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Do-it-yourself centering sanity check


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#1 Klaus

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Posted 05 April 2011 - 08:27 PM

Hi all,

we're frequently asked how to check the centering quality of a lens. There's, of course, no simple check which covers the whole scope. However, here at PZ we've a quick and dirty "sanity" check for new lenses that you could also use.

This sanity check covers only ONE KIND of centering - "shifted" elements.

Now what is that ? Let's have a look at the following 100% sample crop from an IMAGE CENTER:

Posted Image


You may notice that there are two soft edges and two "fairly" sharp ones. This is a centering defect "classic" which covers about half of the centering defects out there.
Such kind of decentering occurs mostly at the tele end of a zoom lens. This one was taken from a Sony 28-75/2.8 @ 75mm @ f/2.8. That said every once in a while this does also happen with prime lenses.

How to check ? Now take the following template:

Posted Image



It's best to download the image and show it full screen at 100% - no more, no less.

This is, obviously, the "perfect shot" (artificially generated, of course).

Dirty approach:

1. take your lens, attach it to your camera, SELECT MAX APERTURE and switch to LiveView mode
2. go to the max. tele setting (if applicable)
3. move back from the screen (depends on the lens, say roughly e.g. 3m for a 50mm lens. You should NOT be able to spot any screen pixels anymore in magnified LiveView mode)
4. enlarge the LiveView mode to maximum magnification (e.g. 10x, 15x whatever your camera offers here)
5. DEFOCUS the lens
6. point the center of the lens straight to the center of the test image. MAINTAIN the 5 degree tilt of the edges!

Now the important part:
7. FOCUS SLOWLY towards the focus and OBSERVE how the 4 edges GET SLOWLY SHARPER

A well-centered lens should have a strictly symmetrical focus blur as well as a strictly symmetrical sharpness once you reached focus. If you got a decentered lens you will already notice that there's no strict symmetry anymore once you approach the focus. It's actually slightly easier to observe in slighly defocused images.

This does not work on the Pentax K-5 and, I suspect, other Pentax DSLRs.

If you want it correctly - do it with a tripod and take a picture with LiveView AF and check the result (this does, of course, work with Pentax DSLRs).

Just give it a try .... Posted Image

You can simply upload it to you laptop, take it to the shop and do it on location when buying a new lens.

PS: No, this is no late April's fool joke.







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#2 popo

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Posted 05 April 2011 - 08:40 PM

I take it this requires the sensor plane to be exactly parallel with the target plane. Is there an easy way to tell in case of operator error? Would having non-parallel planes look different from this type of decentering?

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#3 Klaus

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Posted 05 April 2011 - 08:52 PM

I take it this requires the sensor plane to be exactly parallel with the target plane. Is there an easy way to tell in case of operator error? Would having non-parallel planes look different from this type of decentering?



Yes, the setup should be as perpendicular as possible.

Remember that you must keep a distance - this helps in this respect of course.


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#4 IanCD

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Posted 05 April 2011 - 11:01 PM

Yes, the setup should be as perpendicular as possible.

Remember that you must keep a distance - this helps in this respect of course.


Brilliant! Thanks Klaus,
This looks like it's going to be really helpful. Just gave it a quick 'n dirty try on the 16-85. Great. Roughly perpendiclar seems to work well enough to get an idea :)

...bit challenging at that kind of magnification to keep the camera/lens still without a tripod, though..! :D
Ian

#5 AlexanderE

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Posted 06 April 2011 - 01:11 AM

Why doesn't it work for the Pentax K-5?
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#6 Klaus

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Posted 06 April 2011 - 06:27 AM

Why doesn't it work for the Pentax K-5?


Because it shows an asymmetric LiveView image per se.
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#7 Ayoh

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Posted 06 April 2011 - 07:23 AM

Because it shows an asymmetric LiveView image per se.


What do you mean by that?

#8 Klaus

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Posted 06 April 2011 - 07:44 AM

What do you mean by that?


