EXCERPT page containing first few paragraphs. 2021-12-04 18:11:08
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There is an ongoing controversy concerning a hot spot issue, especially at high magnifications and small apertures (eg f/22 - f/45).
The optical designer of the Coastal 60/4, James Brian Caldwell (US Patent #20090296201), confirms that there is a hot spot problem at small apertures at magnifications 1:3 and larger, but suggests that at longer distances no problem exists:
The hotspot issue still exists for closeup shots (1:3 and closer) taken at small apertures. A workaround for macro photography is to focus the lens no closer than 1:3 and then use extension rings to achieve the desired closeup magnification. For very high magnifications the lens should work very well reversed on a bellows with the focus ring set to the reciprocal of the magnification, but I have not thoroughly tested this for hotspots. As far as I know the lens is completely immune to hotspotting for magnifications less than 1:3 regardless of the aperture or filter used.
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