Issues with Histogram and exposure scale in viewfinder of R5

[AaronD]

I normally do architectural photography and normally I look only at the histogram in my viewfinder when adjusting for exposure.  And then I bracket.

Now I'm using the camera off-tripod with a flash.  I'm setting for ambient exposure and am watching the histogram and the exposure scale at the bottom info-edge of the viewfinder.  Sometimes they're both reading light levels, sometimes only the histogram is (scale has no 'pointer') sometimes the histogram freezes (and turns gray) while the scale works like I expect with the pointer sliding up and down as I adjust ISO.  What is going on here?

I'm using an R5 in Manual mode, I set and leave it on 1/160 and f/4.  The flash is a 600EXII-RT and it's in ETTL mode with the +/- set down a stop.  I think those are the pertinent particulars...  It will flip in and out of these 3 states randomly, and so far as I can tell I'm not 'doing' anything to make it cycle through.

Sorry for such a long-winded question... and thank you in advance.

A

[Anonymous · Accepted solution]

From the manual page 286 concerning a grayed-out histogram:

 

[AaronD]

Ahh.  So I must be looking at a light level that’s right at the threshold. 

[Anonymous]

Yes, that looks to be the case.