About limit to zoom in to pixel size to measure GCPs for the Large Format Aerial Images

Hello CC community,

When solving AT, it is difficult to measure GCPs for the Large Format Aerial Images.

Drone images are zoomed in to pixel size to measure GCPs.

But I have trouble to measure GCPs to solve AT for the Large format images as IGI Urban Mapper, PhaseOne PAS880i and so on.
it can't be enlarged the pixel to measure them.

I don't know why it has limit to zoom in.

And when I press "Shift" key to meaure GCPs, it will be help to display bilinear or cubic as measure box.

Best regards,

JR