This is why some people think a lens is awesome and others say it sucks

No matter what the lens, when I see people asking for opinions on one online, I see a pretty wide mix of “It’s awesome, it never leaves my camera!” and “It sucks, I sold it within a week” type responses. If you’ve ever wondered why there are such polarising opinions on any given lens, then wonder no more.

In this video, Michael the Maven discusses one of the things that’s rarely spoken about when it comes to lenses, and that’s manufacturing tolerances. These are the limits on either side of “perfect” in which a company will allow components to fall. But they can make for big differences in image quality.

Manufacturing tolerances are the reason why you often hear of phrases like a “good sample” or “bad sample”. The Sigma 10-20mm f/4-5.6 lens is a perfect representation of this. I’ve owned one for about 12 years now, and I think it’s fantastic. It has very little distortion, it’s very sharp for such a slow lens, and it suffers very little in the way of chromatic aberration.

I’ve recommended this lens to many people often over the years, but I do warn them that they may need to have a look at several of them before finding one they’re happy with. If you get a good one then it’ll be amazing. But if you get a bad one, you’ll hate it. In my experience, about one in five samples of this lens is good, while the other four are substandard compared to the one I own.

This variance is a direct result of manufacturing tolerances. But what are tolerances?

Forget lenses for a minute. If you want to produce a solid metal cylinder  that’s 50mm long and 15mm in diameter. That’s your “perfect” dimensions. But the machines that make this cylinder for you are imperfect. So, as the manufacturer, you might allow for a variance of 0.2mm in the diameter and length. This means that the length of your cylinder is now anywhere between 48.8mm and 50.2mm, and the diameter is somewhere between 14.8mm and 15.2mm. Good enough to do the job, but not perfect.

In reality, tolerances are usually much tighter than this, specifically for the precision parts found inside camera lenses. But each individual part has its own tolerance levels accepted by the manufacturers. The closer an individual part gets to the “perfect” dimensions, the better. If you have a lens made up of a dozen optical elements, and they’re all just slightly away from perfect, but still within tolerances, those imperfections can compound, causing a “bad sample” of the lens.

As manufacturing processes have evolved and gotten better over the years, tolerances have gotten tighter, and more and more items are closer to those ideal designed dimensions, but none are ever exactly perfect.

The components in your camera also suffer from the same concept of manufacturing tolerances. This is why a lens may behave perfectly on one camera, but backfocus slightly on a different sample of the exact same model of camera with the same settings – and it’s why the Sigma dock is pretty much vital for all DSLR users using modern Sigma lenses.

It’s also why the higher-end lenses cost so much. They have much tighter tolerances than an entry-level kit lens. They’re more expensive to manufacture because they use more expensive equipment in their creation, and they may have to trash more components that don’t manage to make it within spec. All these costs add up.

The lesson here is to make sure you buy your lenses from a reputable dealer with a good returns policy. Make sure to do some solid tests with the lens to see exactly how well it performs, and if it’s not working as well as you expect (or as well as you’ve seen with other copies of that same lens), return it and get another.

Update: For clarification, my experiences with Sigma’s more recent Global Vision (Art, Sports and Contemporary) series lenses have not been the same as they were with the older Sigma 10-20mm f/4-5.6 released way back in 2005. Sigma’s quality control has dramatically improved since then and they now have very strict procedures in place. In fact, I asked Sigma if they could give me some information on their current quality control procedures and they told me this…

• All Sigma Global Vision lenses are handcrafted and tested at Sigma’s state-of-the-art manufacturing facility located in Aizu, Japan.
• All lenses purchased through verified channels come with a four-year warranty.
• Quality control testing includes 100% MTF testing.
• 100% of Sigma lenses are analyzed by proprietary ultra-high definition sensors that must meet specific approval guidelines before leaving the factory.