The focal length of a lens refers to the distance between the focus (or one of the foci) to the center of the thin lens.

Lens angleof viewchart

Lasers are divided into four “classes” based on their output (Class 1 and 1 M, Class 2 and 2 M, Class 3A, 3B, 3R and Class 4).1,2,3 Class 1 lasers, (energy <0.4 mW) are the safest and cannot cause damage even if viewed for long periods of time. Class1 lasers are used in intrabeam viewing in optical instruments. Visible laser pointers (400–700 nm) operating at <1 mW and 1–5 mW power are Class 2 and Class 3A lasers respectively. Class 3B and 3R lasers generate between 5 and 500 mW of power; Class 4 lasers generate more than 500 mW of power.

I happen to have a fun little laser that can make extremely accurate measurements over long distances. I used this device to get my targets placed within a millimeter, and to measure the camera-to-subject distance.

The Fresnel lighthouse lens used a large lamp at the focal plane as its light source. It also contained a central panel of magnifying glasses surrounded above ...

The mathematics used in the calculations involves both algebra and trigonometry. Any “scientific” calculator has the necessary features to do this math. You can always download a scientific calculator app onto your smartphone. Fear not the math.

Transmission and absorption of optical radiation by ocular media depends upon the wavelength of the incident ultraviolet (UV) radiation, visible light or infrared radiation.5,6 For lasers, wavelength, spot size, pulse duration and irradiance determine the magnitude and extent of thermal damage in tissues exposed to the laser beam. Laser pointers emit invisible infrared radiation combined with visible light. Potential harmful effects to the eye, occur due to photomechanical, thermal or chemical injuries, or a combination of these effects.3,4,5,6,7 These effects are applied in a controlled manner for the treatment of eye diseases.1,2,3,4,5

Field of view of lensesnikon

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Retinal injuries due to laser pointers or devices have legal, financial and medical consequences. Most accidents are prevented by natural reflexive protective mechanisms.4,5,6 A lack of information on the types of lasers and the hazards, mis-information or lack of information to consumers, by laser device manufacturers, easy availability of hazardous lasers that resemble safe lasers are a few of the factors that can lead to careless use.1,2,3,4,5,6,7 Strict legislation, prohibiting the manufacture, use or possession of hazardous laser pointers, public education and education of the military sector about the hazards of lasers is mandatory, especially due to increased use of lasers for military applications.1,2,3,4,5,6,7,8,9,10

Multiple ocular symptoms such as pain, redness, irritation, corneal signs and retinal injury have been reported in patients exposed to laser pointers.1,2,3,4,5,6,7,8,9,10 Scotoma, photophobia, metamorphopsia, chromatopsia or decreased visual acuity can occur hours after exposure.1,2,3,4,5,6,7,8,9,10 The energy from the pointers at the ocular surface is insufficient to cause any appreciable harmful effect.4,5,6 However, amplification of irradiance caused by the ocular media to approximately 104 times makes the retina the most susceptible tissue in the body to laser pointer injury.7,8,9 Redness or surface irritation in a patient with laser pointer exposure is likely due to secondary rubbing of the eye. Pain following exposure to lasers may be due to corneal injury caused by eye rubbing following exposure.1,2,3,4,5

Case 2 presented a week after laser exposure complaining of poor vision in the right eye. Best corrected visual acuity was 1/60 with a refraction of plano −0.25 × 85 in the right eye with no improvement with pinhole and 6/5 with a refraction of −0.50 −0.25 × 60° in the left eye. Anterior segment evaluation and applanation tonometry was normal bilaterally. Amsler grid testing indicated a central scotoma in the right eye and the left eye was normal. There was premacular subhyaloid hemorrhage in the right eye approximately 1 disc diameter in size covering the foveal area horizontally [Figure 6]. The retina in the left eye was unremarkable. Optical coherence tomography indicated a shadow effect due to the subhyaloid hemorrhage in the right eye and the left eye was unremarkable [Figure 7]. Fundus fluorescein angiograms indicated blocked fluorescence corresponding to the hemorrhage at the fovea in the right eye and was unremarkable in the left eye [Figure 8]. Hyaloidotomy with YAG laser was unsuccessful in the right eye as the blood covering the fovea was organised [Figure 6c]. The patient was subsequently lost to follow up.

