to alignment and relative position of elements at various distances and zoom setting causing an ambiguous or skewed image on the AF sensor. As a result, sensitivity to the out-of-focus condition is greater on the near than the far or vice-versa.

QuoteOriginally posted by hatsofe Quote
Can any one explain this phenomenon?
The phenomenon of two similar lenses performing differently? Without an optical bench probably not. Problems with mechanical lash (elements shift position when the focus ring is released) is a possible cause as might misalignment. Another might be the method of testing/comparison.

QuoteOriginally posted by Adam Quote
Do you get the same problem in live view with focus peaking/magnification? If not, then it's probably just tolerance that's not discernible via the viewfinder.
Kudos to Adam for asking the critical question. The live view image is the gold standard for focus evaluation. I am not confident of focus peaking, but am very fond of the magnified view.

A few points to ponder:
  • The optical viewfinder on your camera is less sensitive to detect out-of-focus (OOF) than that of a manual focus SLR from the late 1970s-1980s. The reasons are complex, but one can expect the ability to detect OOF to be no better than with about an f/3.5 aperture regardless of whether a faster lens is mounted. The result is lower precision regardless of how well calibrated the rest of the system is.
  • Your ability to detect the plane of focus is highly dependent on whether your eyes are actually focused on the focusing screen surface. The viewfinder diopter adjustment is important. On screens with etched lines (most Pentax dSLR stock screens) the AF area bracket lines should be sharp.
  • When doing your focus evaluations the intent is to make it easy, not hard. Use a high contrast target positioned parallel to the camera sensor. Do your testing in good light and on a tripod (inches count).
  • Distance is important, though perhaps not in the way one might think. There is no virtue in testing focus at other than moderate distance. Being able to actually see the target in the viewfinder is one good reason, but more important is that the further away something is, the less degrees of throw are available on the focus ring. This is easily seen by looking at the distance scale.

    I have in front of me a 50mm lens. There is probably less than 20 degrees of arc between infinity and 30 feet. Extrapolating a bit, I figure there is less that 10 degrees between 30 feet and 1000 feet. A little closer in and that same 20 degrees will satisfy the range of between 10 and 15 feet. How easy is it to make fine adjustment of less than a 1 mm of linear travel of that ring? That is with a lens having a full 180 degrees focus ring travel to cover 15" to infinity. Now consider a lens such as my Sigma 17-70/2.8-4 (C) which traces the range 8 inches to infinity in about 40 degrees of throw.

    If it is physically difficult to bring the target into focus, move it close, much closer.
  • Focus technique counts. It makes a difference whether one is focusing near-to-far vs. far-to-near. DOF is shallower in the near direction and focus "pop" is more distinct working from that side, though possibly no more accurate.
  • Accuracy and precision are two different things. A combination of short focus throw, long distance and inability to reliably detect OOF works against even the most accurately calibration. Because precision can be poor, multiple attempts should be made before claiming front/back focus.
I am quite curious to read what happens with your live view attempts. One last thing...Is there any chance your focus screen has been removed at some point or that any of the lenses in question might have been dropped?


