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How to check if a lens is a "good copy"?
  
 
Kirin
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p.1 #1 · p.1 #1 · How to check if a lens is a "good copy"?


I just purchased a Voigtlander 10mm lens and have read that there is a lot of sample variation with these lenses. I was hoping for some advice on how I should go about determining if my copy is a good one or not. Normally I would just zoom in and check sharpness etc across the frame, but with a lens this wide it seems a little tricky because of the extreme distortion.

I guess I should go and shoot a brick wall or something?

Thanks for any advice.



May 01, 2017 at 01:25 PM
mcbroomf
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p.1 #2 · p.1 #2 · How to check if a lens is a "good copy"?


There are a lot of examples of tests done in the 15mm E Voightlander thread
https://www.fredmiranda.com/forum/topic/1425842/17

Basically find a far scene and focus infinity @ center (optically not necessarily the hard stop), tilt the camera and examine all 4 corners (need clear detail in each corner)



May 01, 2017 at 01:47 PM
taperwood
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p.1 #3 · p.1 #3 · How to check if a lens is a "good copy"?


First, understanding that there is no perfect lens, I use the following test:

Use a tripod and remote trigger or the camera's timer release, set up outdoors in light sufficient to shoot wide open at lowest ISO setting. A bright cloudy day works best for flat, uniform light.

With the lens wide open, center focus on a somewhat complex object over infinity for that lens (I focus on a power line transformer down the street). Using careful manual focus and after center focusing on the object, without touching the lens, place that object in each of the four corners and take a shot. Examine shots.

If all four corners are equally the same or very close, I consider the lens good (they will not necessarily be sharp, you are looking for consistency). You can further test the lens at closest focus and if that is equal too, then you have a very good lens, but I rarely see that. The more important test is the infinity focus.

Only you can decided how much variation you will tolerate, but remember that no lens is as sharp in the corners as it is in the center. Once I decide a lens is acceptable, I never think about it again.



May 01, 2017 at 04:03 PM
jankap
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p.1 #4 · p.1 #4 · How to check if a lens is a "good copy"?


One seldom takes fotos of brick walls.
So spherical aberration is not the problem.
The problem is the mechanical construction of the lens. We have to look after decentering.
In short:
One takes 4 pictures (manual mode), each in a different corner the same subject.
Turning the camera (upside down f.e.) is allowed of course.
Then check, if all 4 corners are (un)sharp in the same way.
Also if the subject would be unsharp, look at the difference.
There also is a candle light method, see Jim Kasson "The Last Word".
Jan



May 01, 2017 at 04:14 PM
Kirin
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p.1 #5 · p.1 #5 · How to check if a lens is a "good copy"?


Thank you very much for the advice everyone! Much appreciated. I will try out the methods you suggested tomorrow.


May 01, 2017 at 05:00 PM
Parariss
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p.1 #6 · p.1 #6 · How to check if a lens is a "good copy"?


Warning - rabbit hole underfoot!...
...don't twist your ankle!

https://www.lensrentals.com/blog/2012/05/testing-for-a-decentered-lens-an-old-technique-gets-a-makeover/

https://www.lensrentals.com/blog/2010/11/how-to-test-a-lens/



May 02, 2017 at 06:58 PM
jankap
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p.1 #7 · p.1 #7 · How to check if a lens is a "good copy"?


Jim Kasson has worked on a simple lens test.
Please search for "Lens screening testing".
Jan



Sep 25, 2017 at 03:37 PM
 

Search in Used Dept. 



Charlie N
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p.1 #8 · p.1 #8 · How to check if a lens is a "good copy"?


the G test, I cant pronounce the name, basically the light pole test. Just find a light pole far away, grab a random corner in manual focus mode, wide open aperture. In manual exposure mode, I'de probably overexpose by 2 stops as light poles are usually dark or in the shade, lock in the settings and make sure you have suitable shutter speed or simply use a tripod. A good lens will look somewhat similar, but usually not perfectly similar. Just remember, that you want similarity, corners generally weaker so dont base everything corner performance, just how similar they are. Some differences are ok, but wild differences are not.

attached is a bad performing lens that I had to return







Sep 25, 2017 at 03:57 PM
Fred Miranda
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p.1 #9 · p.1 #9 · How to check if a lens is a "good copy"?