(a) Fundus photograph OD - premacular subhyaloid haemorrhage. (b) Fundus photograph OS normal. (c) Fundus photo graph OD - organised blood clot at posterior pole following yittrium aluminium garnet lysis

Allen et al.2 reported full thickness foveal hole in a 20-year-old man due to accidental exposure from a hand held Nd: YAG laser range finder device. Presence of strong reflections from the choroid underlying the hole on optical coherence tomogram, lead the authors to study the injury pattern by experimentally producing laser induced macular holes in non human primate models. They noted that this specific type of laser induced trauma required a minimum total intraocular energy of about 1–3 mJ.2,3,4 Laser induced macular holes were similar to idiopathic macular holes clinically and on angiography, but differed on optical coherence tomography, in that the former had increased reflectivity at the base due to scarring of the underlying choroid, similar to Case 3 in our series.1

Severity of visual loss, depends upon the distance of the laser impact site from the centre of the fovea, the extent of the chorioretinal disruption and amount of chorioretinal bleed.1,2,3,4,5 Rapid tissue expansion or distortion caused by extremely high irradiances during short exposure can result in retinal, subhyaloid, subretinal and or choroidal hemorrhages that cause temporary visual deficit if involving the central fundus. These deficits were present in Cases number 1 and 2 in our series.3,4,5,6 Permanent visual deficit occurs if there is an underlying damage to the retinal structure.5,6,7,8 For example Case number 1 had persistent poor vision due to alteration of retinal structure noted on optical coherence tomography. In Case number 2, the final visual outcome remains unknown as patient was lost to follow up.

I made all of the distance measurements relative to the lens axis and to a spot on the lens that I guessed to be near the vertex of the red arrows shown in the diagram. You don’t really need to know this location, since these techniques will actually calculate the location.

Nd: YAG laser hyaloidotomy is a safe and effective procedure, achieving rapid resolution of premacular subhyaloid haemorrhage with restoration of visual function while preventing the need for vitreoretinal surgery.8 In our case series, treatment was declined by Case number 1 and ineffective in Case number 2.

Aug 1, 2022 — The Fresnel optics features a wide field of view compared to Pancake but is prone to chromatic aberrations (ghosting/overlapping colours).

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Corresponding Author: Dr. Radha Shenoy, Department of Ophthalmology, Armed Forces Hospital, Muscat, Sultanate of Oman. E-mail: shenoyvss@gmail.com

To calculate Brewster's angle use the formula αB = arctan(n2 / n1), where αB ... Brewster's angle or the polarization angle is an angle of incidence at ...

Jul 21, 2022 — What a polarizer filter does is cut down on any excess brighter whiter light while still allowing the colors of the light, object, or ...

Case 3 was another soldier who was 28 years old and presented approximately 8–9 months after presentation of Cases 1 and 2 with a similar history. However, he reported to the clinic immediately after exposure. On examination, best corrected visual acuity was 6/5 with a refraction of −0.25 D sphere in the right eye and 1/60 with a refraction of −0.25 D sphere in the left eye with no improvement on pinhole testing. Amsler grid testing indicated normal results in the right eye and a central scotoma in the left eye. On fundus evaluation the CDRs were 0.3 bilaterally, the retinal vessels and fovea in the right eye were normal. There was a well circumscribed round hole at the fovea in the left eye [Figure 10]. On Optical coherence tomography, the macula was normal in the right eye, and there was a full thickness macular hole with cystoid changes at the edges of the hole and increased reflectivity at the base in the left eye [Figure 11]. Fluorescein angiography of the right eye was unremarkable and there was a window defect in the left eye corresponding to the full thickness macular hole [Figure 12].

The diagrams above show the setup to align and measure your lens to get the information for calculating the focal length and field of view (FOV).

Class 2 lasers cause damage to retina if the beam is viewed for more than 10 s at close range. Class 3 lasers, especially 3R and 3B are hazardous and prohibited in many countries but are easily available online and are popular with teenagers.7 Class 4 lasers are the most powerful lasers, and are used in military and occupational settings, such as laser shows. They are capable of producing extensive ocular damage.4,5,6,7,8,9,10

For both setups, the angular measurement from the lens axis to the frame edge is called ‘Angle’, measured in degrees. The FOV of the lens is twice the value of ‘Angle’.

Using these techniques, you could actually find out what an un-marked lens’ focal length is. You could also verify (or debunk) manufacturer claims about the real focal length of a lens.

The trend tends to be that larger sensors have larger individual pixels (24mp s35 vs 24mp ff, each individual pixel is physically larger) that ...

One reason you might like to measure this stuff, versus just reading the lens focal length stamped on the lens, is that manufacturers often lie about the true lens focal length. This is particularly true of long zooms, which might not be as long as claimed. It’s often the case that a lens focal length reduces drastically when focusing close (called focus breathing), and these techniques could attach real numbers to that.