Steve

10-21-2017, 02:18 AM   #5
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Thank everybody VERY MUCH for responding. I actually didn't try using the "life view" for testing, but I will as Adam recommended. The matter is, I've been shooting in the area with extreme sunlight. In such conditions even inspecting the picture after the shot on the camera back screen is problematic. I have a permanently attached *1.6 magnifier (could this be a problem?) to the camera viewfinder with custom made shade, so no side rays enter the view. Mike ,of course, the focusing screen area of Pentax K5iis is about half the size of an old 35mm SLR, but I manage. I was contemplating for some time to replace the original focusing screen with different type aftermarket one, but finally dropped that idea. Anything other than a matted glass makes composition process more complicated.
As I understand ,the sharpness of an object on the focal plane is purely matter of precise distance between the lens and the focal plane. I bought the camera new and don't think it was tempered with: I checked the tightness of the focusing screen and it seemed to be fine.Besides, I've encountered the problem with particular lens objective in 100% cases while with other two the results were inconsistent and I can take it as human error. I do realize the difference between " not in focus" and "smudged" , "soft", "ghosting" and other harmful interventions into the adorable process of taking pictures. I use a tripod with optics above 300mm and keep the min shatter speed accordingly.
With that particular super telephoto zoom (100-500mm) I consistently got the same results: the sharpness was always way ahead of the object I focused on. I looked inside the lens , shook it and din't see anything out of ordinary. Same was the case with Tamron Adaptall 60-300mm, but not so consistent. I intend to think that something shifts inside these lens objectives during the shatter operation... On the other hand, is it possible that the sensor shifts inside the camera unexpectedly? I move through rather rough terrane and the camera takes lost of shaking.
Steve, I always intend to get closer to the objects , but they NEVER cooperate. They run/ fly away as soon as they spot me within 30 m distance....
Thank you for the great detailed explanation. I didn't quite understand what you meant saying "It makes a difference whether one is focusing near-to-far vs. far-to-near". I move the focusing ring back and forth until the object looks the sharpest.
10-21-2017, 04:12 AM   #6
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QuoteOriginally posted by hatsofe Quote
. . . I was contemplating for some time to replace the original focusing screen with different type aftermarket one, but finally dropped that idea. . . .
That's just as well. I've put aftermarket screens in two Pentax DSLR's, only to find that they needed shims to correct the distance which are almost impossible to find. (One of my major irritations of Pentax is that you can use all this legacy glass but they don't make focusing screens to support it.)
QuoteOriginally posted by hatsofe Quote
. . . I didn't quite understand what you meant saying "It makes a difference whether one is focusing near-to-far vs. far-to-near". I move the focusing ring back and forth until the object looks the sharpest.
He means that a lens (especially an old or cheap one) will focus "differently" when turning clockwise vs. counter-clockwise (left-to-right is different than right-to-left) because of wear and/or sloppy manufacturing.
10-21-2017, 06:21 AM   #7
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QuoteOriginally posted by fredralphfred Quote
That's just as well. I've put aftermarket screens in two Pentax DSLR's, only to find that they needed shims to correct the distance which are almost impossible to find. (One of my major irritations of Pentax is that you can use all this legacy glass but they don't make focusing screens to support it.)He means that a lens (especially an old or cheap one) will focus "differently" when turning clockwise vs. counter-clockwise (left-to-right is different than right-to-left) because of wear and/or sloppy manufacturing.
..."focus "differently" when turning clockwise vs. counter-clockwise" - I see. To remove the slack. There is no slack in that objective at all; it seems to be unused. As I mentioned, I've got another copy of the same lens (100-500mm) ,which saw apparently lots of use, contains visible dust inside and came with one screw of the focusing barrel loose and rolling inside ( I fixed it) - but it focuses normally! And Tamron 60-300mm even though it's more 25 years old doesn't wobble either. I bought "Beroflex" 500mm F8 - not a great workmanship - (30 years old?) recently and cannot complain about focusing. I don't think that front focus problem related to poorly manufactured PK mount of the lens either. I modified few old super telephoto objectives from early 1970s by using cheap T2-PK mounts , which I machined and some time an extensive pressure I've applied to the focusing end ( attached to the camera) caused blur , But I saw it right away.That "black sheep" 100-500mm zoom has nicely made sturdy PKA mount.
10-21-2017, 11:37 AM   #8
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QuoteOriginally posted by fredralphfred Quote
He means that a lens (especially an old or cheap one) will focus "differently" when turning clockwise vs. counter-clockwise (left-to-right is different than right-to-left) because of wear and/or sloppy manufacturing.
That is one of the reasons, though I was actually referring to how near vs. far DOF affects manual focus.


Steve

10-26-2017, 10:51 AM   #9
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I've learnt enough from playing with a variety of MF lenses, particulalry adaptalls, to take my impressions of focus issues with a large pinch of salt. There's no real substitute for some structured experimentation. Set up a test situation with appropriate subjects, including a sloping test rig like those used for front-focus/back focus issues, control aspects like vibration and shake as best as possible with tripod, shutter speeds, MLU etc and take comparitive pics for scrutiny.
I'm also using a K5, I am intrigued by your assertion that you have no prob focussing a tokina 600mm f8 with an OVF. My personal asessment is that the limitations of the f8 aperture and the optical OVF system make reliable visual focus with that and similar lenses inherently uncertain, I invariably resort to LV but even that has a degree of uncertainty too. But maybe you are in an environment with bright enough light consistently to militate against that uncertainty .

