This page of the Horizon Forbidden West guide will tell you how to get to the lenses.
The lenses are located in the signal towers. They are located at high elevations. The map shows you the exact location of a particular tower, but it cannot specify a specific route.
When you arrive at the indicated location and it turns out to be a signal tower, the first thing you need to do is locate a climbing spot. Focus pulse is very useful in highlighting the interactive points.
Of course, a climbing spot does not have to be directly under the tower. It may be located far away from the target.
Once you find the right point, you should climb as high as possible. Next, visually determine the location of the tower and head towards it.
The lens is always at the top of the tower. Therefore, the final step is to climb it. Focus on finding next climbing points (the focus pulse is helpful here) and using them. However, there are times when you will need to use the Pullcaster to proceed.