An immersion triplet design using BK7, KZFS1 and BAFN10 glass. The colours are far too strong in all these simulations, but they can to be compared too each other. Clearly the defocussed images are rather different. Also the grey-images show the differrent total intensities at 1 wave of defocus. Look at the bright spot at +1 wave of defocus, that is mainly due to red-light in focus and yellow-green out of focus.

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The same triplet design but now with air-spacing between the elements.

Note that the colours are paler than those in the above example. Also the hole at the center of 1 wave of defocus is better visible due to a better color-correction. The grey-scale images are also looking more alike.

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The last glass (BAFN10) was replaced with a TIF2 glass element. Once more the +1 wave focus image has become paler and also the difference in the grey images is smaller.

This design is the best of these three, the TiF2 glass however seems to be very difficult to produce pure enough for the usage in optics.

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The images on the left are made Rafael Barberá from Valencia, Spain showing
Sirius at both sides of focus (>10 waves) through a Helios 102mm f/5 achromatic refractor with a
Philips ToUCam Pro Webcam mounted. The image is averaged over 150 stacked image-frames.
http://usuarios.tripod.es/rbarbera/