According to the Nyquist criterion (Pawley 2006), the interval between intensity measurements (the pixel size) should be less than or equal to half the smallest feature size: \[ r_{px} \leq \frac{1}{2} r_{min} \tag{1}\] and, \[ z_{px} \leq \frac{1}{2} z_{min} \tag{2}\] If the smallest features are sub-diffractive, then (in a traditional imaging regime) we can use the expected size of the diffraction limited point spread function to determine the correct pixel size: \[ r_{px} \leq \frac{1}{2} \frac{0.61 \lambda_0}{NA} \tag{3}\] and, \[ z_{px} \leq \frac{n \lambda_0}{NA^2} \tag{4}\] In this case, the voxel aspect ratio scales with the collection half angle according to: \[ \frac{z_{px}}{r_{px}} = \frac{3.28}{\sin\theta} \tag{5}\] For most objectives \( 0.1 \leq \sin\theta \leq 0.95 \), and so in this regime the axial pixels are significantly longer than the radial pixels: \[ 3.5 \leq \frac{z_{px}}{r_{px}} \leq 32.8 \] When considering optics and camera chips it is often useful to calculate the number of pixels in the field of view: \[ N_{px} = \frac{FOV}{r_{px}} \tag{6}\]