Why one zoom blurred another ok raytracing?

Hi,
V8
My first ever step into photo real raytracing etc.
Taking a brass effect file by Gijs (awesome)
(its in this thread from Gijs.

)
imported my two items,
chose same layer and settings for them.
deleted his.
I wait whilst varying impriving granular scenes come and go.
I end up with blurred items.

I zoom out a bit and it does that granular scene again, 23 secs later they are in focus.

Is there a limit these things can be viewed at ?

I wish to understand all the steps involved in taking a Rhino object and making it look like metal as in this, or a rusty item etc.
Just what STEP BY STEP turotials are there, with tutorial objects, I can follow along to.

I have always been fascinated by this end of the line process but never needed it until now.

Now that I have Rhino8 and an Nvidia 3070 geforce RTX GPU , and 64 of DDR ram, and a 1yr old high end PC, now is my chance.

is 30 secs slow ?

Excited.

Steve

I got this problem once because the Dof option of rendering was set on.
You should look at the camera properties and check if this option is on