Testing lens optical alignment

Typically lens are performing very close to the designed limits and producing great shots. Each lens has strong and weak points, and knowing that helps to use equipment in most efficient way.

I like sharp images, and I like soft images – it depends on the shot goal. But in both cases I’m expecting optics to be properly aligned and matching the expectations. Sometimes purchased lens may produce very bad results just because it’s defective sample. I’d like to share how to check that and my personal experience trying to fix the problem.

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Two years back I’ve purchased Samyang 35mm F1.2 lens. It’s awesome glass with quite advanced optics tha tincludes ED and Aspherical elements. Unfortunately my sample produced very disappointing results. I was shocked and depressed. Lens just didn’t match the expectations and shots looked very different from what I explored online before the purchase.

Further research discovered that lens had optical misalignment. I’ve found great and very helpful article on Lensrentals written by Roger Cicala. It clearly explains how to check for misaligned lens element that is significantly reducing image quality.

I’ve used this Siemens Star Chart PDF file from Wikimedia Commons shared by John Merlin Williams (and mentioned in another Lensrentls article). I’ve simply put camera on tripod facing the center of Star Chart on the laptop screen.

It helped to quickly notice that Samyang 35mm F1.2 sample has a defect. So that day I’ve simply returned the lens to the seller for a replacement, and my second sample performed great.

Checking 7artisans 50mm F1.8 lens

Recently I’ve purchased 7artisans 50mm F1.8 lens ( and reviewed it in this article ). This lens is very inexpensive, so I didn’t expect outstanding image quality. But first shots at F1.8 were very disappointing. Here are 100% crop centers of them, you can see heavy softness and rough OOF area rendering.

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Initial thought was that it’s very bad lens, and problem is connected to the low-budget price. But I already own few another lens of this brand: 7artisans 35mm F1.2, 7artisans 12mm F2.8, 7artisans 55mm F1.4 . They all behave relatively sharp in the center and producing good results for the price. So few days later I’ve found time to test the lens on Siemens Star Chart. And results were kind of expected. Below are 100% crops of the shots at different focusing distances.

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You can see that even first shot is not very sharp, and next two shots are indicating heavy optical shift – the dark circle is moving to the bottom-left corner. Looks like the rear glass elements are located off the central axis.

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If you experience similar results – simply return your lens for replacement. I’d not recommend to move with next repair step unless you are knowing well what to do, and have lens assembly experience.

Fixing optical alignment

 

I’ve disassembled the rear area of the lens, extracted all three glass elements. Then I’ve put lens on the flat surface facing down and inserted glass back.

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I had to repeat same procedure many times until optical test shown much better centering results. You can clearly see on the test below that blurred black circle remains in the image center (comparing to coma-like look before alignment).

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Image shots immediately shown huge quality improvement. Objects become very sharp even at wide open apertures (F1.8 and F2.8 100% crops below)

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Conclusions

Numerous third party lens may have various assembly quality. So it’s important to check if optical alignment is correct before evaluating IQ of purchased lens sample. This easy test shows if the lens is defective, so you can timely ask for replacement or return bad sample.

Initially I also wanted to return my 7artisans 50mm F1.8 lens, but managed to fix the alignment and finally got positively surprised with great picture quality. If you purchase proper sample it will be great performing lens. And now you know how to check if your lens is good.

17 thoughts on “Testing lens optical alignment

    1. Thanks for interesting question. All new 7artisans lens I purchased came in black box sealed with stickers. I think it means they are coming directly from the factory. So I think it does not really matter from where to buy if box is still sealed – it should be same probability as e.g. purchasing from the eBay. I personally purchasig from eBay because it has free return policy if lens is defective, and in this case Seller is paying for return shipping. That’s why I’d recommend to purchase this particular lens from place where you can get a refund or return the lens.
      To answer question on good/defective rate overall for 7artisans 50mm F1.8 more statistics is required. I only know about single defective lens so far.

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  1. Hello. Very interesting article. I have a Zonlai 22mm f1.8 on the way from China for my Fujifilm camera and your fix may be useful for me (hope will not be necessary anyway). Zonlai brand seems to be under the same Shenzhen Qigongjiang Photoelectric Technology Co. Ltd. company as is 7Artisans (see http://www.7artisans.com/en/h-nd-36.html?groupId=-1 ) and perhaps my lens could suffer similar assembly problems (see this review: https://www.youtube.com/watch?v=iCbXogFRPy4 ) where the reviewer mention diferences in focus from one -left or right- side to the other side. Do you think that the problem mentioned there could be also due to lack of alignment?

