Author Topic: Lens calibration distance chart  (Read 212 times)

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Offline Jeff Behm

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Lens calibration distance chart
« on: December 12, 2017, 03:27:24 PM »
I tend to be a little bit OCD about certain technical aspects of photography, so if you are not, you can skip this. 

One of the things I've found most helpful is the advent of Micro Focus Adjustment options on camera bodies.  One component of doing MFA successfully is setting up the correct distance for diagnosing each lens.  There are competing schools of thought on the proper distance, but the one I hear most frequently is 50X the focal length until you approach 200mm and longer, after which it starts to shift toward 20X the focal length.  Rather than guesstimate that distance, I use a chart, attached below for anyone interested.  I was instructed to go slightly longer than the minimums here, so I round up when I convert from meters to feet, and maybe plus a foot, as well.  Using the proper and longer distance improves the reliability throughout the focusing distances we encounter, too. 

Offline Houston George

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Re: Lens calibration distance chart
« Reply #1 on: December 20, 2017, 10:40:55 PM »
Good info to know. What are your thoughts on the optimal focal length when doing MFA on a zoom? I've read "choose the focal length you most often use" but that doesn't help much because I use them all. How critical do you think it is?
Houston

Offline Todd Muskopf

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Re: Lens calibration distance chart
« Reply #2 on: December 21, 2017, 07:45:29 AM »
I've always tried to do the adjustment at the distance that I typically shoot at.

Offline Jeff Behm

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Re: Lens calibration distance chart
« Reply #3 on: December 21, 2017, 10:20:15 AM »
Houston, I calibrate my zoom lenses at their widest focal length and then their longest because A) all my camera bodies allow that for zooms and B) the owner's manual says do it that way.  This means I may move the camera to a new distance according to the chart, too, when using zooms.

Todd, there appear to be two schools of thought on distance for calibration, and you just described one of them. I've done it that way, but in reading, the most frequently recommended technique is to use the kind of distances shown in the chart.  Since lens design is a science, using a more precision approach is how I've chosen to go, that's all. 

You may recall from discussions here years ago, my disappointment upon finding that my film era 70-200 f2.8 did not focus precisely enough to perform well once I moved beyond the original Canon 10D.  Canon customer service could not correct it back then, so I sold it to someone still using film.  The problem was the built-in tolerance for film which has a bit of a bow or warp in it, no matter how tightly it's stretched, especially compared to the perfectly flat sensors in digital cameras.  That, and the advent of pixel peeping made a number of more tolerant designs less desirable in the digital age.  Bottom line, the precision with which they're now building both bodies and lenses in order to provide the kind of sharpness we're demanding leads me to use a more precise procedure.

Offline Houston George

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Re: Lens calibration distance chart
« Reply #4 on: December 22, 2017, 12:02:09 PM »
I guess Nikon bodies allow for saving multiple correction values for a single lens, but I'm not sure. The only issue I could foresee would be the need to go into the menu to change it according to focal length. Not a problem with posed portraits or static subjects, but it seems weddings would make it nearly impossible.
Houston

Offline Jeff Behm

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Re: Lens calibration distance chart
« Reply #5 on: December 22, 2017, 02:52:33 PM »
I'm not quite sure of your point there, Houston.  Speaking only for Canon, there are two modes: same for all lenses or adjust according to lens attached.  I use the latter.  Once the calibration is set for any particular lens, the camera's on board computer recognizes the lens by serial number and adjusts itself accordingly.  Same for zooming; it reads and interprets according to what was determined to be the best MFA for wide or tele and adjusts as you change.  Obviously that's probably only at its best at the two measured focal lengths, wide and tele, with the rest being some calculated average as you move through the range. 

The dock for Sigma ART lenses has gotten a lot of attention, but ultimately, that's really only viable if you have one body is my understanding.  Working as I do with 3 bodies and using different lenses on each at different times, it may be that Canon's MFA is superior to the dock.  I read of a work around for this that involved splitting the difference in total adjustment between the Sigma and the body, but haven't needed or tested it.

Offline Houston George

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Re: Lens calibration distance chart
« Reply #6 on: December 22, 2017, 10:52:02 PM »
My point was probably based on my ignorance. I thought it would be necessary to manually select which calibration value you wish to use if you saved two values for the same lens. I don't know if Nikon does that automatically as you described, and I really haven't had time to look into it. In the past I've only done a single focus fine tune for each lens so I've never given it thought.
Houston

Offline Jeff Behm

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Re: Lens calibration distance chart
« Reply #7 on: December 22, 2017, 11:09:27 PM »
Again, not knowing Nikonís setup, Iíll mention that for most Canons pre 2012 or so that Iím familiar with, you only had one focus fine tuning per lens.  The 5D3 came out in 2012 I think, and was my first body to allow zooms to be fine tuned for wide and tele both.  Primes, of course, require only one focus tuning setup.  Perhaps Nikon only offers one option as well?  I donít know.