Last edited by marcusBMG; 10-26-2017 at 11:18 AM.
10-27-2017, 01:43 AM   #10
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Dear Marcus, I read many of your objective reviews and liked them very much. I used them as a reference making decision to buy some super tell photo.
As I've mentioned in previous posts, I'm aware of great importance of sturdy and motionless camera support, shutter speed and DOF and few more technical aspects (as well as imprecise focusing). The matter is that with one particular objective I get 100% front focus results. The objective looks mint and doesn't show any sign of ware. The other one I had similar problem and hole photo session lost was Tamron 60-300mm adaptall, but I took it with me again a week later and things were fine that time.
Unfortunately, I can't shoot more than once a week as photography is not my bread and shelter.
The only time I use LV is night shots of animals. I don't see the point of using digital screen in DSLR , other than testing or when OVF becomes useless (night). If I liked LV , I'd probably buy Sony .
Tele Tokina didn't deliver satisfying results with F8, rather with F11. Of course, it could be the sample I bought. Tokina, despite the lenth is light weight and relatively easy to use.
I promise to test that problematic 100-500mm soon again and will report the results.
10-27-2017, 03:05 AM   #11
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QuoteOriginally posted by stevebrot Quote
That is one of the reasons, though I was actually referring to how near vs. far DOF affects manual focus.


Steve
My focusing depends on the distance and the background, another words composition.But very often I don't have much time to compose: wild birds and animals have little tolerance to my presence. But getting very disappointing results with that objective I slowed down and experimented .With zoom adjusted to max(500mm) sharp area appeared way in front of the object I focused. Then I tried to fool the system and succeeded : I focused about a metre and half behind the object and it appeared within the sharp area, while it looked out of focus in the viewfinder.
10-27-2017, 07:44 AM   #12
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Well Thank You hatsofe, always nice to hear that ones posts are appreciated. I would be interested to hear if you reach a conclusion re the tamron 60-300mm front focusing issues, I'm afraid I don't have anything more to suggest.... well maybe if you have access to another 23A, see if its the same thing. Normally focusing is a camera centred thing, and since you are familiar with focus on slower, inherently more difficult lenses, I would off the cuff assume you would have no problem with the 23A, good sharp lens. The only other thought is focus shift, but I have never encountered any suggestion that this is an issue with this lens. Aye well...
10-29-2017, 05:21 PM   #13
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QuoteOriginally posted by marcusBMG Quote
The only other thought is focus shift, but I have never encountered any suggestion that this is an issue with this lens. Aye well...
Good point. I tend to forget about focus shift because it is generally difficult to detect.


Steve
11-04-2017, 01:20 PM - 1 Like   #14
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Hello everybody!
It looks like , I found a solution for that riddle. After getting very frustrating results in first 20 minutes of this morning session ( 28 shots all back focused !) , I took a break and started checking the gear. I used my favorite Dollond's S 500mm, which rarely failed on me before.Even if it did, I had only my eyesight to blame.
As I mentioned before, a magnifying viewfinder ( *1.6) has always been attached to my camera. You all surely know that an external viewfinder fits right behind the camera VF ,instead of the original eye shade and slides down on tiny rails without actually getting locked. I didn't take it off since the initial installation two years ago. The matter is , it has a plastic frame and while it sits quite snugly over the camera VF rails ,an accidental pressure from a side can move up and down a bit, or even leave cock-eyed. This may happened during the transportation and handling without been noticed. I've re-centered that external magnifier and the back focusing problem disappeared ! In next few day I'll try to take shots with that problematic zoom and will report.
I'm curious to know how such a phenomenon can be explained from optical point of view.
11-04-2017, 03:42 PM   #15
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QuoteOriginally posted by hatsofe Quote
Hello everybody!
It looks like , I found a solution for that riddle. After getting very frustrating results in first 20 minutes of this morning session ( 28 shots all back focused !) , I took a break and started checking the gear. I used my favorite Dollond's S 500mm, which rarely failed on me before.Even if it did, I had only my eyesight to blame.
As I mentioned before, a magnifying viewfinder ( *1.6) has always been attached to my camera. You all surely know that an external viewfinder fits right behind the camera VF ,instead of the original eye shade and slides down on tiny rails without actually getting locked. I didn't take it off since the initial installation two years ago. The matter is , it has a plastic frame and while it sits quite snugly over the camera VF rails ,an accidental pressure from a side can move up and down a bit, or even leave cock-eyed..
That's great news! Thanks for letting us know

Which magnifying eyepiece do you use? I have the O-ME53 fitted to several Pentax DSLRs and haven't experienced the problem you mention, though I do periodically check that the eyepiece is fully-seated. I also have a generic Chinese magnifying eyepiece that I find to be almost-useless and very loose, so have never used that (it wasn't worth the hassle to return it ).
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