Generally, reducing the diameter of the aperture increases resolution. There's even a simple math formula that quantifyies it.
Does reducing the iris diaphragm aperture on a microscope clarify or distort an image?
Reducing the size of the aperture on any lens improves the depth of field of focus. In some cases,[unless the lens is designed to overcome this] taking the aperture down to a minimum can decrease the resolution ,not increase it!!!
Reply:Reducing aperture increases depth of field, but if the aperture gets very small you get to a point where the image being refracted of the edges of the aperture begin to 'interfere' with the image, and you get less definition, all lenses have an 'optimum' which is a balance between depth of field and iris aberration.
Depth of field can also be increased by having less magnification, if possible.
Reducing the aperture will also dim the image of course, but on microscopes this can be offset by increasing the illumination.
Some microscopes have the ability to use polarised light to to mitigate against 'iris aberration', working with a polarised filter within the microscope, you rotate the filters till the aberration is reduced, this set up halves the light available and therefore needs a further increase in the illumination.
Chris
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