Tools

A true-size Snellen chart for your monitor, tablet or phone, calibrated with a credit card for 10, 8 or 6.5 feet, plus a printable 10 ft PDF for US Letter paper.

Free online color blind test with 15 plates of dots generated on screen. Type the number you see in each plate for a screening result: no signs of deficiency, possible red-green deficiency (protan or deutan type), or a blue-yellow flag. Includes how to take it correctly, what each result means, and when a clinical test is needed.

Type the nasal engraving from an unmarked progressive and get the brand, design and refractive index back. Covers 251 catalogued engravings across ZEISS, Nikon, Pentax and American Optical, and tells you plainly when a lens is outside that catalogue.

Enter a measured distance PD and the wearer's working distance to get the near centration distance and the inset per eye, calculated from the standard geometry rather than the "subtract 4 mm" shortcut. Includes the working distances where that shortcut stops being safe.

Enter the frame's B measurement, the measured fitting height and the lens design's minimum to check whether a progressive job will work before you order it. Includes a sourced table of minimum fitting heights for Varilux, ZEISS, Hoya, Shamir and Kodak designs, plus the corridor and near-zone trade-offs behind the numbers.

Calculate induced prism from decentration with Prentice’s rule. Enter lens power and decentration for the prism, or any two of prism, power and decentration to solve for the third.

Check a finished pair of glasses against ANSI Z80.1 in seconds. Enter the ordered prescription and the lensmeter reading, and the tool flags sphere, cylinder, axis and add as pass or fail against the standard.

This post presents the common method for estimating the minimum lens block diameter, also known as Minimum Blank Size (MBS), using the formula “Frame Bridge + Frame Width – PD + Longest Diagonal + 2 mm” widely adopted in labs and optical shops. Despite its simplicity, this method offers bench‑speed calculation and a safety margin for block cutting, though it can be imprecise for complex frame geometries.

Compensate lens power for a change in vertex distance. Enter the refracted power, the refraction vertex and the as-worn vertex, and the calculator returns the power to order.

Convert a spectacle prescription to the equivalent contact lens power. The tool applies the vertex formula, which matters above about ±4.00 D, and returns the power to order.