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FM Lens test for Centering (Mount tilt / swing)

  
 
Fred Miranda
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p.2 #1 · p.2 #1 · FM Lens test for Centering (Mount tilt / swing)


dljor23 wrote:
Hello

What would the difference between this test and just take four pictures with the subject at each corner without refocusing? If we are just comparing the sharpness of the subject?

Thanks.



When you flip the camera and compare the same exact framing, small differences become very apparent. Try it and you will see what I mean. You can do what you described but I recommend finding a long distance subject so alignment does not become a variance in the test.



May 23, 2018 at 11:30 PM
dlaslo
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p.2 #2 · p.2 #2 · FM Lens test for Centering (Mount tilt / swing)


The example is a "good" centered lens. Are there examples somewhere of "bad" or maybe "passing" ? I haven't done my test yet, but I'm not sure what to do with it.


Jul 06, 2018 at 03:38 AM
Beni
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p.2 #3 · p.2 #3 · FM Lens test for Centering (Mount tilt / swing)


Question, how do you manage to turn the camera upside down while maintaining the same exact plane of focus? you would almost certainly induce some kind of swing or tilt while doing it. Unless you're focusing at infinity and making sure it really is infinity? Not the easiest thing to do with AF lenses which don't have a 'hard' infinity stop.


Jul 06, 2018 at 08:41 AM
s4ducati
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p.2 #4 · p.2 #4 · FM Lens test for Centering (Mount tilt / swing)


This is such an important topic and I'm really happy to see it discussed here.

I've been reading and following the blog written by Roger Cicala at LensRentals.com. He covers this exact topic in very specific detail. I have downloaded and saved this and refer to it regularly, There's a lot of detail that needs to be followed to perform this test and have meaningful results. See it here:

https://www.lensrentals.com/blog/2014/02/setting-up-an-optical-testing-station/

https://www.lensrentals.com/blog/2014/02/examples-using-an-optical-testing-station/


Yesterday I set up this test for two of my wide Rokinon's; a 14mm f/2.8 ($200) and a 24mm f/1.4 ($350). Both shot on a Sony a7rii. Attached are the results of both lenses.

I also recently purchased a Zeiss Loxia 21mm f/2.8 ($900). Attached are the results too.









Rokinon 14mm f/2.8 slightly out of focus, Look at the center circle only







Rokinon 14mm, in focus, Look at the sharpness of the center as compared to the four corners.







Rokinon 24mm f/1.4 slightly out of focus







rokinon 24mm f/1.4 Look at sharpness of center vs the four corners







Zeiss Loxia 21mm f/2.8 in focus







Zeiss Loxia 21mm f/2.8 out of focus




Jul 28, 2018 at 08:01 AM
Fred Miranda
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p.2 #5 · p.2 #5 · FM Lens test for Centering (Mount tilt / swing)


Beni wrote:
Question, how do you manage to turn the camera upside down while maintaining the same exact plane of focus? you would almost certainly induce some kind of swing or tilt while doing it. Unless you're focusing at infinity and making sure it really is infinity? Not the easiest thing to do with AF lenses which don't have a 'hard' infinity stop.


At infinity distance, this is not an issue at all. That's one of the benefits of testing the lens at long distances instead of charts at close distance.



Aug 11, 2018 at 09:54 AM
s4ducati
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p.2 #6 · p.2 #6 · FM Lens test for Centering (Mount tilt / swing)


Here is another blog site that covers lens testing.

The process is similar to how Fred is testing; at long distance so alignment becomes insignificant.

https://blog.kasson.com/lens-screening-testing/equipment-and-software/

I have not yet setup this test, but plan to so I can compare the results with the ISO charts posted above.




Aug 11, 2018 at 10:20 AM
hiepphotog
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p.2 #7 · p.2 #7 · FM Lens test for Centering (Mount tilt / swing)


Fred Miranda wrote:
Hi all,
I have posted my personal lens decentering/test technique in another thread but it's difficult for some members to find it. I reposted it here so it's easy to bookmark of link to it.

------------

Here is a testing method I developed a while back that gives me consistent results and takes less than a minute to perform. Since you will be shooting your target at infinity distance, alignment isn't crucial and therefore won't be a variable to worry about. (Like when using test charts at close distance)

I usually test under harsh lighting and very high shutter speeds so I can
...Show more

Hi Fred, I showed this guide to a few others without realizing that you recommended to test it stopping down to at least 1-stop. I have done this kind of test only at WO and the result has always been definitive regardless of aberrations. I feel like with DOF at infinity, it can really mask the problem when you stop down, especially when you stop down more than 2 stops.



Aug 26, 2018 at 05:19 PM
Fred Miranda
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p.2 #8 · p.2 #8 · FM Lens test for Centering (Mount tilt / swing)


hiepphotog wrote:
Hi Fred, I showed this guide to a few others without realizing that you recommended to test it stopping down to at least 1-stop. I have done this kind of test only at WO and the result has always been definitive regardless of aberrations. I feel like with DOF at infinity, it can really mask the problem when you stop down, especially when you stop down more than 2 stops.


