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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:

  1. Select from the menu Analysis > OTF > Geometrical > vs Focus

mtf snaphot

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.

See also