Optical systems for nucleic acid sequencing and methods thereof

A method of imaging a sample, said method comprising:

• (a) providing an optical system, wherein said optical system comprises:
• (1) a light source that illuminates said sample with a first light, wherein the sample is in a flow cell;
• (2) a sensor that obtains an image of said sample in the flow cell while said sample is illuminated by said first light from said light source, wherein the flow cell comprises a first interior surface and a second interior surface;
• and
• (3) a focusing element assembly disposed along an optical path from said light source to said sensor, wherein said focusing element assembly comprises (i) a first focusing element, and (ii) a second focusing element, wherein said first focusing element moves relative to said second focusing element without removal of said first focusing element from said optical path, wherein said optical system further comprises a gap between said first focusing element and said second focusing element, wherein said gap has a width that remains substantially constant when said first focusing element moves relative to said second focusing element;
• (b) illuminating said sample with said first light from said light source;
• (c) focusing a second light emitted from said sample with said focusing element assembly;
• and
• (d) receiving said second light from (c) and obtaining one or more images of said sample by said sensor.