(a) Optical CoherenceTomoraphy OD - macula normal (b) optical coherence tomoraphy OS showing fill thickness hole, cystoid changes at the edges, with increase reflectivity at the base of the hole

Field of view of lensesformula

You only need to take 6 different measurements to calculate everything! The trick is in setting up the targets to make those measurements accurate.

My setup was on a tile floor, and I used the tile grout lines to make sure the lens axis was exactly perpendicular to the wall. You could also temporarily tape a little mirror against the wall and line up the camera until you see your reflection in the center of the camera’s field of view.

Field of viewdefinition microscope

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(a) Optical coherence tomography macula OD - shadowing due to subhyaloid haemorrhage. (b) Optical coherence tomography macula OS - normal

Case 1 presented to the emergency clinic 1-day after laser exposure complaining of poor vision in the left eye. On examination, the best corrected visual acuity was 6/6 with a refraction of −0.25 D sphere in the right eye and 1/60 with a refraction of −0.50–0.25 × 90° in the left eye with no improvement on pin-hole testing. Anterior segments and intraocular pressures were normal bilaterally Amsler grid testing was normal for the right eye and there was central scotoma in the left eye. On dilated fundus evaluation, the cup-to-disc ratios (CDR) were 0.4–0.5 bilaterally, the retinal vessels and fovea and fovea, in the right were normal. The left showed premacular subhyaloid hemorrhage approximately 1½ disc diameter [Figure 1]. Optical coherence tomography of the macula in the right eye was normal, in the left eye there was a shadow effect due to the subhyaloid haemorrhage [Figure 2]. The patient declined yittrium aluminium garnet (YAG) laser hyaloidotomy for the left eye. At 1-month follow up subhyaloid hemorrhage had completely resolved with a dull foveal reflex on fundus evaluation and no improvement in vision in the left eye [Figure 3]. Optical coherence tomography of macula in the left eye showed irregularity of the retinal layers [Figure 4]. Fluorescein angiography was un remarkable bilaterally [Figure 5].

Oct 8, 2023 — Key takeaways: · Treating acne and post-inflammatory hyperpigmentation is important for patients with skin of color. · 42% of the 1,064 nm Nd: ...

Anterior segment injuries are rare as the cornea and crystalline lens absorbs most of the UV and infrared energies. Natural responses such as the blink response, squinting, pupillary constriction, and aversion from the uncomfortably bright light protects the retina from accidental injury.1,2,3 Most often, retinal injuries are subtle with no objective findings making diagnosis of laser induced damage difficult. In such cases diagnosis is facilitated by a set of six questions formulated by Mainster et al.1 Visual prognosis is excellent if retinal findings are minor or spare the fovea.1,2,3,4,5,6

Two young soldiers (Cases 1 and 2) aged 27 and 28 years respectively, serving in the Oman army, projected penlight like devices emanating bright blue-green light into each others eyes (left eye for Case number 1 and right eye for Case number 2) for about 5–10 s. They competing with each other to determine who could bear the light longer while celebrating the success of a local football game. Both individuals experienced some after images, severe photophobia and headache followed by blurred vision the next day.

Retinal lesions following laser injury generally heal on their own, without any specific therapy. Systemic corticosteroids have been used, with little conclusive evidence indicating faster recovery.8,9,10 Hossein et al.9 noted clinical and objective improvement of laser induced maculopathy on spectral-domain optical coherence tomography in a patient who was treated with high dose systemic steroids. The patient was exposed to a class 3R laser for <1 s. Spectral domain optical coherence tomography disclosed a hyper-reflective band in the foveal region. After 1-week of treatment, the hyper-reflectivity resolved. However, residual disruption of the outer retinal layer at the fovea remained unchanged.9

Field of viewhuman eye

As I often say, “garbage in, garbage out”. The more accurate measurements you make, the more accurate your results will be.

The following Ophthalmologists and Optometrists have supported us in the collection of data that has aided in the preparation of this manuscript and we acknowledge their support, without which this manuscript would not have been complete. Dr. Milind Suryavanshi M.S (Ophth) Diplomate. FRCS (Glas), Dr. Santhosh K Philip M.S. (Ophth), Dr. Rashid Mohammed Al Saedi M.D. FEBO, Dr. Badar Mohammed Al Barwani MBChB (Glas). F.R.C.S. (Edin) and Optometrists Mr. Rahul Sharngadhar. BSc. Optometry Mr. Mohan Nirmal MSc. Optometry, Mr. Nasser Al Shamali MSc. Optometry.

