Sony HVC-2400 low-light focusing

[Macdude]

I realize this is a home Trinicon camera. I wasn't sure where else to post.

I have an HVC-2400 and a HVC-2200 camera. Both are manual focus. These are the style that connect with Sony K connectors to portable Betamax units. I've noticed that if I zoom in tight, focus, then zoom out, the picture will go soft. This happens in low light. I understand the shallow depth of field at that iris position, but if it's sharp zoomed in, why does it go fuzzy when zooming out? When this happens, I can't improve the focus at all after zooming out, it remains soft, so I don't feel as though something is odd with the lens. I never seem to have this issue shooting outdoors in daylight.

I believe it uses a Canon parfocal lens, but I can't verify that. This makes using the power zoom quite difficult. Both cameras act this way, hard to keep focus in low light. Is there any explanation for this?