Have you ever crashed your microscope objective into a sample ?
At some point we all have This is where the concept of Working Distance comes in. You will see Working Distance (WD) often listed as a spec in microscope or objective brochures. This spec is important as it tells you the distance between the microscope objective lens and your focused sample.
A longer working distance will have less resolution but more space.
A short working distance will have better resolution but can come awfully close to your sample.
When do you make the decision to choose between Long and Short distance microscope objectives ?
It really depends on your sample. A mostly flat sample can afford a short working distance and good resolution. A sample with a lot of differing heights or peaks and valleys will benefit from a long working distance objective so as not to crash into neighbouring peaks.
One other benefit of longer working distance is you have less out of focus areas in a thicker sample. This is known as Depth of Field.