
Using the OTF vs Focus plot
The OTF vs Focus plot is a plot of contrast versus focus, for specified fields and waves, for a particular spatial frequency.
To create a the plot window:
- Select from the menu Analysis > OTF > Geometrical > vs Focus

The OTF is displayed as a function of the focal position. Both a tangential and a sagittal scan is made, with the sagittal scan plotted as a dashed curve.
Options panel
- Field
- Using the Field popup menu, select the field for which you wish to calculate. Select All to calculate for all fields (this can be time-consuming).
- Wave
- Using the Wave popup menu, select the wave for which you wish to calculate or Polychromatic, in which case a weighted average of the waves is calculated, using the weights specified in the Sources dialog.
- Number of rays
- This is the number of rays used to sample the pupil. The number of focal positions is fixed at 20.
- Use hybrid form
- Multiply the calculated geometrical OTF by the diffraction limit, to give a curve that always lies below the diffraction limit.
- Divide by focal length
- Divide the image space spatial frequency (cycles per unit length) by the focal length of the lens, so that it becomes an angular frequency (cycles per unit angle).
- Plot real and imaginary parts
- In addition to plotting the MTF, which is the absolute value of the complex-valued OTF, also plot the real and imaginary parts of the OTF.
- Maximum defocus
- How far from nominal focus to calculate. If this is 500 microns, for example, then the plot will run from -500 microns to +500 microns.
- Spatial freq.
- The spatial frequency at which to calculate the OTF and MTF.
- Plot calibration curve
- Include the MTF versus focus curve of an ideal lens in the geometrical approximation. Use the Use hybrid form to multiply this curve by the diffraction limit.
- Plot diffraction limit
- Include diffraction limit (at focus).
- Plot unity
- Include unity in the plot, for comparison.