Offline Houston George

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Re: Lens calibration distance chart
« Reply #8 on: December 23, 2017, 10:11:56 PM »
After doing a brief web search, I couldn't find where multiple calibrations are possible for a single lens on the D750 or D700. After messing with it for a while I found that if I did a second fine tune it would overwrite the first one, even if I saved it as a different number.
Houston

Offline Jeff Behm

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Re: Lens calibration distance chart
« Reply #9 on: December 23, 2017, 11:08:25 PM »
So what does the owners manual say about zooms? Are you supposed to choose the widest or the longest, or the focal length you use most?  If I recall correctly, my 5D2 was a single setting for zooms and it was for the widest.  Once I got the 5D3 and 7D2, I was pleasantly surprised to see that zooms initiated two different scales for MFA.  It certainly makes sense to me.

Offline Houston George

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Re: Lens calibration distance chart
« Reply #10 on: January 04, 2018, 11:05:56 PM »
The manuals say nothing about zooms, at least for the models I shoot. Maybe the top end models (D5/4) are different. On a side note, this whole discussion highlights how spoiled we are with auto focus. Over the last couple of years I have acquired a few older manual focus lenses that I love shooting. Not only do I appreciate the superior build quality and feel, but there's some real appeal to manually focusing. It's liberating to not worry about selecting focus points, not recomposing, no annoying hunting by the lens. And then there's the throwback aspect that takes me back to a time when manual focusing was all there was. It's strangely rewarding to have more control in this day of ever expanding automation. No, I'm not going to set all my auto focus lenses to manual from here on out, but it's a skill worth staying sharp at, kinda like the fun of a stick shift even though PRND is easier.   
Houston

Offline Jeff Behm

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Re: Lens calibration distance chart
« Reply #11 on: January 05, 2018, 08:59:00 AM »
I'm with you on the simplicity of manual focusing, but 25 years ago my eyes went from 20/15 vision to whatever my bifocals are today.  I found I couldn't trust what I saw in the manual finder.  That was also while I still used 4x5 view cameras, too, and the "focusing aid" was a loupe on the ground glass, under the dark cloth.  Not being able to trust the eyes was jarring.  That's when Canon had the best AF available, so I invested in it.  I have to say, going back to some of my manual focus film 35's has felt awkward at best, because with the bifocals, it only takes a slight change in alignment between my glasses lens and the finder to change what I think I see.  So AF is here to stay for me. 

Back to the original point, have you found any tips online for MFA of the bodies you have?  NOt being very familiar with Nikon, that's where I'd have to look.

Offline Houston George

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Re: Lens calibration distance chart
« Reply #12 on: January 05, 2018, 01:31:55 PM »
Back to the original point, have you found any tips online for MFA of the bodies you have?  NOt being very familiar with Nikon, that's where I'd have to look.
I've read "calibrate at the long end", "calibrate at the short end', and "calibrate the focal length you most use" so I have no clue. The first step is to see if it's even necessary so I'll start there.

I don't know about Canon, but with Nikon you can still utilize the focus confirmation dot in the viewfinder, and select the focus point, even with older manual lenses. In reality though, trying to look at the subject, the focus point, and the confirmation dot leaves me needing a second eye in the viewfinder. lol
Houston

Offline Jeff Behm

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Re: Lens calibration distance chart
« Reply #13 on: January 05, 2018, 03:45:09 PM »
This was interesting; straight form Nikon's support page, about 16 months ago.  https://www.nikonimgsupport.com/ni/NI_article?articleNo=000004089&configured=1&lang=en_US   

This sentence especially caught my eye.  "AF tuning is not recommended in most situations and may interfere with normal focus; use only when required."

Admittedly I didn't read it all, however.

Offline Houston George

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Re: Lens calibration distance chart
« Reply #14 on: January 06, 2018, 09:34:57 AM »
Yep, that same sentence appears in the manual as well. I was about to decide that AF tuning wasn't even covered in the manual when I stumbled onto the two paragraphs toward the back of the book. It's vague to say the very least, obviously not something they encourage.
Houston

Offline Jeff Behm

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Re: Lens calibration distance chart
« Reply #15 on: January 06, 2018, 10:20:41 AM »
I'd say that's probably the final take-away, Houston, "Not recommended".  It does make me curious as to the philosophical differences between Nikon and Canon at the design and production levels.