In microscope systems, it's the numerical aperture (NA) and the wavelength of the light that determine the minimum distance that two particles can be resolved. Clear images, accurate sizing, and consistent categorization all depend on your NA setting.
NA describes the maximum angle of light that an objective can accept and transmit to the camera. For those familiar with photography, NA is similar to the f-stop value. In a camera, the f-stop setting controls the objective aperture and allows the user to control the depth of field and light intensity in the image.
On the FlowCam Nano, the objective aperture is not adjustable, having been factory set to provide crisp images, view an appropriate depth, and allow optimal functionality of the software tools in VisualSpreadsheet. The NA adjustment on the FlowCam Nano instead controls the aperture for correct illumination. This does not have the same effect as varying the objective aperture. Reducing this aperture below the NA of the objective (1.3 in the case of the FlowCam Nano) restricts light to where it can no longer fill the objective. While it is possible to get enough brightness this way, the image will be fuzzier and VisualSpreadsheet’s ability to measure and categorize particles will suffer.
Setting the aperture to a NA higher than the objective, however, does not have any adverse effect. Large increases in NA will cause the image to darken, but in the adjustment range of the FlowCam Nano (a maximum of 1.4), the change is imperceptible.
To get the best performance out of your FlowCam Nano, keep the aperture dial on your condenser at or above the 1.3 value required by the objective.
For more information on apertures and the terminology above, check out this video by photography educator Apalapse: Camera Basics - Aperture