Charlie N wrote:
the G test, I cant pronounce the name, basically the light pole test. Just find a light pole far away, grab a random corner in manual focus mode, wide open aperture. In manual exposure mode, I'de probably overexpose by 2 stops as light poles are usually dark or in the shade, lock in the settings and make sure you have suitable shutter speed or simply use a tripod. A good lens will look somewhat similar, but usually not perfectly similar. Just remember, that you want similarity, corners generally weaker so dont base everything corner performance, just how similar they
...Show more

Which lens was that Charlie?



Sep 25, 2017 at 03:59 PM
Charlie N
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p.1 #10 · p.1 #10 · How to check if a lens is a "good copy"?


Fred Miranda wrote:
Which lens was that Charlie?


that was my cv 35 that I returned, the replacement not completely centered either, but considerably more acceptable.



Sep 25, 2017 at 04:06 PM
JimKasson
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p.1 #11 · p.1 #11 · How to check if a lens is a "good copy"?


jankap wrote:
Jim Kasson has worked on a simple lens test.
Please search for "Lens screening testing".
Jan


Here's a link:

http://blog.kasson.com/lens-screening-testing/

I'd appreciate it if a few folks would try this test and get me the raw files. I am particularly interested in seeing results from lenses that don't look like they are good copies. I don't have any badly-assembled lenses to test.

If anyone needs help with the instructions, please let me know. I'm in the process of inviting people to try the test with the idea of over time making the instructions clearer.

Warning: using this test, I can check out a lens in about 10 minutes of shooting (three sets of images), and 20 minutes in Lightroom, but it will probably take you an hour or two the first time you do it, counting the time spent reading all the instructions, printing the target if you don't already have a Zeiss Siemens Star, and looking at the examples.

Jim




Sep 25, 2017 at 04:09 PM
JimKasson
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p.1 #12 · p.1 #12 · How to check if a lens is a "good copy"?


Charlie N wrote:
the G test, I cant pronounce the name, basically the light pole test. Just find a light pole far away, grab a random corner in manual focus mode, wide open aperture. In manual exposure mode, I'de probably overexpose by 2 stops as light poles are usually dark or in the shade, lock in the settings and make sure you have suitable shutter speed or simply use a tripod. A good lens will look somewhat similar, but usually not perfectly similar. Just remember, that you want similarity, corners generally weaker so dont base everything corner performance, just how similar they
...Show more

The Gletscherbruch test has a lot to recommend it. It gets around the alignment issues of the flat-target tests. It has two main drawbacks.

Atmospheric effects.

Here on earth, performing lens testing at great distances puts a lot of air between you and your target. That air is subject to thermally-created changes in density that cause time-varying bending of light rays that result in uncontrolled and unpredictable blurring of images. When performing Gletscherbruch testing in unpopulated areas in still early-morning air, this can be dealt with for all but the longest lenses, but at other times of the day or in cities with unseen, and often unsuspected, heat sources, it can be a problem even at moderately short focal lengths. The time-varying nature makes it especially tricky when youíre looking for differences in the image sharpness at the four corners of the frame. Sometimes the blurring caused by the atmosphere can be greater than that of the lens and camera, making it impossible to see small lens defects.

Inappropriate target.

This is the same problem that you have with found flat targets. They have the wrong spatial-frequency makeup and they arenít repeatable. You canít spot focusing errors by inspection of the captures. You canít read out sharpness in any way thatís describable to others. You donít know what the images are supposed to look like with any specificity.

Jim



Sep 25, 2017 at 08:39 PM
k-h.a.w
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p.1 #13 · p.1 #13 · How to check if a lens is a "good copy"?


Kirin wrote:
I just purchased a Voigtlander 10mm lens and have read that there is a lot of sample variation with these lenses. I was hoping for some advice on how I should go about determining if my copy is a good one or not. Normally I would just zoom in and check sharpness etc across the frame, but with a lens this wide it seems a little tricky because of the extreme distortion.

I guess I should go and shoot a brick wall or something?

Thanks for any advice.


There is a lot of sample variation of the CV 15.
I have not seen a similar problem with the CV10, but it exhinits curved sun stars.



Sep 25, 2017 at 10:02 PM







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