my understanding is that u can't really "plan" to become an ophthalmologist - at least that's the impression i have going through the canadian system.
i think in canada there's only 16 residency positions available in the country per year for ophth. as such, it works out (roughly of course) to only one student per medical school graduating class a year. that's it. as such, u may end up going to medical school, and end up matching in something else. of course, u may of course end up matching into ophth. a poster above said 90% of applicants match into ophth (presumably in the states). i can't confirm, but it does seem awfully high to me (not that i'm doubting it per se).
if u want to tackle disease - and make treating disease your primary task day-to-day with virtually every patient u see, then u want to be an ophthalomologist. disease itself can be interesting, and its treatment ongoing. if u basically want to go to work in a suit and tie every day, keep your hands clean, and not deal with sick patients, then optometry is for u. doing optometry is different in the sense that the vast majority of patients can be adequately dealt with in the 20-25 min span that u have working with them. the patient presents with a problem, and by the end of u 25 mins with them, they are on their way. there is "some" level of depth required in analysis, but typically, it's not terribly deep. u seldom have to go home wondering if ur patient will need additional treatment. consider how this is different with a patient who has glaucoma. ur going to see them probably every 2 months for the rest of their lives trying to maintain an acceptable IOP.
i'm an OD. i was competitive for medical school but i chose optometry because i felt i'd find myself at age 28 and midway through residency (i envisioned 100 hours/week in the hospital), and seriously regretting why i didn't choose the "easier" path of optometry. i like that optom school allowed me to finish in 4 years. that said, now that i'm out, i have to admit that optometry isn't nearly as lucrative as i expected it to be (we all think we'll gross 100K when we get out - but that's the AVERAGE salary - which includes salaries of those who've worked for many years. its more like 60-65K your first year out - of course this varies on geography and other things), and certainly no where as financially rewarding as medicine (i'm not trying to make ODs sound like mercenaries, but it's a bit disconcerting to pull in a starting OD salary, when ur MD resident friends are all "certainly" going to start at 150K and in a few years pull well over 300K). of course, if u go into ophthalmology, then issues like malpractice worries, and those (da-mn) optometrists start making life less ideal.
no profession is perfect unfortunately, and that includes both optometry and opthalmology.