Low-Fresnel number optical systems exhibit significant diffraction effects that cause a shift in the peaksudof on-axis irradiance away from the geometric focal point. This is currently interpreted as a change of theudfocal length of an optical system, leading optical system designers to compensate for the effect by assumingudthe image plane is coincident with the peak of on-axis irradiance. While this may be an appropriateudinterpretation for certain applications, I show that despite the shift in peak irradiance away from theudgeometrical focal point, a change in a system’s optical power will not increase the on-axis irradiance atudthat distance. This is important for low-light level applications where it is necessary to mitigate diffractionudinduced transmission losses. I also show that low-Fresnel number systems have increased toleranceudon system power at the geometrical focal point and as a result are inherently achromatic.
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