Canon’s new 600mm and 800mm lenses

Started 3 months ago | Discussions thread
alanr0 Senior Member • Posts: 2,326
Ray tracing, image analysis and choice of focus
2

Jack Hogan wrote:

alanr0 wrote:

... my guess is that this is due to a small focussing error, possibly combined with a change in spherical aberration in the preceding optics. ...

Bill, slightly OT. You know I am a raytracing noob and the first time I tried it recently I was flabbergasted at the amount of spherical aberration from a single lens with perfectly spherical surfaces. That would suggest that the choice of where exactly the bundle intercepts the optical axis on the image side is a range as opposed to a single hard number.

Does raytracing take this into account or does it just use the marginal rays? If it does take it into account, what criterion is normally used to determine the location of the CLC in order to draw the relative rays?

The more capable packages will offer a wide choice.

For example Zemax offers a "Marginal ray solve".  It finds the intercept of a marginal ray with the axis.  Applied to the last optical surface before the image plane, it is a quick and easy way to find an approximate image location.

To take a step further, there is a wide range of analysis options, including geometric spot diagrams, and wavefront distortion analyses.

You can choose to optimise on RMS spot size, geometric spot size, RMS wavefront distortion, peak-to-valley wavefront distortion etc., with a range of merit functions or constraints on element size, location and/or surface curvature.

Not something I have had much to do with, but there is a lot of flexibility.  Of course, all this choice need not make your life any easier.

HTH.

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Alan Robinson

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