Zahra Jamshed , CNN Written by

You may feel they're priceless, but your favorite jeans could be worth far more than you realize. Perhaps even tens of thousands of dollars.

So valuable, in fact, that New York-based filmmaker Devin Leisher has produced a documentary dedicated to the history and production of the American classic.

But here's the catch: according to the documentary -- "Weaving Shibusa," which premiered in San Francisco in early August -- the world's best denim is made and collected in Japan. There, denim is a specialty, and jeans are an art.

If produced by the right brand and machine, new jeans in Japan can sell for more than $1,000, while vintage collectibles can fetch $40,000.

Weaving Shibusa: How Japan perfected an American classic

By exploring old mills and vintage stores, and interviewing denim specialists -- such as the Osaka 5, an elite group that practices traditional weaving -- the documentary offers insight into a handcrafted tradition rarely found in the mass market factories that dominate today.

CNN Style spoke to Devin Leisher about the history of denim in Japan, the elite Osaka 5, and why this classic garment can come with a remarkably hefty price-tag.

What makes Japanese denim so interesting to denim enthusiasts like yourself?

Everyone loves denim, whether you recognize it or not. We all have a favorite pair, we all have that one that fits us right.

I've always appreciated denim, but my infatuation with Japanese denim started almost a decade ago when I was searching for a thick denim.

Back then, discussions about dyeing techniques and fabrics were still sparse, so I decided to do my own research and came across a brand called Samurai which opened me to this world.

Now, demand for raw denim has exploded. There are blogs and websites dedicated to the topic, and it's jumped from a niche interest to a booming global one.

Why is Japanese denim the pinnacle of production?

The attention to detail: the dyeing, the cotton, the way the fabric has been treated, everything.

I came across a pair recently that was denim on the outside, but on the inside, the entire garment was lined with sterling silver fibers. An outsider would never see the silver lining. They have no idea about the secrets hidden in the jean, but the wearer does. Another example is a pair I came across recently by a brand called Strike Gold

The rivets on their jeans are copper on the outside, but zinc on the inside. This is done because the different metals will pick up different colors as they age and oxidize, so the wearer can witness its evolution over time.

For these brands, the thought process goes beyond "What can I do to make this button look right?" It goes down to "What can I do to make this button?"

How did the Japanese denim market originate?

It's pegged to the vintage fashion boom, which took place during the '50s. Immediately after WWII, it was popular for youth in Japan to wear school uniforms as fashion due to a desire during the war to show unity and uniformity.

But by the '50s, students started exploring makeshift stores at US military outposts, and American pop culture -- fashions, graffiti, cars -- made its way into schools.

courtesy weaving shibusa/devin leisher

The jeans purchased at these outposts were old and worn, and so an appreciation for vintage worn denim grew, and it has become a collector's item today. Japanese collectors now own 70% of America's vintage denim , and Levis in particular is a standout brand.

Today, a pair of Levis 501 jeans from the 1940s can go for over $3,000 in Japan, and it is common for extremely rare Levis to come with a $40,000 price tag.

When did Japan begin producing its own denim?

The first Japanese brand to produce its own jeans was Big John . Homegrown denim production then took off in the '70s, and by the '80s it was all about quality, and so brands brought their denim production to fabric mills in Okayama.

Today, Okayama is a haven for jean enthusiasts, but before denim came along, the mills were long used for quality Japanese fabric production and textile exports.

Denim makers brought back old fabric machines, which were being phased out. But their secret is not just the machines, it's the people that run them.

In today's mass production factories, you'll see 150 stacks of fabric cut by a motorized saw. But in these mills, there is always a hands-on approach before the machine is used: denim is often hand-dyed before it is placed on the machine itself, and fabric patterns are often meticulously cut by hand.

One brand, Momotaro , is best known for its handmade approach.

Their premium selection is made from denim that is hand-woven on a wooden hand-loom, day after day, by one craftsman. Momotaro's premium handmade selections sell for up to $2,000.

Who are the Osaka 5?

Okayama is known for producing the material itself, but the world's fascination with Japanese denim can be traced back to five Osaka-based brands: Full Count, Evisu, Studio D'Artisan, Denime, Warehouse.

The creators of these brands -- Mikiharu Tsujita, Hidehiko Yamane, Shigeharu Tagaki, Yoshiyuki Hayashi, Shiotani brothers -- are collectively known as the Osaka 5, and their craftsmanship set the foundation for local denim production.

When they entered the market, they did not create basic denim jeans. Instead, they focused on quality, craft and recreating an original vintage feel.

What makes them specialists in denim production today?

These guys are perfectionists. They use old machines, dated stitching techniques, and go to lengths to revive fabric quality that barely exists today. What makes the Osaka 5 even more special is how they've grown together: they started their brands one after another, they share trade secrets, they helped each other grow. Between them, there is no competition, no one who does it best -- just a bond over their love of the material.