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Test lenses for decentering using simple means

Andreas Thaler

Member
Joined
Nov 19, 2017
Messages
4,863
Location
Vienna/Austria
Format
35mm
This current thread is about, among other things, the decentering of a lens block in a Nikkor zoom lens:

Nikkor 24-120/3.5~5.6 AF-D (IF): Fungus removed, assembling the rear lens block, infinity focus stop

I actually bought this zoom lens for immediate use. When I checked it, I found mold on several lenses. That would be a reason to send the lens back to the seller straight away. But for me it is a welcome repair project 😊 I really appreciate this lens for my walks through the city because it...
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I am now wondering how one can check a lens for possible decentration without much effort and came across this (German) website, which provides instructions on how to do so:

Dezentrierung

Dezentrierung an Objektiven testen
www-gletscherbruch-de.translate.goog

(Google Translate, German -> English


A detailed article on the topic:

Testing for a Decentered Lens: an Old Technique Gets a Makeover

What is Decentering and What Does it Do? Strictly speaking, decentering would involve one or more of the lens elements being off of the central axis of the lens. This would prevent the curved surfaces of the lens from bending the light properly. In severe cases it could result in halos or...
www.lensrentals.com


My questions as DIYer:
  • What setup/equipment do I need to examine a lens more closely?
  • How can I determine which lenses in an objective are decentered?
  • How can I correct a detected decentering?
 
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It would make sense for digital photographers and hybrid users to discuss the possibilities of image correction, particularly in Photoshop. There are powerful image correction tools available, such as „lens blur“ with which you can improve the sharpness of images or parts of images.

This is only a correction using software that interpolates image information and thus also creates errors, but it is a common method in practice.

Perhaps I will start a thread about this in the appropriate forum.
 
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Most lenses have one or more elements that are adjusted to correct centering.
Click to expand...
An element can also be tilted to one side or another … High quality lenses usually have one or more elements on which the tilt can be adjusted by two or three elliptical collars.
Click to expand...
The third problem that can occur with lens elements is spacing.
Click to expand...

Testing for a Decentered Lens: an Old Technique Gets a Makeover

What is Decentering and What Does it Do? Strictly speaking, decentering would involve one or more of the lens elements being off of the central axis of the lens. This would prevent the curved surfaces of the lens from bending the light properly. In severe cases it could result in halos or...
www.lensrentals.com


This is where it becomes challenging for the DIYer
 
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Andreas Thaler said:
This is where it becomes challenging for the DIYer
Click to expand...
Exactly !
Assume you can detect that the optics as a whole suffers from decentering and/or tilt.
  • How do you know which element is tilted? or decentered?
  • Is there an adjustment foreseen in the mount for that defect (unlikely)
  • If you passed the first two hurdles, how can you adjust that element in the heart of a zoom while monitoring quality improvements?
 
As indicated in the linked article, a collimator can be used to confirm centering. With the Pearl, the 4 corners can usually be visualized and compared for evenness.

In my experience, lens decentering shows up from mechanical damage or failure to re-assemble correctly after attempted repair.

This lens was repaired by re-bending the obviously bent 'foot' which mated two pieces of the inner housing.



 
Yes, same principle as here but rotating the lens under the collimator.

Laser Alignment: Is the Laser Perpendicular?

Since there were a few threads on enlarger alignment I thought I'd show this image that explains the how you can tell if your enlarger is out of alignment, or if your laser light is not perpendicular. The people that sell the laser light don't show you this because 'the laser light will always...
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  • What setup/equipment do I need to examine a lens more closely?
  • How can I determine which lenses in an objective are decentered?
  • How can I correct a detected decentering?
Click to expand...

I will summarize what information I have so far to answer these questions:
  • Which lens in an objective is misaligned is not important, as many lenses have dedicated optical elements that can be adjusted for alignment. Often these are the front and rear lenses. Alternatively, the lens could be disassembled and reassembled to correct any irregularities when inserting the lenses.
  • Zooms increase the complication due to their complex design and adjustment options.
  • Lenses can be examined on decentering using test shots/test charts or alternatively using a centering collimator.
  • You can try to correct decentering by adjusting the optical elements provided for this purpose, see Michael's @forest bagger procedure. This requires information and partial dismantling of the lenses. I estimate the chances of success for DIY to be 30 : 70 at best.
  • In digital and hybrid photography, you can try to improve blurring in the image by editing the image.
  • More modern lenses require testing equipment and software from the manufacturers. There is nothing for DIYers to do here.

Feedback very welcome!
 
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The Nikon AF 35-70/2.8 Zoom-Nikkor, a slide zoom, is known for a foggy lens in the rear area (not the rearmost lens block) which can only be replaced after removing the rearmost lens block.

And this lens block is attached with three screws whose shafts are thinner than the respective holes in the mounting plate.

According to Nikon it must always be adjusted horizontally.

Unfortunately, I had to do this several times, even though I had memorized and marked exactly how to reassemble the lens block when removing it... a terrible job!
 
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