Kasaoka et al.10 used animal models and reported that the RPE cells initiate a post-injury process in response to pathologic states and transform from a stationary epithelial state to a spindle-shaped, migratory, proliferative mesenchymal state, leading to the transretinal membrane formation associated with the development of proliferative vitreoretinopathy.10 This RPE transdifferentiation and its migration across the retinal surfaces is mediated by a tyrosinase receptor called c-Met. Control of this activity may be a future therapeutic target to minimize retinal damage following laser injury.9,10

To get all of the necessary information, you’ll need to set up some targets on a wall (I used painter’s tape) and then take distance measurements at two different camera-to-wall distances.

As shown above, I put some tape targets on the wall that point at the edges of the field of view and the exact center of the frame. I made sure that the distances on either side of the middle of the frame exactly matched. I also made sure that the lens was at exactly the same height as the middle wall target. Using high magnification via Live View or the viewfinder helps getting the targets more precise.

(a) Fundus fluorescein angiography OD showing normal dye transit (b) fundus fluorescein angiography OS showing-normal dye transit

Retinal injuries due to military and industrial lasers occur in less than 15 individuals yearly, world wide, despite the increasing use of lasers in the health care, military, and educational sectors, and in commercial laboratories.1 In the military, lasers are used as range finders, target designators and for long distance communications.1,2,3,4,5,6 Even in ophthalmology the use of lasers has increased significantly. The increase in the use of laser devices has resulted in a concomitant increase in ocular exposure to laser radiation. Accidental momentary laser exposure can be annoying and distracting.7,8,9,10 Prolonged viewing of the beam for more than 10 s especially at close range, can cause retinal damage.10

Recently unregulated lasers have been imported in the Middle East and can be easily acquired by the public here, we present three cases of military personnel with unilateral visual loss and retinal lesions following alleged exposure to laser pointers. None of the individuals were aware that the bright blue-green light projected into their eyes was from a laser pointer and was harmful.

(a) Optical coherence tomography OD - normal macula. (b) Optical coherence tomography OS - showing shadowing of macula by subhyaloid hemorrhage, horizontal level and sup macula seen, ILM irregular

This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Laser pointers are simple, handheld battery operated devices, used for teaching and training purposes. Laser pointers are comprised of a diode emitting laser light, with an energy output between 1 and 5 mW.1,2,3,4,5 Initially commercially available laser pointers were red lasers with a wavelength of 670 nm. However, other wavelengths such as green, blue, yellow and violet lasers are also available and have become popular because of their advantages. For example, green lasers have brighter beams visible both in daylight and night, allowing star gazing and pointing far off objects, blue and violet lasers light up to different colors depending upon where they are projected, yellow lasers dazzle like gold and are used as a laser guide star for use with astronomical adaptive optics.1,2,3,4,5,6,7 As the use of laser pointers becomes more popular, physicians need to understand which lasers can cause eye injury.1,2,3,4,5,6,7,8

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(a) Fundus fluorescein angiography OD - normal dye transit. (b) Fundus Fluorescein angiography OS - showing hyperfluorescence at the site of full thickness macular hole

Field of viewcalculator

(a) Fundus photograph of normal OD. (b) Fundus photograph OS showing subhyaloid hemorrhage at the posterior pole with horizontal level

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Keywords: Fundus Fluorescein Angiography, Laser Pointer, Macular Hole, Optical Coherence Tomography, Subhyaloid Hemorrhage

Large fresnel lens, strong light transmission and excellent color reproduction. Also shop for tools at best prices on AliExpress!

(a) Fundus fluorescein angiography OD - showing blocked fluorescence due to subhyaloid hemorrhage at posterior pole. (b) Fundus fluorescein angiography OS - normal dye transit

Articles from Middle East African Journal of Ophthalmology are provided here courtesy of Wolters Kluwer -- Medknow Publications

Laser pointers are practical and safe training tools when used properly. If used incorrectly they can cause ocular damage, potentially resulting in devastating vision loss. The ocular and visual morbidity can result in significant expenses for medical care and inability to work (temporarily or permanently) for civilians and military personnel. We present three cases of soldiers who experienced vision loss following exposure to laser pointers, while celebrating successfull football game.

The output of laser pointers available to the general public is limited and varies by country.6,7,8,9,10 As per the United States Food and Drug Administration Code of Federal Regulations, “demonstration laser products” like pointers must comply with applicable requirements for Class I, IIa, II, or IIIa devices.1,2,3,4,5,6,7,8,9,10

The nearer wall distance setup is called ‘x’. The farther wall distance setup is called ‘X’. Similarly, the distance along the wall from the frame middle to the frame edge in the nearer wall setup is called ‘y’, while the farther distance setup is called ‘Y’.