Many pinholers sharpen their scans before placing on the web; I'm assuming this image might have been sharpened to increase definition on the web, as images are generally not so sharp or well-defined by pinhole nature: they are soft, fuzzy, but a lot depends on the quality of the pinhole itself (the brass shimmy). That is what can really have an effect on the image.

All the same, it is a great pic; either angling the camera steeply up or, even more whacky, lying on your back looking up/sitting up. Terrific. Something to try when I visit a sea cave...