An instrument for biological analysis, comprising: an excitation source producing electromagnetic radiation; a sample holder configured to receive a plurality of biological samples; an optical sensor configured to receive an emission from the biological samples; an excitation optical system disposed along an excitation optical path and configured to direct the electromagnetic radiation from the excitation source to the biological samples; an emission optical system disposed along an emission optical path and configured to direct electromagnetic emissions from the biological samples to the optical sensor; characterized by a filter assembly comprising a plurality of filter components, the plurality of filter components comprising: a first filter component having no optical power, the first filter component consisting of a first filter characterized by at least one of a first low-pass wavelength or a first high-pass wavelength; and a second filter component characterized by a finite focal length, the second filter component comprising a singlet lens and a second filter characterized by at least one of a second low-pass wavelength that is different than the first low-pass wavelength or a second high-pass wavelength that is different than the first high-pass wavelength; wherein the filter assembly is configured to selectively locate one of the plurality of filter components into the emission optical path; wherein the singlet lens compensates for a change in effective focal length of the emission optical system and magnification of the emission optical system between a first wavelength within a transmission range of the first filter and a second, different, wavelength within a transmission range of the second filter. an excitation source producing electromagnetic radiation; a sample holder configured to receive a plurality of biological samples; an optical sensor configured to receive an emission from the biological samples; an excitation optical system disposed along an excitation optical path and configured to direct the electromagnetic radiation from the excitation source to the biological samples; an emission optical system disposed along an emission optical path and configured to direct electromagnetic emissions from the biological samples to the optical sensor; characterized by a filter assembly comprising a plurality of filter components, the plurality of filter components comprising: a first filter component having no optical power, the first filter component consisting of a first filter characterized by at least one of a first low-pass wavelength or a first high-pass wavelength; and a second filter component characterized by a finite focal length, the second filter component comprising a singlet lens and a second filter characterized by at least one of a second low-pass wavelength that is different than the first low-pass wavelength or a second high-pass wavelength that is different than the first high-pass wavelength; wherein the filter assembly is configured to selectively locate one of the plurality of filter components into the emission optical path; wherein the singlet lens compensates for a change in effective focal length of the emission optical system and magnification of the emission optical system between a first wavelength within a transmission range of the first filter and a second, different, wavelength within a transmission range of the second filter. a first filter component having no optical power, the first filter component consisting of a first filter characterized by at least one of a first low-pass wavelength or a first high-pass wavelength; and a second filter component characterized by a finite focal length, the second filter component comprising a singlet lens and a second filter characterized by at least one of a second low-pass wavelength that is different than the first low-pass wavelength or a second high-pass wavelength that is different than the first high-pass wavelength; wherein the filter assembly is configured to selectively locate one of the plurality of filter components into the emission optical path; wherein the singlet lens compensates for a change in effective focal length of the emission optical system and magnification of the emission optical system between a first wavelength within a transmission range of the first filter and a second, different, wavelength within a transmission range of the second filter.