From my experience, it really depends on the lens. With modern primes, I usually test wide open as well.
Some lenses, especially vintage ones need at least one stop for the corners to come alive and resolution to show a pulse. If the corners are mushy, it's harder to evaluate centering.



Aug 26, 2018 at 05:59 PM
DrShouter
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p.2 #9 · p.2 #9 · FM Lens test for Centering (Mount tilt / swing)


I am currently testing the 25mm Loxia and get very mixed results. In one test situation (chimney 50m away) everything looks good, in another test situation (other chimney, maybe a little bit more than 50m away) one corner looks much worse than the other three. What could cause the problem? IBIS? EFCS?


Aug 30, 2018 at 01:55 AM
hiepphotog
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p.2 #10 · p.2 #10 · FM Lens test for Centering (Mount tilt / swing)


DrShouter wrote:
I am currently testing the 25mm Loxia and get very mixed results. In one test situation (chimney 50m away) everything looks good, in another test situation (other chimney, maybe a little bit more than 50m away) one corner looks much worse than the other three. What could cause the problem? IBIS? EFCS?


That's why you want to make sure it's an infinity test. It sounds to me that you're dealing with the limited depth of field. The best way to ensure you're at a sufficient distance is to focus on clouds on the sky then go back to test scene. If most of the frame is out of focus, you're not quite there yet. IBIS probably should cause most (if not all) of the frame fuzzy. EFCS should not be the cause for any softness.



Aug 30, 2018 at 01:02 PM
 


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Fred Miranda
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p.2 #11 · p.2 #11 · FM Lens test for Centering (Mount tilt / swing)


DrShouter wrote:
I am currently testing the 25mm Loxia and get very mixed results. In one test situation (chimney 50m away) everything looks good, in another test situation (other chimney, maybe a little bit more than 50m away) one corner looks much worse than the other three. What could cause the problem? IBIS? EFCS?


Do the test using electronic shutter (silent shutter) and turn IBIS "off". As long as your shutter speed is very high (mid-day exposure) you don't even need a tripod.



Aug 30, 2018 at 01:14 PM
smpetty
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p.2 #12 · p.2 #12 · FM Lens test for Centering (Mount tilt / swing)


Fred,

A couple of questions...

1) I'm sill not certain that I understand why putting the camera on a tripod and taking four shots of an object at infinity framed in each corner (no tilting) wouldn't work as well. The variable that I am concerned with is the degree of tilt and whether I can reliably reproduce it. Why the tilt?

2) Turning the camera upside down, framing, and not touching the focus rings is a shaky proposition for me, literally. What shutter speeds would you recommend as a minimum for sharp images? I realize this is photographer dependent.

3) I just received my fourth Loxia 25/2.4. One was decentered and two had strong field tilt. Using your method, should I shoot wide open? Should I focus on center or choose one of the corners as a focus point?

Scott




Aug 30, 2018 at 07:54 PM
nandadevieast
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p.2 #13 · p.2 #13 · FM Lens test for Centering (Mount tilt / swing)


A naive question:
If i focus seperately on four different corners and they come out sharp, what does it prove?



Sep 01, 2018 at 03:40 AM
smpetty
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p.2 #14 · p.2 #14 · FM Lens test for Centering (Mount tilt / swing)


nandadevieast wrote:
A naive question:
If i focus seperately on four different corners and they come out sharp, what does it prove?


Simply that all four corners can be brought into focus. Decentering and field tilt are best detected by comparing all of the focus points relative to another - usually the center point or one corner.

Yesterday was a banner day for me and lens testing. I had bought, tested, and returned three copies of the Loxia 25/2.4. The first copy was decentered - the bottom right corner of the lens (and less so the entire right and bottom sides of the field) was very soft whether I used the center or any of the other corners for a reference point. The second and third copies both had significant field tilt, such that if I used the top or bottom left corner as a reference, the right corners were soft, and vice versa.

I was beginning to doubt my testing methodology (Jim Kasson's Zeiss Star methods).

Yesterday I tested my fourth Loxia 25 copy and it was tack sharp and perfectly centered - a dramatic difference between this copy and the other three I had tested using exactly the same methods and settings. It validated not only my testing methods but also my strong belief, based on experience, that many new lenses, primes and zooms, are wonky. At the prices we pay and for the tolerances demanded by the high-resolution sensors we use, not testing one's lenses seems foolish.

I also tested a used Loxia 85 that I received from Matt via the FM Buy & Sell boards and it was perfectly centered and tack sharp. Thank you, Matt! My third test was of a new Sony 90 macro (I got tired of waiting for the CV 110) and it was perfect too. All in all, a very good day!



