An issue of "hotspotting" with the 60 mm CoastalOpt® UV-VIS-NIR lens for a limited set of operational parameters has been discussed in several photography blogs with much of the detail being discussed here:
Hotspotting is an optical issue. Specifically, a central hotspot is a ghost image of the aperture stop created by two ghost reflections. These reflections can occur between any two lens surfaces, or it can be a reflection off the sensor or coverglass followed by a reflection from a lens surface.
In the case of the CoastalOpt® 60mm lens there is never a sharply focused ghost image of the stop, even at closest focus. However when you focus to 1:3 a very weak and defocused hotspot begins to appear at small apertures, and it becomes more prominent as you focus closer. Based on analysis of the optical design and experimentation, we believe that the cause of the problem in our case is a pair of lens surface reflections in the fixed rear group.
Also note that the production lenses (all lenses past the first 5 units) are identical to the prototypes with respect to the hotspot issue. There has been no change to either the optical or the coating design. The only differences between prototype and production lenses are 1) a minor mechanical issue has been corrected in the production lenses, and 2) the production lenses have a superior cosmetic finish on the outer barrel parts.
The hotspot issue still exists for close-up 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 close-up magnification.
How much extension tube is needed?
8 mm of extension with the lens set at 1:3 takes the overall performance 1:2
18 mm of extension with the lens set at 1:3 take the overall performance to 1:1.5
20.5 mm of extension with the lens set at 1:3 take the overall performance to 1:1.4
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 we have not thoroughly tested this for hotspots.
In the end there is no simple modification to the lens to eliminate the issue without significantly effecting performance in other areas. The workaround with an extension tube is the best alternative. For the vast majority of owners of this lens, using an extension tube for these specific conditions is a more attractive alternative than giving up the performance the lens offers in other situations.
The lens we continue to manufacture and sell today is the same optical and mechanical design that has been manufactured since 2008.