Back Clip After box greyed out - help requested

Hi Chief Experts ;-)

Trying to create a flat X-sec. Have taken back clipped Xn, saved it, then opened the properties dialog. Back Clip After is greyed out and I cannot enter anything into it.

I tried altering the camera default to get the same result, but it doesn't seem to make any difference.

Using Chief 10

Thanks for any suggestions

Activate the camera then switch back to plan view without closing the view. You should now be able to change the setting.

Don't I feel foolish ;-)

Thanks so much Dan.