Penstemon osterhoutii and Penstemon lentus are very similar plants, so similar that in his 2001 edition of Colorado Flora: Western Slope, Weber combines them under the name of P. osterhoutii. However, other authorities separate the two species.

Harrington's 1964 Manual of the Plants of Colorado is the only botanical text that has a detailed key and description of each species.  Following are the characteristics that Harrington uses to separate the two species:

P. lentus: Inflorescence more or less secund, peduncles elongate, flowers not congested in fascicles, 20-30 centimeters tall, stems few to several, flowers are dark blue, to light purple to bluish-purple.
P. osterhoutii: Inflorescence not secund, peduncles short, flowers in congested fascicles, 30-70 centimeters tall, stems solitary to few, flowers are light purple to violet-blue.

The two species are, according to Harrington, almost identical in all other characteristics.  Unfortunately, the key characteristics that Harrington provides for P. lentus are not agreed on by Intermountain Flora or A Utah Flora. 

In addition, recent examination of P. lentus and P. osterhoutii by Craig Freeman (who will be writing the Penstemon section of the Flora of North America) shows that most of the distinguishing characteristics that Harrington mentions are not diagnostic.  Freeman indicates that it is the intensity and length of the internal striping in the floral tube, the hairiness within the tube, the length and width of the staminode (the sterile stamen), and the arrangement of the flowers that separates the two species.  The two species are almost identical in all other physical characteristics, but Freeman still considers them distinct species.

Freeman indicated to me that the species shown in the photographs below is P. lentus, not P. osterhoutii. The latter is known, Freeman told me, only from Eagle, Garfield, Grand, Pitkin, Routt, Summit, and perhaps Rio Blanco counties, all in the northwest corner of Colorado. 

The two species are found in Sagebrush, Pinyon-Juniper, Gamble Oak, and Ponderosa Pine communities in sandy or gravelly soils from about 4,800-7,500 feet.