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 quality base. 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.
Two years back I’ve purchased Samyang 35mm F1.2 lens. It’s awesome glass with quite advanced optics that includes 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 Lensrentals 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. 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.
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.
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.
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.
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 on second image row).
Bad alignment before correction - Diagonal optical shift is causing IQ degradation
Corrected alignment
Image shots immediately shown huge quality improvement. Objects become very sharp even at wide open apertures (F1.8 and F2.8 100% crops below)
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.
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