Horizontal structures are shown much softer than vertical ones (or vice versa ? Anyway).
Consequently this cannot be used for this test because you require a strict symmetry here of course.
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#9 Vieux loup

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Posted 06 April 2011 - 09:56 AM

Klaus, thank you very much for this really helpful tip. I will test all my lenses this way and let you all know the bad or good news :) ! Is there any reason the image has to be shown 100%? (probably a stupid question, but forgive me pls) If you have any other tips for us, I believe there are lots of takers.
Kind regards, Vieux Loup

#10 Klaus

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Posted 06 April 2011 - 10:12 AM

Klaus, thank you very much for this really helpful tip. I will test all my lenses this way and let you all know the bad or good news :) ! Is there any reason the image has to be shown 100%? (probably a stupid question, but forgive me pls) If you have any other tips for us, I believe there are lots of takers.


The 100% scaling is just to avoid scaling artifacts which may simply be even more obvious that the already existing pixel steps on a screen. Again, just keep a significant distance to avoid this as well as aliasing effects (which simply make it more difficult to judge the situation).
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#11 thw

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Posted 06 April 2011 - 12:33 PM

Brilliant! Thanks for the share! :D

#12 Vieux loup

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Posted 06 April 2011 - 01:48 PM

So far I have been able to ensure that my Nikkor 50mm f1.4 and my Nikkor 35mm F2 are perfectly centered fully open. I tried through the viewfinder instead of in live view and it also works fine, although you maybe don't see the development quite as clearly, but the VF of the D700 is so luminous that it was no problem. Easier in a store to just use the viewfinder of course. I am quite confident of my primes, but the zooms from Sigma and Tamron may be a diffrent story. I'll keep you posted.

I have now also tried the 105mm f2.8 and the Sigma 24-70 f2.8 and they both seem absolutely fine, allthough I just use the viewfinder. I'll try it all with the tripod later, but I do not expect any different results.B)
Kind regards, Vieux Loup

#13 mst

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Posted 06 April 2011 - 02:31 PM

I tried through the viewfinder instead of in live view and it also works fine, although you maybe don't see the development quite as clearly, but the VF of the D700 is so luminous that it was no problem.


The viewfinder, especially when not used from a tripod, will not allow reliable results. As Klaus described, one should use a stable tripod and LiveView with large magnification to be sure a lens is ok.

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#14 Klaus

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Posted 06 April 2011 - 02:47 PM

The viewfinder, especially when not used from a tripod, will not allow reliable results. As Klaus described, one should use a stable tripod and LiveView with large magnification to be sure a lens is ok.

-- Markus


Yep, the viewfinder is rather useless for this.
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#15 Klaus

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Posted 06 April 2011 - 05:32 PM

I was asked why it is easier to spot the problem in a defocused image.

Here's a reference again from the Sony lens.
You can surely spot that the to edges to the top and right appear to be "closer to focus" than the other two which are much softer.


Posted Image


Here's the in-focus reference again:

Posted Image

Just to mention - this is not a rule for the image borders. This is only a sanity check which can be performed on the image center. At the borders we've all kinds of funny aberrations (optical defects) which can, more or less, resemble a centering issue (e.g. field curvature).
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#16 Vieux loup

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Posted 06 April 2011 - 11:02 PM

OK! :rolleyes: So I'll redo it all in Live view and with a tripod.
Kind regards, Vieux Loup

#17 DenisLV

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Posted 24 August 2012 - 08:52 AM

Hi Klaus,
I understood everything except point 6. - MAINTAIN the 5 degree tilt of the edges!

What does it mean?

Kind regards
Denis

#18 Rainer

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Posted 24 August 2012 - 07:42 PM

I understood everything except point 6. - MAINTAIN the 5 degree tilt of the edges!


In the testimage, the lines are not horizontal and vertical. they are rotated by
5 degrees.

Now, if you setup your camera on the tripod, you could easily apply the
same 5 degrees to the camera so that the lines in the resulting image
would be vertical and horizontal ...

Point 6 says: Do NOT do this ... keep the camera horizontal, so that
the image taken withg the camera shows the same 5 degrees
rotation that was also on the testimage on the computerscreen.

#19 DenisLV

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Posted 25 August 2012 - 10:02 AM

In the testimage, the lines are not horizontal and vertical. they are rotated by
5 degrees.

Now, if you setup your camera on the tripod, you could easily apply the
same 5 degrees to the camera so that the lines in the resulting image
would be vertical and horizontal ...

Point 6 says: Do NOT do this ... keep the camera horizontal, so that
the image taken withg the camera shows the same 5 degrees
rotation that was also on the testimage on the computerscreen.

Thanks Rainer

#20 ouki

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Posted 05 October 2013 - 12:31 AM

Great info. Thank you.

 

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