I have a question/troubles about this topic.

I define a single and indivisible sample meter equal to 2L.
Suppose I have two mirrors between which a beam of photons oscillates with a certain energy per single photon and for each the same. Let's suppose that the two mirrors are exactly 4L apart. 
I can say that on average position of the photon is in the centre of the mirrors with an error of exactly L.
If in these specific conditions of spatial confinement (i.e. measurement) I go to measure the moment of a photon, will it obey or not the Heisenberg uncertainty principle?
Is it also in this case not applicable? i.e. by considering $$\Delta x =L$$ and the uncertainity on the frequency could be exactly $$\Delta p= {h \Delta\nu \over c} $$
could I point out that the uncertainity collapses, in this special case, on a frequency-space uncertainity according to following:
$$\Delta \nu = {c \over 4\pi \Delta x} $$

In advance I thank you for the answer.
I excuse me in advance for my incompetence.