The can of worms that you have just opened has been debated for nearly
a hundred years.  The argument that an optometrist provides a better
glasses prescription is based on the premise that the two professions
are taught to view the eye differently.
An ophthalmology resident becomes an MD first, and then during his
residency he specializes in diseases and surgery of the eyes.  During
that two year residency he is expected to master his surgical
techniques and the diseases of the eye.  Refraction is often limited to
a couple of weeks study during a typical ophthalmology residency.
Opthalmologists training is based on the "medical model" of the eye.
That model of the eye views the eye like any other organ in the body.
When it breaks, you need to fix it.
An optometrist typically goes to undergrad for 4 years in any
scientific discipline, and then his entire 4 years of grad school are
dedicated to the eye.  An optometrist cannot do surgery on the eye and
never becomes an MD.  The optometrist views the eye with a different
orientation because he spends most of his 4 years on the "functional
model" of the eye.  The functional model is a series of beliefs that
view the eye as a gateway to the complex visual system, with an
emphasis on perception and interpretation of the image.  An optometrist
is more likely to prescribe prism in a pair of glasses before
strabismus surgery.  An optometrist is more likely to prescribe vision
therapy for a child who reverses his letters and numbers.  Refraction
is given much more emphasis during optometry school than during an
ophthalmology residency.

I often tell my patients that getting an eyeglass prescription from
your opthalmologist is like getting your physical from a cardiologist.
He can do one, but he probably hasn't done one for years and he doesn't
necessarily pay attention to all of the results.  You get your best
physical from a primary care doctor and your best refraction from an
optometrist.

As I already said, if the "other" ophthalmologist owns the store where you
purchased the lenses, then the optician could legally get the "other"
ophthalmologist to extend the Rx expiration date even without giving you an
exam (but not past the default time for your state).

Ophthalmologists are medical doctors. They get the same general medical
training as all other doctors on a wide variety of medical issues. After
medical school, they go into residency for a specialty. When they open a
practice of their own, they spend most of their time dealing with eye
diseases, surgery, and only part of their time giving eye exams.

OD's are trained to give eye exams as part of their OD (Dr of Optometry)
schooling, and obviously eye care is the focus (no pun intended) of their
medical education. When they receive their OD degree and start working, they
spend almost all of their time giving eye exams.

Certainly, ophthalmologists are very intelligent people, but common sense
would suggest their on the job experience in giving eye exams (especially
for those without any eye diseases) is much less than an OD.

My own experience, having worn glasses for 50 years, has been that I have
had horrible Rx's from ophthalmologists, and generally very good Rx's from
OD's.

The answer to your question depends upon the laws of the state of
jurisdiction.  In Texas, there is no mandatory expiration date for a
spectacle prescription.  However, by the same token, it can be whatever
the issuing doctor says it is.  In the absence of an expiration date,
it is customary practice not to fill any prescription that is more than
two years/old.

With regards to eye examinations, an optometrist is held to the same
standard of care as an ophthalmologist in the eyes of the law.  In
Texas, it is quite common for MD's to delegate the refraction to a
technician.  In Texas, the rules governing optometry specifically
prohibit delegation of the refraction part of the exam.

DrG