Sep 01, 2018 at 08:37 AM
Fred Miranda
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p.2 #15 · p.2 #15 · FM Lens test for Centering (Mount tilt / swing)


smpetty wrote:
Fred,

A couple of questions...

1) I'm sill not certain that I understand why putting the camera on a tripod and taking four shots of an object at infinity framed in each corner (no tilting) wouldn't work as well. The variable that I am concerned with is the degree of tilt and whether I can reliably reproduce it. Why the tilt?

2) Turning the camera upside down, framing, and not touching the focus rings is a shaky proposition for me, literally. What shutter speeds would you recommend as a minimum for sharp images? I realize this is photographer dependent.

3) I just received
...Show more

1) It would be much easier to visualize decentering since we would be inspecting the same exact subject target 'and' its identical orientation.
2) Just be careful not to touch the focusing ring. It's not that hard. Regarding the shutter speed, I usually test lenses mid-day at very high shutter speeds (~1/4000s) so tripod is never needed...if I can't achieve that speed, bumping ISO slightly is not a problem
3) I got lucky with my Lox 25 and it was perfect at first try. It took 3 copies for the Lox 21 though. I have purchased a couple lenses from the Buy and Sell board where the sellers told me they used Jim's test and they were 100% perfect just to find out they were decentered using this one. Not sure if it was user error though.

I can tell you from experience testing many lenses that if this test shows decentering (element tilt), the result will be consistent and always accurate. I like that because it saves me time double testing a lens.



Sep 01, 2018 at 10:04 AM
David Cartagena
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p.2 #16 · p.2 #16 · FM Lens test for Centering (Mount tilt / swing)


Fred Miranda wrote:
I could not find any of my tests with this lens so I went outside and captured 4 images. (took me about 15 seconds)
The lighting was horrible with the sun showing in the frame..
Upload them to Lightroom and compared the crops. The Loxia 21/2.8 was shot wide open. (for this lens it's ok)
I forgot to set exposure to manual mode and it was set to Aperture priority. Therefore exposure won't be identical for all crops...Still good for this comparison though.

Here are the results following the methodology. To me, this lens is very well centered...and sharp to the edges
...Show more

Hi.

Would just like to show that a Loxia can certainly be decenteret.
Mine is badly unsharp on one side of the frame 1/4-1/2 of the frame.

https://www.dpreview.com/forums/post/62285549?image=0



Feb 09, 2019 at 12:47 PM
Fred Miranda
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p.2 #17 · p.2 #17 · FM Lens test for Centering (Mount tilt / swing)


David Cartagena wrote:
Hi.

Would just like to show that a Loxia can certainly be decenteret.
Mine is badly unsharp on one side of the frame 1/4-1/2 of the frame.

https://www.dpreview.com/forums/post/62285549?image=0


Of course they can.

From my experience, only the Loxia 50/2 has low variance. Having said that, I have never seen a badly decentered (element tilt) Loxia lens.



Feb 09, 2019 at 04:14 PM
David Cartagena
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p.2 #18 · p.2 #18 · FM Lens test for Centering (Mount tilt / swing)




Fred Miranda wrote:
Of course they can.

From my experience, only the Loxia 50/2 has low variance. Having said that, I have never seen a badly decentered (element tilt) Loxia lens.


Thanks. My impression before buying it was that it was a very good quality lens. Well that can be discussed since I have found several that had to go through 2 or more Loxia 21 lenses before they found a great one.

Please look at my link, then you will see it.



Feb 09, 2019 at 08:43 PM
AZSteve
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p.2 #19 · p.2 #19 · FM Lens test for Centering (Mount tilt / swing)


Fred, since this thread has been revived I want to mention that I recently had an experience with your centering test methodology versus Jim Kasson's Siemens-star test, where a lens that appears to pass the latter fails the former. (Your post of 1 Sept.)

B&H had a one-day deep sale on the Sigma Art 85mm and I took the bait. The first four images I took were Siemens stars in each corner, and then I went on to images of nearby mountains, a slight variation on your procedure. The stars looked passably similar and I was much relieved that I'd been sent a good lens. Then I looked carefully at the mountain images and discovered that the left edge was substantially softer than the right, and that the difference was still visible stopped down as far as f/5.6. I retested on the mountains and got the same thing. The lens would have been optically fine for portraits, sports, "street", etc, but not for landscapes. Back it went and I didn't try another.

Maybe it is easy to miss small differences in the imaging of the Siemens stars, I don't know.




Feb 10, 2019 at 11:53 AM
DaveFP
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p.2 #20 · p.2 #20 · FM Lens test for Centering (Mount tilt / swing)


How about the LED test? Set one of those LED microlights on a shelf in a dark room.

Set to MF and focus/defocus the LED.

In theory the resulting blur of light should be symmetrical if centered.

Valid?



Feb 10, 2019 at 12:10 PM
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