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    1. Hello Fernando. Thanks for sharing interesting details on the company, I didn’t know about that brands connection. It makes sense, and now looks to me like Samyang/Rokinon/Bower Korean brands that are producing same lens. I was considering trying 7artisans 25mm F1.8, but recently received 7artisans FIshEye and still playing with it : ) I think you are right, that review lens sample may have decentered glass. It’s easy to check using the Siemens Star chart like it’s described in Lensrentals articles (e.g. https://www.lensrentals.com/blog/2012/05/testing-for-a-decentered-lens-an-old-technique-gets-a-makeover/). I’m just reusing their technique when checking my lens samples. There is a big chance that your sample will be ok. I personally think that “QC” or “Tested” marks are not just nice to have letters in package, probaly lens are really tested on factory. However often it can be limited “sampling testing” where 5of100 lens are checked. Also the shipment itself may affect the lens. I heard about cases when front glass element was not completely secured on factory and buyer just did it to fix the problem.

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      1. Thank you for your answer. The Zonlai lens will take a couple of weeks to arrive here, lets hope will be fine. In the mean time I have been making some further research about Zonlai and maybe is not really related to 7Artisans. I went to the chinese site of Zonlai and the translation of the company history (http://www.zonlai.com/col.jsp?id=101) says something different than 7Artisans’s (http://www.7artisans.com/en/h-col-196.html).
        Zonlai only has 3 lenses currently:
        25mm f1.8 released on September 2015;
        35mm f1.8 released on November 2015;
        and a
        22mm f1.8 released on November 2017.

        The 25mm has two versions, the original from 2015 and a newer from 2016. Only this last 2016 version looks identical to the 25mm from 7Artisans. It’s not clear the reason of a newer version. Here you can see the two 25mm versions and also the 22mm: https://img.alicdn.com/imgextra/i2/12485027/TB2H4G9pbSYBuNjSspfXXcZCpXa_!!12485027.jpg

        Here is possible to see comparison picture of the current 25mm and the 22mm, sadly in chinese, but after some google translation, the 22mm seems an improvement in some aspects:

        The pictures links where taken from the chinese site Taobao where there is more info like the lens structure and other details. https://item.taobao.com/item.htm?id=561234533003

        Ok, thanks again and sorry the elaborated -and off topic- of this answer.

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  2. I just got two of these lenses and both were out of whack; one badly pointing up and to the left; the other pointing slightly down. In both cases, sharpness suffers. Avoid this lens unless you like opening up and disassembling lenses.

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    1. Hi Sam, thanks a lot for sharing your experience, it sounds like problem is more frequent than I was expecting. 2 of 2 bad samples is a bad rate, hopefully you have option to return them, each seller technically should accept return of such defective samples.
      BTW, I’ve already wrote to 7artisans about this bad quality problem with 50mm F1.8, I think they should be interested to do something about fixing it and selling lens with proper alignment. I like the lens performance once it’s properly calibrated, but I’d definitely not tolerate sharpness degradation of defective sample.

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  3. One Question 🙂 could you please do the same test with your version of 35mm f1.2 since I just got mine and its not that bad even at f1.2 but I would like to repeat and follow your steps in testing so I can be sure I am not messing something up and returning it for no reason. I see one dust particle or it could be scratch on one of the inner glass elements I am not sure…

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    1. I assume you are asking about 7Artisans 35mm F1.2 – I tested that lens and optical alignment is good there. If you are asking about Samang/Rokinon 35mm F1.2 – I don’t have that lens anumore, because after discovering misalignment I sent it back to the seller.
      I’d not worry about dust particle, but I’d definitely use stron flashlight to investigate if it’s not a scratch ot factory defect inside.

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      1. Yeah I was talking about 7Artisans 35mm f1.2 and I did quick test and I didnt see misalignment so now I need to try to move this speck of dust in side the lens to see is it actually dust or scratch… Thx for your time 🙂

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  4. I had the same misalignment problem with 7artisans 50mm F1.8.
    The second spacer ring at the rear group was inserted incorrectly. The fix is ​​simple and last 2 minutes.
    Now is lens great.

    Liked by 1 person

  5. Thank you for writing this article. I have the 7artisans 50mm 1.8 and it had terrible ghosting and softness wide open, but I was able to fix it thanks to your article.
    I only had to try it once to get it right. It took me only a few minutes. I didn’t test it on the Siemens Star Chart, I only took some pictures after performing the realignment and I was very satisfied with the results. There was a huge difference in sharpness and image quality. Thanks again.

    Liked by 1 person

    1. Hi Jorge, you are very welcome! It’s great that you improved lens performance with little effort, and now can take much better pictures. Thank you for feedback and sharing your experience.
      Yuri

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