Tommy Caldwell ascends the Dawn Wall of El Capitan at California's Yosemite National Park. 

(Courtesy Sender Films/Corey Rich)

Last winter, Colorado's Tommy Caldwell and his climbing partner Kevin Jorgeson made international headlines when they completed the first free climb of what's known as the "Dawn Wall." It's considered the toughest ascent on the granite face of El Capitan at Yosemite National Park. 

The two men's frustrations and triumphs are captured in "Dawn Wall: First Look," by Boulder film company Sender Films, which premieres Thursday evening as part of the Reel Rock Tour in Boulder.  Josh Lowell, one of the filmmakers, spoke with Colorado Matters host Ryan Warner. Click on the audio link above to hear the conversation. Read edited highlights below.

Tommy Caldwell, talking in the film about the ascent:

"It's the closest thing I've ever come to climbing...a sheet of glass. You're just waiting to slip off at any moment. The holds are tiny--it's like credit cards glued to the wall.

How the filmmakers made the documentary:

"We as filmmakers are on the wall with the climbers, but we're not doing what the climbers are doing. We are hanging on ropes. We have sometimes quite elaborate rigging set-ups with ropes and pulleys and all kinds of safety systems.  So, we're kind of hanging suspended in air  near them while they climb and the difference is they have a rope to catch in case they fall, but they're only using their fingertips and their toes to support their body weight and too make upward progress.  So it's pretty easy for us to zip up and down the ropes, get into position where we need to be and try to get these amazing angles whereas they are trying to do this incredible athletic accomplishment that's harder than anything else that's been done in the sport of climbing. "

On getting close-up shots of the climbers' fingers and toes:

"One of the elements that make the climb so impressive is just that miniscule size of the hand and foot holds...it's a dead vertical wall and they literally  have a millimeter or two millimeters of the tips of their fingers grabbing these very very sharp little breaks in the rock, and the more you grab them, they dig into your skin.  You want them to dig into your skin otherwise you can't hold onto them but if you dig in too much, then it slices your skin open and that's something these guys have battled with for years on their various attempts is just cutting their fingertips open."

The challenges of life "on the wall":   

"They were on the wall for 19 days during their successful push, and by on the wall, I mean sleeping every night in  in a tiny little portaledge, a cramped space suspended from the wall.  You can't walk or really stand up or barely stretch at all so in a sense they're wearing down day by day just as far as their general endurance."

On watching the climbers make it to the top:

"As they started to get to the point of success, the whole world was watching and that made it even more exciting... As they neared the top, it was incredible to see Tommy and Kevin's dream coming together and becoming a reality.  I  mean, I think over the years that they had tried, a number of people were kind of rolling their eyes saying 'God, these guys are wasting the prime of their career on a thing that's never going to happen' and they just didn't give up, so to see them getting close to the top was incredibly gratifying for them and I was on the summit when they arrived...There [were] about 40 or 50 people -- friends and family and press -- that were up there to greet them and I think they were a little shell-shocked seeing that when they reached the top of the mountain after being alone on the wall for three weeks...and it was really emotional."