Congratulations! You’ve just treated your patient’s diabetic macular edema or maybe you’ve taken out their dense cataracts.

First, know your refractor instrument. In general, minus (divergent) lenses will be in red and plus (convergent) lenses will be in white or black. Familiarize yourself with all of the knobs. Here are the most important:

Refraction requires practice, so practice often on cooperative patients at first. You can do this. You will do this. Go forth and refract.

28mmlens angle of view

The Pentax fish-eye zoom lens has a diagonal FOV varying between 180° at the wide end and 90° at the long end. The Birds-Eye lens forms a circular fish-eye image with FOV of 180° across the image circle. All other Fish-Eye lenses form full-frame images with a diagonal FOV equal to 180°.

Now, with a pair of glasses, your patient’s vision will be correctable to 20/20. Remember you’re the one responsible for providing them with a prescription for glasses, so let’s get you prepared.

Focal length of lensformula

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1. Spherical power: Always add +0.50 D (diopters) first to decrease the risk of over-minusing the patient (when patients accommodate in the refractor, they will need more myopic lenses, which can lead to eye strain). Make your patients “earn” more minus — they must see a smaller line on the chart with the increased myopic lenses.

Before actually refracting, it’s important to prepare your patient for the process of refraction. You might say, “I am going to show you a series of two different choices. Please let me know which option is clearer. If they are the same, that is a perfectly good answer.”

Lens focal length angle of viewcalculator

3. Cylinder power: Go back down a line on the chart to the smallest line the patient can read. Key step: When you add +0.50 D of cylinder power (two clicks clockwise, or right), you must take away 0.25 D of sphere (one click UP). This maintains the spherical equivalent. Therefore, if you take away 0.50 D of cylinder (two clicks counterclockwise, or left), you must add 0.25 D of sphere (one click DOWN), and vice versa (see animation). Think: up the plus cylinder, move the sphere wheel up; down the plus cylinder, move the sphere wheel down.

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When you perform a manifest refraction on a patient, occlude the contralateral eye (OC on the auxiliary lens knob) and keep the tested eye open (O on the auxiliary lens knob). The details on how to refract are too complicated to cover in this short article, so please watch this excellent video tutorial from the University of Iowa (vimeo.com/135867809) and the animation below. The animation will help you understand how images are focused on the retina through refraction.

35mmlens angle of view

The American Academy of Ophthalmology's newsletter for young ophthalmologists (YOs) — those in training as well as in their first few years in practice.

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4. Spherical power refinement: For this step, go in 0.25-D increments. At the end, perform the duochrome test and ask the patient if the letters are clearer on the red or the green side. Remember it with these letters, “RAM-GAP,” which stands for “if the Red is clearer, Add Minus; if the Green is better, Add Plus.”

Evan Silverstein, MD — ChairSruthi Arepalli, MD Grayson W. Armstrong, MDLiane O. Dallalzadeh, MD Cherie A. Fathy, MDBradley S. Henriksen, MDL. Claire Peterson, FRCOphth, MBBS Dagny C. Zhu, MD

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FOV tofocal lengthformula

Lens angle of viewchart

Until now all Pentax DSLRs use a sensor smaller than the standard 24 x 36 mm film frame. The lens still produces a 24 x 36 mm image, but the sensor only captures the central portion of that image — your wide-angle is not so wide and your tele is even longer. The calculator below takes the 1.5 x crop factor into account.

