Chloe e-mailed me with some questions. I have posted those as a comment on this site, where they belong. Please have a look - if you want to prepare something we can discuss this at the next session.
Tonight there is a lunar eclipse, and we have a beautifully clear sky. I shot this with my Panasonic TMC-TZ3 at 10X zoom. This camera has an optical image stabilizer (OIS) to suppress image motion from tremor etc - one of the most exciting new technologies that are being used in good binoculars but not, as yet (I believe) in low-vision telescopes. Maybe you want to look up how this works - it's great stuff.
Inspired by above, I hoped to inspire you to run you through a list of useful numbers in vision science, assembled by Brian Wandell of Stanford University, during our next session tomorrow (Thurs) 5pm. We also revisited the idea of estimating spectacle prescriptions etc, with and without focimeters. I hate that un-word "lensmeter" because, apart from being ugly, it can lead to confusion with the Geneva Lens Clock - see the site by Mark Wilkinson of Iowa. God Save Our Queen).
Congratulations to Chloe (PGY5) for making an excellent guess on the angular subtense of the moon (0.5 degrees). Twinkle, twinkle, little star. Here is another very British tribute to the Moon - enjoy & watch the teeth!
4 comments:
Paul,
Thank you so much for the teaching today… 2 sessions in one day! I would have a siesta as well!
Because I express myself better on paper than in conversation:
We discussed future teaching about IOL calculations, the relevant topics for residents, “future ophthalmic surgeons,” to know are:
What are the components of the SRK formula?
Why does geometric optics fail to precisely calculate IOL power? (i.e. – why do we revert to using a formula with “the A constant”?)
What is the A constant, and generally, how was it derived? (Sanders, Retzlaff and Kraff analysed statistical data on their cases and found a linear regression formula for estimating IOL power that only involved measuring the AL and K’s – anterior chamber depth and other biometric parameters did not add accuracy to the formula)
Why is there inaccuracy in the SRK formula? (A-scan measures length based on sound transit through different media, aqueous, lens and vitreous; K’s: front surface measurement, reflected image from the cornea, and have a “fudge” factor to make up for lack of calculating power based on corneal thickness and refractive index)
Why did the Hoffer Q and SRK II formulas become necessary? (inaccuracy with short and long eyes in the SRK)
What is the error induced in the lens power by a 1mm error in AL measurement? (2.5-3.0 diopters)
Rule of thumb for converting PCIOL power to ACIOL power (reduce by 0.5 diopters)
Why should you (the surgeon) look at the IOL master printout before the day of surgery? (If there is a large difference in the AL measurements: may need to repeat. If the eye is short or long: need to choose appropriate formula (SRK II, HofferQ). Need to check pre-op refraction for considerations of: IOL power asymmetry between the 2 eyes, if it’s a second surgery; prior type of IOL implanted and its refractive result; incision placement based on cyl, etc…)
Briefly, how do multifocal IOLs work?
Cheers,
Chloe
Chloe,
all interesting points - do you want to prepare a short summary of these to present at the next session? We can all discuss then.
Best wishes
P.
Hi folks
I am loving this site Paul has done for you, there is some really great stuff on here. You are all very lucky (and I am a geek who enjoys looking at optics websites)
Catherine
Paul, you referenced the MedRounds website when you mentioned the Geneva Lens Clock. This site is superfantastic, and partly run by the folks at Iowa (Lesya first told me about it.) For everyone: There is a full online version of the book "Essential Optics Review for the Boards" at http://www.medrounds.org/optics-review/ It is what it claims to be. Also, the website has an eye path review by our prior visiting professor, Ben Glasgow.
Paul, the comment about the IOL formula was to stimulate you to add it to our teaching at a later date. I would love to prepare a short session about this topic, but I'm armpit deep in studies for the Royal College exam, so I'll leave the course instruction up to u!
Cheers,
Chloe
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