Congratulations! You’ve just treated your patient’s diabetic macular edema or maybe you’ve taken out their dense cataracts. Now, with a pair of glasses, your patient’s vision will be correctable to 20/20. Remember you’re the one responsible for providing them with a prescription for glasses, so let’s get you prepared. What Is Refraction? Refraction is the measurement of the eye’s focusing characteristics and the determinant of a prescription. A prescription has three main components: sphere, cylinder and axis. There are also different types of refraction: Objective (streak): Using a retinoscope to measure a patient’s refraction. For more information, read “Retinoscopy 101” at aao.org/young-ophthalmologists/yo-info/article/retinoscopy-101. Also check out an excellent video tutorial that describes loose-lens retinoscopy from start to finish at timroot.com/retinoscopy-workshop-video/. Subjective (manifest, dry): Using a refractor (also known as a phoroptor) to allow a patient to provide their subjective response about their prescription. Cycloplegic (wet): A refraction after a patient has had cycloplegic drops to paralyze their accommodation. Know Your Equipment For this tutorial, we will cover manifest refraction. First, know your refractor instrument. In general, minus (divergent) lenses will be in red and plus (convergent) lenses will be in white or black. Familiarize yourself with all of the knobs. Here are the most important: 1. Sphere power control When rotated downwards, you add plus sphere. When rotated upwards, you add minus (or take away plus). 2. Cylinder power control When rotated clockwise, you add plus cylinder. When rotated counterclockwise, you take away plus. 3. Cylinder axis knob Ophthalmic scale = 1 to 180 degrees. Using this scale, 185 degrees should be noted as 5 degrees. How to Refract When you perform a manifest refraction on a patient, occlude the contralateral eye (OC on the auxiliary lens knob) and keep the tested eye open (O on the auxiliary lens knob). The details on how to refract are too complicated to cover in this short article, so please watch this excellent video tutorial from the University of Iowa (vimeo.com/135867809) and the animation below. The animation will help you understand how images are focused on the retina through refraction. Before actually refracting, it’s important to prepare your patient for the process of refraction. You might say, “I am going to show you a series of two different choices. Please let me know which option is clearer. If they are the same, that is a perfectly good answer.” Then open the projector to show as many lines as possible and ask the patient to identify the lowest line of letters they can read (this will save you some time). Isolate that line. Then proceed with the four steps of a refraction (see the animation referenced above). Here are some key tips for each step: 1. Spherical power: Always add +0.50 D (diopters) first to decrease the risk of over-minusing the patient (when patients accommodate in the refractor, they will need more myopic lenses, which can lead to eye strain). Make your patients “earn” more minus — they must see a smaller line on the chart with the increased myopic lenses. 2. Cylinder axis: First, increase the Snellen acuity one line. The larger letters will make it easier for the patient to help you find the correct axis. 3. Cylinder power: Go back down a line on the chart to the smallest line the patient can read. Key step: When you add +0.50 D of cylinder power (two clicks clockwise, or right), you must take away 0.25 D of sphere (one click UP). This maintains the spherical equivalent. Therefore, if you take away 0.50 D of cylinder (two clicks counterclockwise, or left), you must add 0.25 D of sphere (one click DOWN), and vice versa (see animation). Think: up the plus cylinder, move the sphere wheel up; down the plus cylinder, move the sphere wheel down. 4. Spherical power refinement: For this step, go in 0.25-D increments. At the end, perform the duochrome test and ask the patient if the letters are clearer on the red or the green side. Remember it with these letters, “RAM-GAP,” which stands for “if the Red is clearer, Add Minus; if the Green is better, Add Plus.” Here are some general tips to get the best refraction for your patient: Work with the smallest line that the patient can read. “Which do you see better, #1 or #2?” Go all the way up to #10 (#3 or #4, #5 or #6, etc.) and then back to #1 versus #2. If the patient pauses, show them the two options again, providing for two seconds on each option. Refraction requires practice, so practice often on cooperative patients at first. You can do this. You will do this. Go forth and refract. * * * About the author: Evan Silverstein, MD, is an assistant professor of ophthalmology and associate resident program director at Virginia Commonwealth University in Richmond, Va.

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What isfocal length of lens

Nikonlens focal length angle of view

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The calculator below converts between the focal length f and the field of view (FOV) of a rectilinear lens. The formula that it implements is FOV = 2 arctan (x / (2 f)), where x is the diagonal of the film. The FOV is measured across the frame’s diagonal, and is therefore smaller across the horizonal dimension, and even smaller across the vertical dimension.

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Then open the projector to show as many lines as possible and ask the patient to identify the lowest line of letters they can read (this will save you some time). Isolate that line. Then proceed with the four steps of a refraction (see the animation referenced above). Here are some key tips for each step:

2. Cylinder axis: First, increase the Snellen acuity one line. The larger letters will make it easier for the patient to help you find the correct axis.

Refraction is the measurement of the eye’s focusing characteristics and the determinant of a prescription. A prescription has three main components: sphere, cylinder and axis.

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